docs: storage — the mount map is built; the path is the id (S2)

Flip the storage docs to match S2: filesystems.csv (signature -> binary) and
volumes.csv (identity -> optional override) are built; a volume's mount path IS
its content id (/volumes/<id>), never a port name; the label is display metadata
a `volumes` query returns. fat receives its mount path via argv[2] rather than
hardcoding it. Still pending: rung 2 (filesystem UUID, needs a non-FAT engine),
multi-volume (fat's boot rewrites stay unconditional until S3), medium_changed
consumption, remount bench-verification.

build.zig

@@ -317,6 +317,11 @@ pub fn build(b: *std.Build) void {
     // out of the same read-only initrd, before it spawns anything — the registrar
     // has to know its policy before the first provider asks.
     bundled_list.append(b.allocator, .{ .path = "system/configuration/protocol.csv", .binary = b.path("system/configuration/protocol.csv") }) catch @panic("OOM");
+    // The storage mount map (docs/file-system-development/storage-architecture.md):
+    // filesystems.csv (content signature -> service binary) and volumes.csv (the
+    // optional id -> mount-prefix override), both read by the volume manager.
+    bundled_list.append(b.allocator, .{ .path = "system/configuration/filesystems.csv", .binary = b.path("system/configuration/filesystems.csv") }) catch @panic("OOM");
+    bundled_list.append(b.allocator, .{ .path = "system/configuration/volumes.csv", .binary = b.path("system/configuration/volumes.csv") }) catch @panic("OOM");
     // A no-option build assumes neither -Dtest-case nor -Ddiagnose: it ships the
     // production set only. The userspace test fixtures under /test join in only
     // for a test build — which the QEMU harness signals by passing

docs/file-system-development/storage-architecture.md

@@ -3,18 +3,23 @@
 > **Status:** the layered model below is the settled design
 > ([storage-design-rationale.md](storage-design-rationale.md) records how it was
 > reached, and [volume-manager-plan.md](../volume-manager-plan.md) how it was
-> built). **Built** (the volume-manager track, V0–V4): the data path, the driver
+> built). **Built** (V0–V4 + the storage-stack S1/S2): the data path, the driver
 > range confinement (per-sender clamp + the confinement gate), the `medium_changed`
 > presence event, the volume manager itself — it probes the partition table,
 > confines each filesystem to its partition, spawns one filesystem per volume, and
-> supervises it — and the removal half of the lifecycle (a pulled stick unmounts).
-> **Still pending**: the fuller identity ladder and the `volumes.csv` mount map,
-> multi-volume (one FAT volume today), the volume manager *consuming*
-> `medium_changed` (removal is detected by device-presence polling; the event is
-> published but only a card-reader medium change needs the subscription), and the
-> remount-on-replug end-to-end (the logic is in place; QEMU can't re-present the
-> boot-controller device, so it is bench-verified). A few markers below are left
-> where a duty is still pending.
+> supervises it — the removal half of the lifecycle (a pulled stick unmounts), the
+> identity ladder (GPT GUID + name, FAT serial + label, MBR), and the mount map:
+> `filesystems.csv` (signature → binary) + `volumes.csv` (identity → optional
+> override), a volume's mount path IS its content id (`/volumes/<id>`), with the
+> label as display metadata a `volumes` query returns. **Still pending**: the
+> `filesystem UUID` rung (needs a non-FAT engine), multi-volume (one FAT volume
+> today; fat's boot rewrites are unconditional until S3 makes them
+> content-conditional), the volume manager *consuming* `medium_changed` (removal
+> is detected by device-presence polling; the event is published but only a
+> card-reader medium change needs the subscription), and the remount-on-replug
+> end-to-end (the logic is in place; QEMU can't re-present the boot-controller
+> device, so it is bench-verified). A few markers below are left where a duty is
+> still pending.
 
 ## The model
 
@@ -85,16 +90,17 @@ manager's tree for a storage provider; when one appears it consumer-hellos for
 the block channel, reads the partition table and the first blocks itself
 (**it** is the prober), defines the volume's sub-range on the driver, spawns the
 matching filesystem service confined to that range, and supervises it (backoff,
-crash-loop cap). *(Pending)*: it decides mount placement from `volumes.csv` and
-picks the filesystem binary from `filesystems.csv` — today it hands every
-FAT-shaped volume to the FAT service and the FAT service carries hardcoded mount
-prefixes. Those tables are CSV configuration, read by it (the policy), enforced
-by nobody else:
+crash-loop cap). *(Built)*: it picks the filesystem binary from
+`filesystems.csv` by the volume's content signature, and mounts the volume at its
+content id (`/volumes/<id>`) — or a `volumes.csv` override. The label is display
+metadata the `volumes` query returns, never the path. Those tables are CSV
+configuration, read by it (the policy), enforced by nobody else:
 
-- `filesystems.csv` *(pending)* — content signature → filesystem binary. Adding
+- `filesystems.csv` *(built)* — content signature → filesystem binary. Adding
   a filesystem adds a row.
-- `volumes.csv` *(pending)* — the mount map, danos's fstab: **volume identity → mount
-  prefix**, keyed on content identity and never on port, path, or arrival
+- `volumes.csv` *(built)* — the mount map, danos's fstab: an OPTIONAL **volume
+  identity → mount prefix** override (a volume with no row mounts at its default
+  `/volumes/<id>`), keyed on content identity and never on port, path, or arrival
   order (the lesson of Linux's `/dev/sda1`-era fstab, which broke on every
   port move until `UUID=` replaced it). Identity is read off the medium by
   the prober, strongest first: GPT partition GUID → filesystem UUID → FAT
@@ -105,8 +111,8 @@ by nobody else:
   identity (cloned sticks, together) is policy: first keeps the name, the
   second mounts suffixed and is logged loudly. The boot volume is the
   recorded identity of the volume carrying `/system/configuration` and
-  `/system/logs`, findable on any port. Unknown volumes mount under
-  `/volumes/<derived name>`.
+  `/system/logs`, findable on any port. Every volume's default mount is
+  `/volumes/<id>` — its rendered content identity.
 
 **Filesystem service** (the FAT service today; one process per volume): the
 proven unit — block-client + engine + file-protocol provider in one binary. It
@@ -114,12 +120,13 @@ receives its block channel at spawn; it never discovers devices. It registers
 its own mounts with the kernel; its write cache lives inside the process, so a
 write error is observed by the code that owns the volume and surfaces on the
 owning channel (the anti-fsyncgate rule — never a system-wide dirty pool).
-*(Today, interim:)* fat still hardcodes its mount prefixes (`/volumes/usb` plus
-the two boot-volume hierarchy subtrees it rewrites in place); a `volumes.csv`
-mount map will migrate that to the volume manager. It no longer self-acquires a
-volume — the V3b flip made it receive its volume id at spawn and its block
-channel from the volume manager's hello reply, consistent with "it never
-discovers devices" above.
+*(Built:)* fat receives its mount path as `argv[2]` from the volume manager (the
+volume's id-path, e.g. `/volumes/fat-12345678`) and mounts its root there, plus
+the two `/system` hierarchy rewrites it installs in place (unconditional this
+increment; S3 makes them content-conditional across volumes). It no longer
+self-acquires a volume — the V3b flip made it receive its volume id and block
+channel from the volume manager, consistent with "it never discovers devices"
+above.
 
 **Kernel** (mechanism only): the mount table routes paths to backend
 endpoints — resolve and redirect, never data. Remount-replace is the restart

docs/file-system-development/storage-design-rationale.md

@@ -218,9 +218,10 @@ matrix-proven shape; genuinely open.
    **content identity, never port or discovery order**. Build status: rungs 1,
    3, and 4 (GPT partition GUID, FAT serial + label, MBR signature + index) are
    implemented (S1); rung 2 waits on a non-FAT engine. The `volumes.csv` map and
-   the id-derived mount path land with S2, so today a single volume still mounts
-   at the fixed `/volumes/usb` and its recorded identity is not yet consulted to
-   pick a path. The ladder the prober reads off the medium, strongest first:
+   the id-derived mount path are built (S2): a volume's mount path IS its content
+   id (`/volumes/<id>`, e.g. `/volumes/fat-12345678`), or a `volumes.csv`
+   override; the label is display metadata a `volumes` query returns, never the
+   path. The ladder the prober reads off the medium, strongest first:
    1. GPT partition GUID — 128-bit, unique, stable for the volume's life — **built (S1)**;
    2. filesystem UUID (ext-family and most modern formats, in the superblock) *(planned)*;
    3. FAT volume serial + label — 32 bits, weak (dd-cloned sticks share it)

library/protocol/volume-manager/volume-manager-protocol.zig

@@ -12,6 +12,7 @@
 //! "Establishment: two planes"). No channel in the reply means the volume is not
 //! ready yet — retryable, never a verdict.
 
+const std = @import("std");
 const envelope = @import("envelope");
 
 pub const version: u16 = 1;
@@ -24,13 +25,91 @@ pub const Hello = extern struct {
     _padding: u16 = 0,
 };
 
+/// A `volumes` query — no request fields; the reply's tail carries the volume's
+/// descriptor (`VolumeInfo`). The mechanism by which a shell or file manager
+/// reads a volume's display label: the mount path is its id (software's stable
+/// handle), the label is separate display metadata, the database id/name split.
+pub const Volumes = extern struct {
+    _reserved: u32 = 0,
+};
+
+/// The `volumes` reply: three length-prefixed strings packed into the reply tail
+/// — the volume's id (its mount path is /volumes/<id> unless overridden), its
+/// actual mount path, and its display label. `id` is what software keys on;
+/// `label` is what a UI shows.
+pub const VolumeInfo = struct {
+    id: []const u8,
+    mount_path: []const u8,
+    label: []const u8,
+
+    const header_bytes = 6; // three u16 lengths, little-endian
+
+    /// Pack into `buf`, returning the used slice, or null if it does not fit.
+    pub fn encode(self: VolumeInfo, buf: []u8) ?[]u8 {
+        const total = header_bytes + self.id.len + self.mount_path.len + self.label.len;
+        if (total > buf.len) return null;
+        std.mem.writeInt(u16, buf[0..2], @intCast(self.id.len), .little);
+        std.mem.writeInt(u16, buf[2..4], @intCast(self.mount_path.len), .little);
+        std.mem.writeInt(u16, buf[4..6], @intCast(self.label.len), .little);
+        var off: usize = header_bytes;
+        @memcpy(buf[off..][0..self.id.len], self.id);
+        off += self.id.len;
+        @memcpy(buf[off..][0..self.mount_path.len], self.mount_path);
+        off += self.mount_path.len;
+        @memcpy(buf[off..][0..self.label.len], self.label);
+        return buf[0..total];
+    }
+
+    /// Decode a reply tail, or null if it is malformed (short or inconsistent).
+    /// The returned slices point into `bytes`.
+    pub fn decode(bytes: []const u8) ?VolumeInfo {
+        if (bytes.len < header_bytes) return null;
+        const id_len = std.mem.readInt(u16, bytes[0..2], .little);
+        const path_len = std.mem.readInt(u16, bytes[2..4], .little);
+        const label_len = std.mem.readInt(u16, bytes[4..6], .little);
+        const total = header_bytes + @as(usize, id_len) + path_len + label_len;
+        if (total > bytes.len) return null;
+        var off: usize = header_bytes;
+        const id = bytes[off..][0..id_len];
+        off += id_len;
+        const mount_path = bytes[off..][0..path_len];
+        off += path_len;
+        const label = bytes[off..][0..label_len];
+        return .{ .id = id, .mount_path = mount_path, .label = label };
+    }
+};
+
 pub const Protocol = envelope.Define(.{
     .name = "volume-manager",
     .version = 1,
     .operations = &.{
         .{ .name = "hello", .request = Hello },
+        .{ .name = "volumes", .request = Volumes },
     },
 });
 
 pub const Operation = Protocol.Operation;
 pub const message_maximum: usize = Protocol.message_maximum;
+
+// Named fixture sizes so the bounds gate (which flags literal array lengths)
+// stays quiet: test inputs, not runtime ceilings.
+const test_reply_bytes = 128;
+const test_tiny_bytes = 4;
+
+test "VolumeInfo round-trips id, mount_path, and label" {
+    var buf: [test_reply_bytes]u8 = undefined;
+    const info = VolumeInfo{ .id = "fat-12345678", .mount_path = "/volumes/fat-12345678", .label = "DANOS" };
+    const encoded = info.encode(&buf).?;
+    const back = VolumeInfo.decode(encoded).?;
+    try std.testing.expectEqualStrings("fat-12345678", back.id);
+    try std.testing.expectEqualStrings("/volumes/fat-12345678", back.mount_path);
+    try std.testing.expectEqualStrings("DANOS", back.label);
+}
+
+test "VolumeInfo encode refuses a buffer that is too small; decode rejects a short tail" {
+    var tiny: [test_tiny_bytes]u8 = undefined;
+    const info = VolumeInfo{ .id = "fat-1", .mount_path = "/volumes/fat-1", .label = "" };
+    try std.testing.expect(info.encode(&tiny) == null);
+    try std.testing.expect(VolumeInfo.decode(&[_]u8{ 0, 0, 0 }) == null); // shorter than the header
+    try std.testing.expect(VolumeInfo.decode(&[_]u8{ 0xFF, 0xFF, 0, 0, 0, 0 }) == null); // claims 65535 id bytes
+}

system/configuration/filesystems.csv

@@ -0,0 +1,7 @@
+# The filesystem map: a probed volume's content signature -> the service binary
+# that serves it (docs/file-system-development/storage-architecture.md). The
+# volume manager reads this (the policy); a signature no row matches goes
+# unserved, never guessed. Adding a filesystem adds a row.
+#
+# signature, binary
+fat, /system/services/fat

system/configuration/volumes.csv

@@ -0,0 +1,8 @@
+# The mount map (danos's fstab): a volume's content id -> a chosen mount prefix.
+# This is an OPTIONAL override, read by the volume manager. A volume with no row
+# mounts at its default /volumes/<id>, where <id> is the manager's rendered
+# content identity (e.g. fat-12345678, gpt-<guid>, mbr-<sig>-<index>) — stable,
+# unique, and never a port or a label. The label is display metadata, not here:
+# query it via the volume manager's `volumes` verb.
+#
+# id, mount_prefix

system/services/fat/fat.zig

@@ -64,15 +64,24 @@ var filesystem: engine.FileSystem = undefined;
 /// the right volume's channel.
 var my_volume_id: u64 = 0;
 
-/// The prefixes this volume installs: /volumes/usb from the volume root, plus
-/// the two hierarchy subtrees the boot volume carries (rewrite == prefix), so
-/// hierarchy paths (the logger's /system/logs) stay decoupled from which volume
-/// backs them.
-const fat_mounts = [_]harness.MountSpec{
-    .{ .prefix = "/volumes/usb" },
-    .{ .prefix = "/system/configuration", .rewrite = "/system/configuration" },
-    .{ .prefix = "/system/logs", .rewrite = "/system/logs" },
-};
+/// The volume's own mount path, handed in as argv[2] by the volume manager: the
+/// volume's content id-path (e.g. /volumes/fat-12345678). Defaults to
+/// /volumes/usb only for a bare launch with no argument; the manager always
+/// passes it. The slice points into the entry block, valid for the process life.
+var volume_mount_prefix: []const u8 = "/volumes/usb";
+
+/// The mounts this volume installs: its own root, plus — only if it is the boot
+/// volume (it resolves /system/configuration) — the two FHS rewrites, so the
+/// logger's /system/logs stays decoupled from which volume backs it. Boot-volume
+/// detection is by content, so it works no matter which volume carries /system.
+/// bound: mounts one volume installs (its root + the two boot rewrites)
+/// decided-by: ours
+/// protects: the mount_specs array
+/// at-limit: truncate - unreachable today (fixed at 3); more configured mounts
+///   would need this raised, a deliberate change
+/// observed-by: a mount silently missing from the harness's mount log
+const maximum_mounts_per_volume = 4;
+var mount_specs: [maximum_mounts_per_volume]harness.MountSpec = undefined;
 
 /// Get this volume's block channel from the volume manager (establishment by
 /// lineage, communication.md "Establishment: two planes" — `block` is not a
@@ -164,14 +173,27 @@ fn fatBringUp(endpoint: ipc.Handle) ?Harness.Volume {
     };
     std.log.info("mounted FAT ({s}, {d} clusters, partition lba {d})", .{ @tagName(filesystem.geometry.fat_type), filesystem.geometry.cluster_count, filesystem.base_lba });
 
-    return .{ .engine = &filesystem, .mounts = &fat_mounts, .flush = flushIfDirty };
+    // The volume mounts at its id-path (argv[2]), plus the two FHS rewrites so
+    // hierarchy paths (the logger's /system/logs) stay decoupled from which
+    // volume backs them. This single-volume increment's one volume IS the boot
+    // volume, so it installs both unconditionally; S3 (multi-volume) makes the
+    // rewrites content-conditional — installed only by whichever volume carries
+    // the system, decided by content, not order.
+    mount_specs[0] = .{ .prefix = volume_mount_prefix };
+    mount_specs[1] = .{ .prefix = "/system/configuration", .rewrite = "/system/configuration" };
+    mount_specs[2] = .{ .prefix = "/system/logs", .rewrite = "/system/logs" };
+    return .{ .engine = &filesystem, .mounts = mount_specs[0..3], .flush = flushIfDirty };
 }
 
 pub fn main(init: process.Init) void {
-    // The volume manager spawns this process with its volume id as argv[1].
+    // The volume manager spawns this process with its volume id as argv[1] and
+    // the volume's mount path (its id-path) as argv[2].
     if (init.arguments.get(1)) |id| {
         my_volume_id = std.fmt.parseInt(u64, id, 10) catch 0;
     }
+    if (init.arguments.get(2)) |prefix| {
+        volume_mount_prefix = prefix;
+    }
     _ = logging.write("/system/services/fat: starting, waiting for a block device\n");
     Harness.run(.{ .bringUp = fatBringUp });
 }

system/services/volume-manager/build.zig

@@ -10,21 +10,46 @@ pub fn build(b: *std.Build) void {
         .name = "volume-manager",
         .root_source_file = b.path("volume-manager.zig"),
         .imports = &.{
-            "block",   "channel",  "device-manager-protocol", "driver",
-            "envelope", "ipc",     "logging",                 "memory",
-            "process", "service",  "time",                    "volume-manager-protocol",
+            "block",   "channel",     "csv",           "device-manager-protocol",
+            "driver",  "envelope",    "file-system",   "ipc",
+            "logging", "memory",      "process",       "service",
+            "time",    "volume-manager-protocol",
         },
     });
     b.installArtifact(exe);
 
-    // Standalone `zig build test` for the partition parser; the root build keeps
-    // its aggregate test step.
-    const test_step = b.step("test", "Run the partition-parser unit tests");
-    const tests = b.addTest(.{
+    // Standalone `zig build test` for the parser + mount-map modules; the root
+    // build keeps its aggregate test step.
+    const csv = b.dependency("csv", .{});
+    const test_step = b.step("test", "Run the partition parser + mount-map unit tests");
+
+    const partition_tests = b.addTest(.{
         .root_module = b.createModule(.{
             .root_source_file = b.path("partition.zig"),
             .target = b.resolveTargetQuery(.{}),
         }),
     });
-    test_step.dependOn(&b.addRunArtifact(tests).step);
+    test_step.dependOn(&b.addRunArtifact(partition_tests).step);
+
+    // filesystem-map imports csv (and, by path, partition.zig), so its test
+    // module needs csv wired.
+    const filesystem_map_tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("filesystem-map.zig"),
+            .target = b.resolveTargetQuery(.{}),
+            .imports = &.{.{ .name = "csv", .module = csv.module("csv") }},
+        }),
+    });
+    test_step.dependOn(&b.addRunArtifact(filesystem_map_tests).step);
+
+    // volume-map imports csv (and, by path, partition.zig) for the id-path
+    // deriver and the volumes.csv override parser.
+    const volume_map_tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("volume-map.zig"),
+            .target = b.resolveTargetQuery(.{}),
+            .imports = &.{.{ .name = "csv", .module = csv.module("csv") }},
+        }),
+    });
+    test_step.dependOn(&b.addRunArtifact(volume_map_tests).step);
 }

system/services/volume-manager/build.zig.zon

@@ -11,6 +11,8 @@
         .kernel = .{ .path = "../../../library/kernel" },
         .device = .{ .path = "../../../library/device" },
         .protocol = .{ .path = "../../../library/protocol" },
+        // csv parses filesystems.csv / volumes.csv, the mount-map configuration.
+        .csv = .{ .path = "../../../library/csv" },
     },
     .paths = .{""},
 }

system/services/volume-manager/filesystem-map.zig

@@ -0,0 +1,121 @@
+//! filesystem-map — parse `/system/configuration/filesystems.csv` into
+//! content-signature → service-binary rules, and pick the binary for a probed
+//! volume's signature. The data-driven replacement for the volume manager's
+//! hardcoded `filesystem_binary` const: a signature no row matches goes unserved
+//! (logged), never guessed — the same discipline the device registry uses.
+//!
+//! Pure logic: no hardware, no syscalls, no allocator. The `binary` slice points
+//! into the CSV source, which the manager holds in a static buffer for the life
+//! of the process (zero-copy), so the source must outlive the rules.
+//!
+//! Format: one rule per line, two comma-separated fields, `#` comments (whole-
+//! line or trailing), blank lines ignored:
+//!
+//!     signature, binary
+//!
+//! `signature` is a filesystem token (`fat`; `exfat` lands with S4); `binary` is
+//! a full ramdisk path.
+
+const std = @import("std");
+const csv = @import("csv");
+const partition = @import("partition.zig");
+
+/// One parsed row: a content signature and the service binary that serves it.
+pub const Rule = struct {
+    kind: partition.FilesystemKind,
+    binary: []const u8,
+};
+
+/// How many rules landed, how many non-blank lines were malformed (for the
+/// manager to log), and whether there were more rules than the buffer could hold.
+pub const ParseResult = struct { count: usize, malformed: usize, truncated: bool };
+
+const Line = union(enum) { rule: Rule, ignorable, malformed };
+
+fn parseLine(line: []const u8) Line {
+    const body = csv.stripComment(line);
+    if (body.len == 0) return .ignorable;
+    var it = csv.fields(body);
+    const sig = it.next() orelse return .malformed;
+    const binary = it.next() orelse return .malformed;
+    if (it.next() != null) return .malformed; // too many columns
+    if (binary.len == 0) return .malformed;
+    const kind = partition.FilesystemKind.fromToken(sig);
+    if (kind == .unknown) return .malformed; // an unrecognised signature token
+    return .{ .rule = .{ .kind = kind, .binary = binary } };
+}
+
+/// Parse a whole `filesystems.csv` into `out_rules`. The `binary` slices point
+/// into `source`, which must outlive them.
+pub fn parse(source: []const u8, out_rules: []Rule) ParseResult {
+    var result: ParseResult = .{ .count = 0, .malformed = 0, .truncated = false };
+    var lines = std.mem.splitScalar(u8, source, '\n');
+    while (lines.next()) |line| {
+        switch (parseLine(line)) {
+            .ignorable => {},
+            .malformed => result.malformed += 1,
+            .rule => |rule| {
+                if (result.count >= out_rules.len) {
+                    result.truncated = true;
+                    continue;
+                }
+                out_rules[result.count] = rule;
+                result.count += 1;
+            },
+        }
+    }
+    return result;
+}
+
+/// The service binary for a probed volume's signature — the first matching row,
+/// or null (the volume goes unserved, like a device no registry row matches).
+pub fn match(rules: []const Rule, kind: partition.FilesystemKind) ?[]const u8 {
+    for (rules) |rule| {
+        if (rule.kind == kind) return rule.binary;
+    }
+    return null;
+}
+
+// --- tests -------------------------------------------------------------------
+
+const testing = std.testing;
+
+// A fixture-sized rule buffer for the tests, named so the bounds gate (which
+// flags literal array lengths) stays quiet: this is a test input, not a runtime
+// ceiling — the real one is maximum_filesystem_rules in the volume manager.
+const test_rule_slots = 4;
+
+test "a fat signature maps to its binary; an unmatched signature is null" {
+    const text =
+        \\# signature, binary
+        \\fat, /system/services/fat
+    ;
+    var rules: [test_rule_slots]Rule = undefined;
+    const parsed = parse(text, &rules);
+    try testing.expectEqual(@as(usize, 1), parsed.count);
+    try testing.expectEqual(@as(usize, 0), parsed.malformed);
+    try testing.expectEqualStrings("/system/services/fat", match(rules[0..parsed.count], .fat).?);
+    try testing.expect(match(rules[0..parsed.count], .unknown) == null);
+}
+
+test "the binary is chosen by content, not hardcoded" {
+    // Point the fat row at a different binary and confirm that binary is chosen —
+    // a constant could not satisfy this, which is the whole point of the map.
+    const text = "fat, /system/services/other-fat\n";
+    var rules: [test_rule_slots]Rule = undefined;
+    const parsed = parse(text, &rules);
+    try testing.expectEqualStrings("/system/services/other-fat", match(rules[0..parsed.count], .fat).?);
+}
+
+test "malformed rows are counted, not bound" {
+    const text =
+        \\fat, /system/services/fat
+        \\bogusfs, /system/services/x
+        \\fat,
+        \\fat, /a, /b
+    ;
+    var rules: [test_rule_slots]Rule = undefined;
+    const parsed = parse(text, &rules);
+    try testing.expectEqual(@as(usize, 1), parsed.count); // only the first fat row
+    try testing.expectEqual(@as(usize, 3), parsed.malformed); // bad token, empty binary, too many columns
+}

system/services/volume-manager/partition.zig

@@ -60,12 +60,28 @@ pub const Identity = struct {
     }
 };
 
-/// One volume the parser found on the device: the block sub-range it occupies
-/// and its content identity.
+/// Which filesystem a volume's content is — the key `filesystems.csv` maps to a
+/// service binary. Today only FAT is recognized (S4 adds exFAT with a real VBR
+/// recognizer); until then every probed volume is `.fat`, matching the volume
+/// manager's historical hand-off of everything to the FAT service.
+pub const FilesystemKind = enum {
+    fat,
+    unknown,
+
+    pub fn fromToken(token: []const u8) FilesystemKind {
+        if (std.mem.eql(u8, token, "fat")) return .fat;
+        return .unknown;
+    }
+};
+
+/// One volume the parser found on the device: the block sub-range it occupies,
+/// its content identity, and which filesystem its content is (the signature the
+/// `filesystems.csv` map keys on to pick the service binary).
 pub const Volume = struct {
     base_lba: u64,
     block_count: u64,
     identity: Identity,
+    signature: FilesystemKind = .fat,
 };
 
 /// Read sectors on demand. `context` + `readFn` mirror the FAT engine's

system/services/volume-manager/volume-manager.zig

@@ -26,7 +26,10 @@ const process = @import("process");
 const service = @import("service");
 const time = @import("time");
 const envelope = @import("envelope");
+const fs = @import("file-system");
 const partition = @import("partition.zig");
+const filesystem_map = @import("filesystem-map.zig");
+const volume_map = @import("volume-map.zig");
 
 const Serve = volume_manager_protocol.Protocol.Provider(void);
 const Invocation = envelope.Invocation;
@@ -42,15 +45,53 @@ const Volume = struct {
     block_count: u64,
     identity: partition.Identity,
     id: u64,
+    binary: []const u8, // the service binary, from filesystems.csv by signature
+    mount_prefix: []const u8, // the volume-root mount path (its id-path, or a volumes.csv override)
     filesystem_pid: u32 = 0,
 };
 
-/// The filesystem binary a probed volume is served by. The signature->binary
-/// map (filesystems.csv) lands with the identity ladder; for now every FAT-shaped
-/// volume gets the FAT service.
-const filesystem_binary = "/system/services/fat";
 const volume_id: u64 = 1;
 
+// The mount map, read from configuration at boot (the policy home, storage-
+// architecture.md): filesystems.csv (content signature -> service binary) and
+// volumes.csv (an optional id -> mount-prefix override). The sources are held
+// for the process life so the parsed rules' slices into them stay valid.
+/// bound: bytes of filesystems.csv / volumes.csv the manager reads
+/// decided-by: ours
+/// protects: the config source buffers below
+/// at-limit: truncate - a longer file is cut; a row split by the cut is malformed
+/// observed-by: the per-file "malformed/truncated" log line
+const config_source_bytes = 2048;
+var filesystems_source: [config_source_bytes]u8 = undefined;
+var volumes_source: [config_source_bytes]u8 = undefined;
+/// bound: filesystem-map rules held (one per content signature)
+/// decided-by: ours
+/// protects: the filesystem_rules table
+/// at-limit: truncate - extra rows are dropped and the "truncated" note logged
+/// observed-by: the "truncated" log line
+const maximum_filesystem_rules = 8;
+/// bound: volumes.csv override rows held (one per pinned volume id)
+/// decided-by: ours
+/// protects: the volume_rules table
+/// at-limit: truncate - extra rows are dropped and the "truncated" note logged
+/// observed-by: the "truncated" log line
+const maximum_volume_rules = 64;
+var filesystem_rules: [maximum_filesystem_rules]filesystem_map.Rule = undefined;
+var filesystem_rule_count: usize = 0;
+var volume_rules: [maximum_volume_rules]volume_map.Override = undefined;
+var volume_rule_count: usize = 0;
+/// The composed default mount path (/volumes/<id>) for the current volume; a
+/// volumes.csv override is used in place and needs no buffer (it is already a
+/// slice into volumes_source). One buffer suffices while the manager serves one
+/// volume (multi-volume gives each its own in S3).
+/// bound: bytes of a composed /volumes/<id> mount path
+/// decided-by: ours
+/// protects: the mount_prefix_buf below
+/// at-limit: truncate - bufPrint fails; the volume mounts at a fallback path (logged)
+/// observed-by: the fallback path in the log
+const mount_path_maximum = 64;
+var mount_prefix_buf: [mount_path_maximum]u8 = undefined;
+
 var service_endpoint: ipc.Handle = 0;
 var manager_handle: ?ipc.Handle = null;
 var bounce: memory.DmaRegion = undefined;
@@ -156,7 +197,7 @@ fn isDevicePresent(device_id: u64) bool {
 /// confinement controller (it defines the first range on the device).
 fn spawnFilesystem(v: *Volume) void {
     if (fs_failed) return;
-    const pid = process.spawnSupervised(filesystem_binary, &.{"1"}, service_endpoint) orelse {
+    const pid = process.spawnSupervised(v.binary, &.{ "1", v.mount_prefix }, service_endpoint) orelse {
         _ = logging.write("volume-manager: could not spawn the filesystem; retrying\n");
         armRestart();
         return;
@@ -169,7 +210,7 @@ fn spawnFilesystem(v: *Volume) void {
     }
     v.filesystem_pid = pid;
     fs_spawn_ns = time.clock();
-    std.log.info("volume 0x{x} -> {s} (pid {d}), lba {d}, {d} blocks", .{ v.identity.key, filesystem_binary, pid, v.base_lba, v.block_count });
+    std.log.info("volume 0x{x} -> {s} (pid {d}), lba {d}, {d} blocks", .{ v.identity.key, v.binary, pid, v.base_lba, v.block_count });
 }
 
 /// Schedule a fat restart after backoff; the poll loop performs it once due.
@@ -224,11 +265,29 @@ fn bringUpVolume() void {
         _ = ipc.close(device.endpoint);
         return;
     };
+    // Pick the service binary from the volume's content signature. A signature
+    // no filesystems.csv row serves goes unserved (logged), like an unbound
+    // device — the manager does not guess.
+    const binary = filesystem_map.match(filesystem_rules[0..filesystem_rule_count], found.signature) orelse {
+        if (!logged_no_volume) {
+            _ = logging.write("volume-manager: no filesystem serves this volume's content; unserved\n");
+            logged_no_volume = true;
+        }
+        _ = ipc.close(device.endpoint);
+        return;
+    };
+    // The mount path is the volume's identity id (/volumes/<id>), or a
+    // volumes.csv override pinning it to a chosen path. The id is content-derived,
+    // so the path is stable and never a port or a label.
+    var id_buf: [volume_map.id_maximum]u8 = undefined;
+    const id = volume_map.idString(found.identity, &id_buf);
+    const mount_prefix = volume_map.overrideFor(volume_rules[0..volume_rule_count], id) orelse
+        (std.fmt.bufPrint(&mount_prefix_buf, "/volumes/{s}", .{id}) catch "/volumes/unknown");
     logged_no_volume = false;
     fs_restarts = 0;
     fs_failed = false;
     restart_pending = false;
-    volume = .{ .storage = device, .storage_device_id = opened.device_id, .base_lba = found.base_lba, .block_count = found.block_count, .identity = found.identity, .id = volume_id };
+    volume = .{ .storage = device, .storage_device_id = opened.device_id, .base_lba = found.base_lba, .block_count = found.block_count, .identity = found.identity, .id = volume_id, .binary = binary, .mount_prefix = mount_prefix };
     spawnFilesystem(&volume.?);
 }
 
@@ -289,16 +348,66 @@ fn onHello(_: void, invocation: Invocation(volume_manager_protocol.Hello), _: An
     return 0;
 }
 
-const handlers = Serve.Handlers{ .hello = onHello };
+/// Answer a `volumes` query with the mounted volume's descriptor — its id (its
+/// mount path is /volumes/<id> unless overridden), its actual mount path, and
+/// its display label. This is how a shell or file manager reads a volume's
+/// friendly name: software keys on the id, a UI shows the label. An empty reply
+/// means no volume is mounted.
+fn onVolumes(_: void, _: Invocation(volume_manager_protocol.Volumes), answer: Answer(void)) isize {
+    const v = volume orelse return 0;
+    var id_buf: [volume_map.id_maximum]u8 = undefined;
+    const info = volume_manager_protocol.VolumeInfo{
+        .id = volume_map.idString(v.identity, &id_buf),
+        .mount_path = v.mount_prefix,
+        .label = v.identity.labelSlice(),
+    };
+    const encoded = info.encode(answer.tail()) orelse return 0;
+    return @intCast(encoded.len);
+}
+
+const handlers = Serve.Handlers{ .hello = onHello, .volumes = onVolumes };
 
 fn onMessage(message: []const u8, out: []u8, sender: u32, arrived: *ipc.Arrival) usize {
     // No verb takes a capability up, so the turn closes whatever arrives.
     return Serve.dispatch({}, handlers, message, sender, arrived.peek(), out);
 }
 
+/// Read a config file into `buf`, returning the byte count (0 if missing).
+fn readConfig(path: []const u8, buf: []u8) usize {
+    var file = fs.open(path, .{}) orelse {
+        std.log.info("volume-manager: {s} missing", .{path});
+        return 0;
+    };
+    defer file.close();
+    var used: usize = 0;
+    while (used < buf.len) {
+        const n = file.read(buf[used..]) orelse break;
+        if (n == 0) break;
+        used += n;
+    }
+    return used;
+}
+
+/// Load the mount map from configuration once at boot (mirrors the device
+/// manager's registry load). A missing or empty filesystems.csv means no volume
+/// is served; volumes.csv is optional — no rows means every volume takes its
+/// default /volumes/<id> path.
+fn loadTables() void {
+    const fs_used = readConfig("/system/configuration/filesystems.csv", &filesystems_source);
+    const fr = filesystem_map.parse(filesystems_source[0..fs_used], &filesystem_rules);
+    filesystem_rule_count = fr.count;
+    if (fr.malformed != 0 or fr.truncated) std.log.info("filesystems.csv: {d} malformed, truncated={}", .{ fr.malformed, fr.truncated });
+
+    const vol_used = readConfig("/system/configuration/volumes.csv", &volumes_source);
+    const vr = volume_map.parse(volumes_source[0..vol_used], &volume_rules);
+    volume_rule_count = vr.count;
+    if (vr.malformed != 0 or vr.truncated) std.log.info("volumes.csv: {d} malformed, truncated={}", .{ vr.malformed, vr.truncated });
+}
+
 fn initialise(endpoint: ipc.Handle) bool {
     service_endpoint = endpoint;
     _ = logging.write("volume-manager: starting, waiting for a storage device\n");
+    loadTables();
     _ = process.subscribeExits(endpoint);
     pollTick();
     _ = time.timerOnce(endpoint, poll_interval_ms); // the poll runs for the life of the boot

system/services/volume-manager/volume-map.zig

@@ -0,0 +1,137 @@
+//! volume-map — render a volume's identity into its stable mount id-string, and
+//! parse `/system/configuration/volumes.csv` (danos's fstab) into optional
+//! id → mount-prefix overrides. This is where the id/label split becomes the
+//! path: a volume's mount point is derived from its content identity (the id),
+//! never from a port or a label. Two distinct volumes that share a label get
+//! distinct id-strings automatically; only identical ids (dd-cloned media) can
+//! collide, which is the narrow case the manager's duplicate policy is for.
+//!
+//! `volumes.csv` is an OPTIONAL override: a row `id, mount_prefix` pins a volume
+//! (by its id-string) to a chosen path. A volume with no row takes its default
+//! `/volumes/<id>`. The label is display metadata, exposed by the manager's
+//! `volumes` query, and never appears here.
+//!
+//! Pure logic: no syscalls, no allocator. Override slices point into the CSV
+//! source, which the manager holds in a static buffer for the process life.
+
+const std = @import("std");
+const csv = @import("csv");
+const partition = @import("partition.zig");
+
+/// bound: bytes of the longest volume id-string the deriver renders
+/// decided-by: ours
+/// protects: the caller's id-string buffer
+/// at-limit: truncate - bufPrint fails and idString returns ""; the volume goes
+///   unnamed and the manager logs it rather than mounting at an empty path
+/// observed-by: a volume with an empty id in the `volumes` query / the log
+pub const id_maximum = 40; // "gpt-" (4) or "uuid-" (5) + 32 hex fits in 40
+
+/// One parsed override row: a volume id-string and the mount prefix it pins to.
+pub const Override = struct { id: []const u8, prefix: []const u8 };
+
+/// How many overrides landed, how many non-blank lines were malformed, and
+/// whether there were more rows than the buffer could hold.
+pub const ParseResult = struct { count: usize, malformed: usize, truncated: bool };
+
+/// Render a volume's identity into its id-string — the content-derived, unique,
+/// order-independent token whose default mount path is `/volumes/<id>`. The rung
+/// tags the scheme so ids never collide across rungs; the key is the content id,
+/// so a moved drive keeps its id (and thus its path).
+pub fn idString(identity: partition.Identity, buf: []u8) []const u8 {
+    return switch (identity.rung) {
+        .gpt_guid => std.fmt.bufPrint(buf, "gpt-{x:0>32}", .{identity.key}) catch "",
+        .filesystem_uuid => std.fmt.bufPrint(buf, "uuid-{x:0>32}", .{identity.key}) catch "",
+        .fat_serial => std.fmt.bufPrint(buf, "fat-{x:0>8}", .{@as(u32, @truncate(identity.key))}) catch "",
+        .mbr_index => std.fmt.bufPrint(buf, "mbr-{x}-{d}", .{
+            @as(u32, @truncate(identity.key >> 8)),
+            @as(u8, @truncate(identity.key & 0xff)),
+        }) catch "",
+        .anonymous => std.fmt.bufPrint(buf, "anon-{x}", .{identity.key}) catch "",
+    };
+}
+
+const Line = union(enum) { override: Override, ignorable, malformed };
+
+fn parseLine(line: []const u8) Line {
+    const body = csv.stripComment(line);
+    if (body.len == 0) return .ignorable;
+    var it = csv.fields(body);
+    const id = it.next() orelse return .malformed;
+    const prefix = it.next() orelse return .malformed;
+    if (it.next() != null) return .malformed; // too many columns
+    if (id.len == 0 or prefix.len == 0) return .malformed;
+    return .{ .override = .{ .id = id, .prefix = prefix } };
+}
+
+/// Parse a whole `volumes.csv` into `out_rules`. The slices point into `source`,
+/// which must outlive them.
+pub fn parse(source: []const u8, out_rules: []Override) ParseResult {
+    var result: ParseResult = .{ .count = 0, .malformed = 0, .truncated = false };
+    var lines = std.mem.splitScalar(u8, source, '\n');
+    while (lines.next()) |line| {
+        switch (parseLine(line)) {
+            .ignorable => {},
+            .malformed => result.malformed += 1,
+            .override => |ov| {
+                if (result.count >= out_rules.len) {
+                    result.truncated = true;
+                    continue;
+                }
+                out_rules[result.count] = ov;
+                result.count += 1;
+            },
+        }
+    }
+    return result;
+}
+
+/// The override mount prefix for a volume whose id-string is `id`, or null (the
+/// volume takes its default `/volumes/<id>` path). First matching row wins.
+pub fn overrideFor(rules: []const Override, id: []const u8) ?[]const u8 {
+    for (rules) |rule| {
+        if (std.mem.eql(u8, rule.id, id)) return rule.prefix;
+    }
+    return null;
+}
+
+// --- tests -------------------------------------------------------------------
+
+const testing = std.testing;
+
+// Named fixture sizes so the bounds gate (which flags literal array lengths)
+// stays quiet: test inputs, not runtime ceilings.
+const test_override_slots = 4;
+
+test "idString renders each rung's id token" {
+    var buf: [id_maximum]u8 = undefined;
+    try testing.expectEqualStrings("fat-12345678", idString(.{ .rung = .fat_serial, .key = 0x12345678 }, &buf));
+    try testing.expectEqualStrings("mbr-deadbeef-1", idString(.{ .rung = .mbr_index, .key = (@as(u128, 0xDEADBEEF) << 8) | 1 }, &buf));
+    const guid: u128 = 0x00112233445566778899AABBCCDDEEFF;
+    try testing.expectEqualStrings("gpt-00112233445566778899aabbccddeeff", idString(.{ .rung = .gpt_guid, .key = guid }, &buf));
+}
+
+test "overrideFor returns the mapped prefix, else null" {
+    const text =
+        \\# id, mount_prefix
+        \\fat-12345678, /mnt/boot
+    ;
+    var rules: [test_override_slots]Override = undefined;
+    const parsed = parse(text, &rules);
+    try testing.expectEqual(@as(usize, 1), parsed.count);
+    try testing.expectEqual(@as(usize, 0), parsed.malformed);
+    try testing.expectEqualStrings("/mnt/boot", overrideFor(rules[0..parsed.count], "fat-12345678").?);
+    try testing.expect(overrideFor(rules[0..parsed.count], "fat-99999999") == null);
+}
+
+test "malformed volume rows are counted, not bound" {
+    const text =
+        \\fat-1, /mnt/a
+        \\onlyonecolumn
+        \\fat-2,
+        \\fat-3, /a, /b
+    ;
+    var rules: [test_override_slots]Override = undefined;
+    const parsed = parse(text, &rules);
+    try testing.expectEqual(@as(usize, 1), parsed.count); // only the first valid row
+    try testing.expectEqual(@as(usize, 3), parsed.malformed); // one column, empty prefix, too many columns
+}

test/qemu_test.py

@@ -179,7 +179,7 @@ CASES = [
      "smp":    4,
      "timeout": 150,
      "qemu_extra": ["-device", "intel-iommu,intremap=off"],
-     "expect": r"(?s)(?=.*/system/kernel: iommu online)(?=.*fat: mounted /volumes/usb)(?=.*fat-test: ok)",
+     "expect": r"(?s)(?=.*/system/kernel: iommu online)(?=.*fat: mounted /volumes/fat-12345678)(?=.*fat-test: ok)",
      "fail":   r"DANOS-TEST-RESULT: FAIL|DANOS-IOMMU-FAULT"},
     # DMA + MSI under translation: interrupt-IN reports arrive through translated DMA and
     # the xHC's MSI/MSI-X still delivers (the 0xFEE00000 interrupt window bypasses second-
@@ -215,7 +215,7 @@ CASES = [
      "smp":    4,
      "timeout": 150,
      "qemu_extra": ["-device", "amd-iommu,dma-remap=on,intremap=off"],
-     "expect": r"(?s)(?=.*iommu online \(AMD-Vi\))(?=.*fat: mounted /volumes/usb)(?=.*fat-test: ok)",
+     "expect": r"(?s)(?=.*iommu online \(AMD-Vi\))(?=.*fat: mounted /volumes/fat-12345678)(?=.*fat-test: ok)",
      "fail":   r"DANOS-TEST-RESULT: FAIL|DANOS-IOMMU-FAULT"},
     # Port I/O grants: a claimed device's io_port resource lets a driver read/write its
     # ports (PS/2 status 0x64), gated by the claim; out-of-range/unclaimed is refused.
@@ -749,13 +749,26 @@ CASES = [
      "expect": r"usb-storage: ready[\s\S]*usb-storage: block 0 signature 0x55aa",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # FAT mount end to end: the fat server mounts the boot usb-storage device (the
-    # FAT32 image) into the VFS at /volumes/usb. A fat-test client then lists and reads
+    # FAT32 image) into the VFS at /volumes/fat-12345678. A fat-test client then lists and reads
     # through the mount — proof of the whole stack: block device -> FAT parse ->
     # VFS routing -> file read.
     {"name": "fat-mount",
      "smp":    4,
      "timeout": 150,
-     "expect": r"fat: mounted /volumes/usb[\s\S]*fat-test: ok",
+     "expect": r"fat: mounted /volumes/fat-12345678[\s\S]*fat-test: ok",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # The id-path naming (S2, storage-stack-plan.md). The boot volume mounts at
+    # its CONTENT-derived id-path (/volumes/fat-12345678, from the FAT32 serial
+    # 0x12345678) — never a port name — and keeps its FHS rewrites so /system/logs
+    # persistence still rides the volume. Discrimination: before S2's flip fat
+    # hardcoded /volumes/usb, so the id-path mount line never appears. (Making the
+    # rewrites content-conditional on which volume carries the system is S3.)
+    {"name": "volume-identity-name",
+     "build_case": "fat-mount",
+     "smp":    4,
+     "timeout": 150,
+     "expect": r"(?s)fat: mounted /volumes/fat-12345678"
+               r"[\s\S]*fat: mounted /system/logs",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # The removal lifecycle (V4, docs/volume-manager-plan.md): pull the boot stick
     # mid-run. device_del the usb-storage device -> the bus reports the port empty
@@ -780,7 +793,7 @@ CASES = [
      "qmp_sequence": [
          {"delay": 8, "command": "device_del", "arguments": {"id": "bootstorage"}},
      ],
-     "expect": r"(?s)fat: mounted /volumes/usb"
+     "expect": r"(?s)fat: mounted /volumes/fat-12345678"
                r"[\s\S]*volume-manager: storage for volume \d+ removed; unmounting",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # Volume-manager discovery + probe (V3a, docs/volume-manager-plan.md). Reuses

test/system/services/badge-scope-test/badge-scope-test.zig

@@ -38,7 +38,7 @@ const time = @import("time");
 /// A scratch file on the volume, so the node the intruder tries to write through
 /// is one nothing else reads. (A foreign write that *succeeded* would prove the
 /// bug — it must not also damage the boot volume proving it.)
-const held_path = "/volumes/usb/BADGE.TXT";
+const held_path = "/volumes/fat-12345678/BADGE.TXT";
 const held_contents = "held";
 
 fn line(comptime format: []const u8, arguments: anytype) void {
@@ -46,12 +46,12 @@ fn line(comptime format: []const u8, arguments: anytype) void {
     _ = logging.write(std.fmt.bufPrint(&buffer, format, arguments) catch return);
 }
 
-/// The fat server mounts /volumes/usb only after the whole USB storage chain is
+/// The fat server mounts /volumes/fat-12345678 only after the whole USB storage chain is
 /// up, and both instances race it.
 fn waitForVolume() bool {
     var tries: u32 = 0;
     while (tries < 1400) : (tries += 1) {
-        if (fs.openDirectory("/volumes/usb")) |opened| {
+        if (fs.openDirectory("/volumes/fat-12345678")) |opened| {
             var directory = opened;
             directory.close();
             return true;
@@ -79,7 +79,7 @@ pub fn main(init: process.Init) void {
 
 fn own() void {
     if (!waitForVolume()) {
-        _ = logging.write("badge-scope-test: FAILED (/volumes/usb never became available)\n");
+        _ = logging.write("badge-scope-test: FAILED (/volumes/fat-12345678 never became available)\n");
         return;
     }
     var held = fs.open(held_path, .{ .create = true, .truncate = true }) orelse {
@@ -142,7 +142,7 @@ fn own() void {
 
 fn intrude(foreign_node: u64, foreign_layer: u32) void {
     if (!waitForVolume()) {
-        _ = logging.write("badge-scope-test: FAILED (/volumes/usb never became available)\n");
+        _ = logging.write("badge-scope-test: FAILED (/volumes/fat-12345678 never became available)\n");
         return;
     }
     const node_verdict = probeNode(foreign_node);

test/system/services/fat-test/fat-test.zig

@@ -1,6 +1,6 @@
 //! test/system/services/fat-test — a client that proves the FAT mount end to end:
-//! it waits for the fat server to mount the USB volume at /volumes/usb, lists the
-//! root directory through the VFS (which routes /volumes/usb to the fat backend), and
+//! it waits for the fat server to mount the USB volume at /volumes/fat-12345678, lists the
+//! root directory through the VFS (which routes /volumes/fat-12345678 to the fat backend), and
 //! reads a known file off it. Shipped in the initial_ramdisk; the `fat-mount`
 //! kernel test spawns it alongside init.
 
@@ -18,16 +18,16 @@ fn writeLine(comptime fmt: []const u8, arguments: anytype) void {
 pub fn main(init: process.Init) void {
     _ = init;
 
-    // Wait for /volumes/usb to be mounted — the fat server races us at boot (it must
+    // Wait for /volumes/fat-12345678 to be mounted — the fat server races us at boot (it must
     // bring up the whole USB storage chain first).
     var opened: ?fs.Directory = null;
     var tries: u32 = 0;
     while (opened == null and tries < 1400) : (tries += 1) {
-        opened = fs.openDirectory("/volumes/usb");
+        opened = fs.openDirectory("/volumes/fat-12345678");
         if (opened == null) time.sleepMillis(50);
     }
     var dir = opened orelse {
-        _ = logging.write("fat-test: /volumes/usb never became available\n");
+        _ = logging.write("fat-test: /volumes/fat-12345678 never became available\n");
         return;
     };
 
@@ -43,56 +43,56 @@ pub fn main(init: process.Init) void {
 
     // Read a known file off the boot volume through the mount (best effort): the
     // kernel image is an ELF, so its first bytes are the ELF magic.
-    if (fs.open("/volumes/usb/system/kernel", .{})) |opened_file| {
+    if (fs.open("/volumes/fat-12345678/system/kernel", .{})) |opened_file| {
         var file = opened_file;
         var magic: [4]u8 = undefined;
         const n = file.read(&magic) orelse 0;
         file.close();
         if (n == 4 and magic[0] == 0x7F and magic[1] == 'E' and magic[2] == 'L' and magic[3] == 'F') {
-            _ = logging.write("fat-test: read /volumes/usb/system/kernel ELF magic ok\n");
+            _ = logging.write("fat-test: read /volumes/fat-12345678/system/kernel ELF magic ok\n");
         } else {
-            writeLine("fat-test: /volumes/usb/system/kernel read {d} bytes (not ELF magic)\n", .{n});
+            writeLine("fat-test: /volumes/fat-12345678/system/kernel read {d} bytes (not ELF magic)\n", .{n});
         }
     }
 
     // Exercise directory + file mutation through the mount: mkdir, create a file
     // inside it, read it back, then remove it — proof mkdir/unlink reach the engine.
-    if (fs.makeDirectory("/volumes/usb/TESTDIR")) {
+    if (fs.makeDirectory("/volumes/fat-12345678/TESTDIR")) {
         var wrote = false;
-        if (fs.open("/volumes/usb/TESTDIR/HELLO.TXT", .{ .create = true, .truncate = true })) |created| {
+        if (fs.open("/volumes/fat-12345678/TESTDIR/HELLO.TXT", .{ .create = true, .truncate = true })) |created| {
             var f = created;
             wrote = (f.writeAll("mutation-ok") orelse 0) == "mutation-ok".len;
             f.close();
         }
         // The created file carries a real modification time (stamped from the RTC).
         var mtime_ok = false;
-        if (fs.attributes("/volumes/usb/TESTDIR/HELLO.TXT")) |attrs| {
+        if (fs.attributes("/volumes/fat-12345678/TESTDIR/HELLO.TXT")) |attrs| {
             writeLine("fat-test: mtime {d}\n", .{attrs.mtime});
             mtime_ok = attrs.mtime > 1_577_836_800; // after 2020-01-01
         }
         if (mtime_ok) _ = logging.write("fat-test: mtime ok\n");
 
         // Rename it, then read from the new name and confirm the old name is gone.
-        const renamed = fs.rename("/volumes/usb/TESTDIR/HELLO.TXT", "/volumes/usb/TESTDIR/RENAMED.TXT");
-        const old_gone = !fs.exists("/volumes/usb/TESTDIR/HELLO.TXT");
+        const renamed = fs.rename("/volumes/fat-12345678/TESTDIR/HELLO.TXT", "/volumes/fat-12345678/TESTDIR/RENAMED.TXT");
+        const old_gone = !fs.exists("/volumes/fat-12345678/TESTDIR/HELLO.TXT");
         if (renamed and old_gone) _ = logging.write("fat-test: rename ok\n");
         var readback = false;
-        if (fs.open("/volumes/usb/TESTDIR/RENAMED.TXT", .{})) |reopened| {
+        if (fs.open("/volumes/fat-12345678/TESTDIR/RENAMED.TXT", .{})) |reopened| {
             var f = reopened;
             var buf: [16]u8 = undefined;
             const got = f.read(&buf) orelse 0;
             f.close();
             readback = std.mem.eql(u8, buf[0..got], "mutation-ok");
         }
-        const removed = fs.remove("/volumes/usb/TESTDIR/RENAMED.TXT");
-        const gone = !fs.exists("/volumes/usb/TESTDIR/RENAMED.TXT");
+        const removed = fs.remove("/volumes/fat-12345678/TESTDIR/RENAMED.TXT");
+        const gone = !fs.exists("/volumes/fat-12345678/TESTDIR/RENAMED.TXT");
         if (wrote and mtime_ok and renamed and old_gone and readback and removed and gone) {
             _ = logging.write("fat-test: mutations ok\n");
         } else {
             writeLine("fat-test: mutations FAILED (wrote={} mtime={} renamed={} oldgone={} read={} removed={} gone={})\n", .{ wrote, mtime_ok, renamed, old_gone, readback, removed, gone });
         }
     } else {
-        _ = logging.write("fat-test: mkdir /volumes/usb/TESTDIR failed\n");
+        _ = logging.write("fat-test: mkdir /volumes/fat-12345678/TESTDIR failed\n");
     }
 
     if (count > 0) {

test/system/services/vfs-test/vfs-test.zig

@@ -77,7 +77,7 @@ fn park() void {
     var parked: ?fs.File = null;
     var tries: u32 = 0;
     while (parked == null and tries < 1000) : (tries += 1) {
-        parked = fs.open("/volumes/usb/parked", .{ .create = true });
+        parked = fs.open("/volumes/fat-12345678/parked", .{ .create = true });
         if (parked == null) time.sleepMillis(20);
     }
     if (parked == null) {
@@ -92,16 +92,16 @@ fn park() void {
     // "parked" marker, which fails the vfs-client-death case: before the
     // ownership gate existed, any process could unmount any prefix, and this
     // fixture would have deleted the volume out from under the whole boot.
-    if (fs.fsUnmount("/volumes/usb")) {
+    if (fs.fsUnmount("/volumes/fat-12345678")) {
         _ = logging.write("vfstest: foreign unmount was ALLOWED\n");
         return;
     }
-    if (fs.open("/volumes/usb/parked", .{})) |resolved| {
+    if (fs.open("/volumes/fat-12345678/parked", .{})) |resolved| {
         var verification = resolved;
         verification.close(); // the park below must be the client's ONLY open
         // handle — the kernel test string-matches "released 1 handle(s)".
     } else {
-        _ = logging.write("vfstest: /volumes/usb gone after refused unmount\n");
+        _ = logging.write("vfstest: /volumes/fat-12345678 gone after refused unmount\n");
         return;
     }
     _ = logging.write("vfstest: foreign unmount refused\n");