danos/build-support/build.zig

//! The danos build API (docs/build-packages-plan.md, phase 0): the one shared
//! recipe for building a user-space binary, extracted from the root build so
//! cross-cutting build changes have a single home. 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,
    });
}

/// What `userBinaryFromImports` 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,
    /// The shared root shim supplying the compilation-root declarations
    /// (`main` re-export, panic handler, `_start` pull): library/kernel/root.zig.
    shim_source_file: std.Build.LazyPath,
    /// The shared user link script. Its PHDRS (segment permissions) are
    /// authoritative — the kernel's W^X user-ELF loader requires exact perms.
    linker_script: std.Build.LazyPath,
    target: std.Build.ResolvedTarget,
    /// The default set of importable modules every user binary sees — the
    /// library/kernel concern modules (ipc, memory, process, time, logging,
    /// file-system, ...), the device/service clients (driver, block, display,
    /// input), mmio, acpi-ids, and xkeyboard-config. Must include `start` and
    /// `logging` (the root shim reaches those two directly).
    default_imports: []const std.Build.Module.Import,
    /// 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,
};

/// The module registered under `name` in `imports` — the root shim reaches the
/// couple of concern modules it needs (start, logging) out of the default set.
fn findImport(imports: []const std.Build.Module.Import, name: []const u8) *std.Build.Module {
    for (imports) |import| {
        if (std.mem.eql(u8, import.name, name)) return import.module;
    }
    @panic("default_imports is missing a module the root shim needs");
}

/// 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
/// (options.shim_source_file), 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 imports.
pub fn userBinaryFromImports(b: *std.Build, options: UserBinaryOptions) *std.Build.Step.Compile {
    // 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 = options.default_imports,
    });
    const exe = b.addExecutable(.{
        .name = options.name,
        .root_module = b.createModule(.{
            .root_source_file = options.shim_source_file,
            .target = options.target,
            .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); the program's own file reaches the full default set.
            .imports = &.{
                .{ .name = "start", .module = findImport(options.default_imports, "start") },
                .{ .name = "logging", .module = findImport(options.default_imports, "logging") },
                .{ .name = "program", .module = program_module },
            },
        }),
    });
    exe.setLinkerScript(options.linker_script);
    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 `userBinaryFromImports` — the
/// module rooted at the program's own source file. Per-binary imports
/// (protocol modules, bus ABIs) 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").?;
}