danos/build-support/build.zig

//! The danos build API (docs/build-packages-plan.md): the one shared recipe
//! for building a user-space binary. A binary package's build.zig names its
//! binary and EXACTLY the modules its source imports — the moral equivalent
//! of a C file's include list — and `userBinary` resolves each name from the
//! library domain package that exports it. Nothing is pre-wired: an @import
//! the package did not declare is a compile error, and a domain none of the
//! imports come from never appears in the package's manifest. The only
//! implicit dependency is the kernel package, because the shared root shim
//! (root.zig, user.ld) lives there and itself reaches start + logging.
//!
//! Consumers declare this package in their build.zig.zon (as "build-support")
//! and @import its build.zig from their own build.zig; nothing is compiled
//! from this package itself — it exports build-time functions only.

const std = @import("std");

pub fn build(b: *std.Build) void {
    _ = b; // nothing to build: this package exports build-time functions only
}

/// The freestanding x86-64 target every danos binary (kernel and user) is
/// built for. SSE2 is part of the x86_64 baseline and UEFI leaves it enabled
/// at handoff, so we keep it: disabling it forces soft-float and makes the
/// compiler unable to encode the vector ops that std's formatting/runtime
/// still emit.
pub fn freestandingTarget(b: *std.Build) std.Build.ResolvedTarget {
    return b.resolveTargetQuery(.{
        .cpu_arch = .x86_64,
        .os_tag = .freestanding,
        .abi = .none,
    });
}

/// Resolve one imported module by searching the packages this binary DECLARED
/// in its own build.zig.zon — the C include path made literal: an import can
/// only be satisfied by a domain the binary claims, and each domain's own
/// build.zig (its addModule exports) is the single statement of who owns
/// what. There is no name table here to drift.
fn moduleFromDeclaredDependencies(b: *std.Build, name: []const u8) *std.Build.Module {
    for (b.available_deps) |declared| {
        const dependency = b.dependency(declared[0], .{});
        if (dependency.builder.modules.get(name)) |module| return module;
    }
    @panic(b.fmt(
        "no declared dependency exports a module named '{s}' — declare the domain that owns it in this package's build.zig.zon",
        .{name},
    ));
}

/// What `userBinary` needs to know about one user binary.
pub const UserBinaryOptions = struct {
    name: []const u8,
    /// The program's own source file — it becomes the `program` module the
    /// root shim imports; a program only defines `pub fn main`.
    root_source_file: std.Build.LazyPath,
    /// Exactly the modules the program's source @imports (directly or through
    /// its same-directory files) — no more, no less. Order is free; sorted
    /// reads best. An undeclared @import fails the compile; a name no
    /// declared domain exports fails the build graph, naming the miss.
    imports: []const []const u8,
    /// Built multi-threaded (`single_threaded = false`) so real atomics/TLS
    /// work — required before a binary may call `Thread.spawn`
    /// (docs/threading.md). Threads are a deliberate per-binary opt-in.
    threaded: bool = false,
};

/// Build one user-space binary the same way for every program (init, the
/// services, the drivers): freestanding, ReleaseSmall, `.large` code model
/// (the image base is above 4 GiB — smaller models emit 32-bit relocations
/// that can't reach), linked with the shared user link script. Pinned to
/// LLVM + LLD so the script's PHDRS (segment permissions) are authoritative —
/// the kernel's W^X user-ELF loader requires exact perms.
///
/// The compilation root is not the program's own file but the shared shim
/// (the kernel package's root.zig), which supplies the root declarations
/// (`main` re-export, panic handler, `_start` pull) so a program only defines
/// `pub fn main`. The program's file becomes the `program` module the shim
/// imports; reach it through `programModule` to add per-binary non-library
/// modules (compile-time options).
pub fn userBinary(b: *std.Build, options: UserBinaryOptions) *std.Build.Step.Compile {
    const kernel = b.dependency("kernel", .{});
    var imports: std.ArrayListUnmanaged(std.Build.Module.Import) = .empty;
    for (options.imports) |name| {
        imports.append(b.allocator, .{
            .name = name,
            .module = moduleFromDeclaredDependencies(b, name),
        }) catch @panic("OOM");
    }
    // Settings (target, optimize, code model, ...) live on the root module
    // only; the program module inherits them.
    const program_module = b.createModule(.{
        .root_source_file = options.root_source_file,
        .imports = imports.items,
    });
    const exe = b.addExecutable(.{
        .name = options.name,
        .root_module = b.createModule(.{
            .root_source_file = kernel.path("root.zig"),
            .target = freestandingTarget(b),
            .optimize = .ReleaseSmall,
            .code_model = .large,
            .single_threaded = !options.threaded, // a threaded binary needs real atomics/TLS
            .sanitize_c = .off,
            .stack_check = false,
            .stack_protector = false,
            // The root shim itself imports only start (_start + panic) and
            // logging (std_options) — straight from the kernel package, so a
            // program's own import list stays exactly its own.
            .imports = &.{
                .{ .name = "start", .module = kernel.module("start") },
                .{ .name = "logging", .module = kernel.module("logging") },
                .{ .name = "program", .module = program_module },
            },
        }),
    });
    exe.setLinkerScript(kernel.path("user.ld"));
    exe.entry = .{ .symbol_name = "_start" };
    exe.image_base = 0x7000_0000_0000;
    exe.use_llvm = true;
    exe.use_lld = true;
    return exe;
}

/// The `program` module of a binary built by `userBinary` — the module rooted
/// at the program's own source file. Per-binary non-library modules (an
/// addOptions build_options) go here, not on the root shim: module imports
/// are not transitive, so an import added to the root would be invisible to
/// the program's code.
pub fn programModule(exe: *std.Build.Step.Compile) *std.Build.Module {
    return exe.root_module.import_table.get("program").?;
}