compiler: support building openbsd crt0 and stub shared libraries

closes #2878

CMakeLists.txt

@@ -553,6 +553,7 @@ set(ZIG_STAGE2_SOURCES
     src/libs/freebsd.zig
     src/libs/glibc.zig
     src/libs/netbsd.zig
+    src/libs/openbsd.zig
     src/introspect.zig
     src/libs/libcxx.zig
     src/libs/libtsan.zig

src/Compilation.zig

@@ -29,6 +29,7 @@ const glibc = @import("libs/glibc.zig");
 const musl = @import("libs/musl.zig");
 const freebsd = @import("libs/freebsd.zig");
 const netbsd = @import("libs/netbsd.zig");
+const openbsd = @import("libs/openbsd.zig");
 const mingw = @import("libs/mingw.zig");
 const libunwind = @import("libs/libunwind.zig");
 const libcxx = @import("libs/libcxx.zig");
@@ -236,6 +237,7 @@ fuzzer_lib: ?CrtFile = null,
 glibc_so_files: ?glibc.BuiltSharedObjects = null,
 freebsd_so_files: ?freebsd.BuiltSharedObjects = null,
 netbsd_so_files: ?netbsd.BuiltSharedObjects = null,
+openbsd_so_files: ?openbsd.BuiltSharedObjects = null,
 
 /// For example `Scrt1.o` and `libc_nonshared.a`. These are populated after building libc from source,
 /// The set of needed CRT (C runtime) files differs depending on the target and compilation settings.
@@ -306,6 +308,7 @@ const QueuedJobs = struct {
     glibc_crt_file: [@typeInfo(glibc.CrtFile).@"enum".fields.len]bool = @splat(false),
     freebsd_crt_file: [@typeInfo(freebsd.CrtFile).@"enum".fields.len]bool = @splat(false),
     netbsd_crt_file: [@typeInfo(netbsd.CrtFile).@"enum".fields.len]bool = @splat(false),
+    openbsd_crt_file: [@typeInfo(openbsd.CrtFile).@"enum".fields.len]bool = @splat(false),
     /// one of WASI libc static objects
     wasi_libc_crt_file: [@typeInfo(wasi_libc.CrtFile).@"enum".fields.len]bool = @splat(false),
     /// one of the mingw-w64 static objects
@@ -314,6 +317,7 @@ const QueuedJobs = struct {
     glibc_shared_objects: bool = false,
     freebsd_shared_objects: bool = false,
     netbsd_shared_objects: bool = false,
+    openbsd_shared_objects: bool = false,
     /// libunwind.a, usually needed when linking libc
     libunwind: bool = false,
     libcxx: bool = false,
@@ -1391,6 +1395,8 @@ pub const MiscTask = enum {
     freebsd_shared_objects,
     netbsd_crt_file,
     netbsd_shared_objects,
+    openbsd_crt_file,
+    openbsd_shared_objects,
     mingw_crt_file,
     windows_import_lib,
     libunwind,
@@ -1426,6 +1432,9 @@ pub const MiscTask = enum {
     @"netbsd libc Scrt0.o",
     @"netbsd libc shared object",
 
+    @"openbsd libc Scrt0.o",
+    @"openbsd libc shared object",
+
     @"mingw-w64 crt2.o",
     @"mingw-w64 dllcrt2.o",
     @"mingw-w64 libmingw32.lib",
@@ -2592,6 +2601,14 @@ pub fn create(gpa: Allocator, arena: Allocator, io: Io, diag: *CreateDiagnostic,
                     }
 
                     comp.queued_jobs.netbsd_shared_objects = true;
+                } else if (target.isOpenBSDLibC()) {
+                    if (!std.zig.target.canBuildLibC(target)) return diag.fail(.cross_libc_unavailable);
+
+                    if (openbsd.needsCrt0(comp.config.output_mode)) |f| {
+                        comp.queued_jobs.openbsd_crt_file[@intFromEnum(f)] = true;
+                    }
+
+                    comp.queued_jobs.openbsd_shared_objects = true;
                 } else if (target.isWasiLibC()) {
                     if (!std.zig.target.canBuildLibC(target)) return diag.fail(.cross_libc_unavailable);
 
@@ -2733,6 +2750,10 @@ pub fn destroy(comp: *Compilation) void {
         netbsd_file.deinit(gpa);
     }
 
+    if (comp.openbsd_so_files) |*openbsd_file| {
+        openbsd_file.deinit(gpa);
+    }
+
     for (comp.c_object_table.keys()) |key| {
         key.destroy(gpa);
     }
@@ -4720,6 +4741,11 @@ fn performAllTheWork(
         comp.link_task_wait_group.spawnManager(buildNetBSDSharedObjects, .{ comp, main_progress_node });
     }
 
+    if (comp.queued_jobs.openbsd_shared_objects) {
+        comp.link_task_queue.startPrelinkItem();
+        comp.link_task_wait_group.spawnManager(buildOpenBSDSharedObjects, .{ comp, main_progress_node });
+    }
+
     if (comp.queued_jobs.libunwind) {
         comp.link_task_queue.startPrelinkItem();
         comp.link_task_wait_group.spawnManager(buildLibUnwind, .{ comp, main_progress_node });
@@ -4777,6 +4803,14 @@ fn performAllTheWork(
         }
     }
 
+    for (0..@typeInfo(openbsd.CrtFile).@"enum".fields.len) |i| {
+        if (comp.queued_jobs.openbsd_crt_file[i]) {
+            const tag: openbsd.CrtFile = @enumFromInt(i);
+            comp.link_task_queue.startPrelinkItem();
+            comp.link_task_wait_group.spawnManager(buildOpenBSDCrtFile, .{ comp, tag, main_progress_node });
+        }
+    }
+
     for (0..@typeInfo(wasi_libc.CrtFile).@"enum".fields.len) |i| {
         if (comp.queued_jobs.wasi_libc_crt_file[i]) {
             const tag: wasi_libc.CrtFile = @enumFromInt(i);
@@ -5989,6 +6023,31 @@ fn buildNetBSDSharedObjects(comp: *Compilation, prog_node: std.Progress.Node) vo
     }
 }
 
+fn buildOpenBSDCrtFile(comp: *Compilation, crt_file: openbsd.CrtFile, prog_node: std.Progress.Node) void {
+    defer comp.link_task_queue.finishPrelinkItem(comp);
+    if (openbsd.buildCrtFile(comp, crt_file, prog_node)) |_| {
+        comp.queued_jobs.openbsd_crt_file[@intFromEnum(crt_file)] = false;
+    } else |err| switch (err) {
+        error.AlreadyReported => return,
+        else => comp.lockAndSetMiscFailure(.openbsd_crt_file, "unable to build OpenBSD {s}: {s}", .{
+            @tagName(crt_file), @errorName(err),
+        }),
+    }
+}
+
+fn buildOpenBSDSharedObjects(comp: *Compilation, prog_node: std.Progress.Node) void {
+    defer comp.link_task_queue.finishPrelinkItem(comp);
+    if (openbsd.buildSharedObjects(comp, prog_node)) |_| {
+        // The job should no longer be queued up since it succeeded.
+        comp.queued_jobs.openbsd_shared_objects = false;
+    } else |err| switch (err) {
+        error.AlreadyReported => return,
+        else => comp.lockAndSetMiscFailure(.openbsd_shared_objects, "unable to build OpenBSD libc shared objects: {s}", .{
+            @errorName(err),
+        }),
+    }
+}
+
 fn buildMingwCrtFile(comp: *Compilation, crt_file: mingw.CrtFile, prog_node: std.Progress.Node) void {
     defer comp.link_task_queue.finishPrelinkItem(comp);
     if (mingw.buildCrtFile(comp, crt_file, prog_node)) |_| {

src/libs/openbsd.zig

@@ -0,0 +1,699 @@
+const std = @import("std");
+const Allocator = std.mem.Allocator;
+const mem = std.mem;
+const log = std.log;
+const fs = std.fs;
+const path = fs.path;
+const assert = std.debug.assert;
+const Version = std.SemanticVersion;
+const Path = std.Build.Cache.Path;
+
+const Compilation = @import("../Compilation.zig");
+const build_options = @import("build_options");
+const trace = @import("../tracy.zig").trace;
+const Cache = std.Build.Cache;
+const Module = @import("../Package/Module.zig");
+const link = @import("../link.zig");
+
+pub const CrtFile = enum {
+    scrt0_o,
+};
+
+pub fn needsCrt0(output_mode: std.builtin.OutputMode) ?CrtFile {
+    // https://github.com/ziglang/zig/issues/23574#issuecomment-2869089897
+    return switch (output_mode) {
+        .Obj, .Lib => null,
+        .Exe => .scrt0_o,
+    };
+}
+
+fn includePath(comp: *Compilation, arena: Allocator, sub_path: []const u8) ![]const u8 {
+    return path.join(arena, &.{
+        comp.dirs.zig_lib.path.?,
+        "libc" ++ path.sep_str ++ "include",
+        sub_path,
+    });
+}
+
+fn csuPath(comp: *Compilation, arena: Allocator, sub_path: []const u8) ![]const u8 {
+    return path.join(arena, &.{
+        comp.dirs.zig_lib.path.?,
+        "libc" ++ path.sep_str ++ "openbsd" ++ path.sep_str ++ "lib" ++ path.sep_str ++ "csu",
+        sub_path,
+    });
+}
+
+/// TODO replace anyerror with explicit error set, recording user-friendly errors with
+/// lockAndSetMiscFailure and returning error.AlreadyReported. see libcxx.zig for example.
+pub fn buildCrtFile(comp: *Compilation, crt_file: CrtFile, prog_node: std.Progress.Node) anyerror!void {
+    if (!build_options.have_llvm) return error.ZigCompilerNotBuiltWithLLVMExtensions;
+
+    const gpa = comp.gpa;
+    var arena_allocator = std.heap.ArenaAllocator.init(gpa);
+    defer arena_allocator.deinit();
+    const arena = arena_allocator.allocator();
+
+    const target = &comp.root_mod.resolved_target.result;
+    const target_version = target.os.version_range.semver.min;
+
+    // In all cases in this function, we add the C compiler flags to
+    // cache_exempt_flags rather than extra_flags, because these arguments
+    // depend on only properties that are already covered by the cache
+    // manifest. Including these arguments in the cache could only possibly
+    // waste computation and create false negatives.
+
+    switch (crt_file) {
+        .scrt0_o => {
+            var cflags = std.array_list.Managed([]const u8).init(arena);
+            try cflags.appendSlice(&.{
+                "-w", // Disable all warnings.
+            });
+
+            // See `Compilation.addCommonCCArgs`.
+            try cflags.append(try std.fmt.allocPrint(arena, "-D___OpenBSD={d}", .{
+                202510,
+            }));
+            try cflags.append(try std.fmt.allocPrint(arena, "-DOpenBSD{d}_{d}", .{
+                target_version.major,
+                target_version.minor,
+            }));
+
+            try cflags.appendSlice(&.{
+                "-I",
+                try includePath(comp, arena, try std.fmt.allocPrint(arena, "{s}-{s}-{s}", .{
+                    std.zig.target.openbsdArchNameHeaders(target.cpu.arch),
+                    @tagName(target.os.tag),
+                    @tagName(target.abi),
+                })),
+                "-I",
+                try includePath(comp, arena, "generic-openbsd"),
+                "-I",
+                try csuPath(comp, arena, switch (target.cpu.arch) {
+                    .mips64el => "mips64",
+                    .x86 => "i386",
+                    .x86_64 => "amd64",
+                    else => |t| @tagName(t),
+                }),
+                "-Qunused-arguments",
+            });
+
+            const sources = [_]struct {
+                path: []const u8,
+                flags: []const []const u8,
+            }{
+                .{
+                    .path = "crt0.c",
+                    .flags = cflags.items,
+                },
+                .{
+                    .path = "crtbegin.c",
+                    .flags = cflags.items,
+                },
+            };
+
+            var files_buf: [sources.len]Compilation.CSourceFile = undefined;
+            var files_index: usize = 0;
+            for (sources) |file| {
+                files_buf[files_index] = .{
+                    .src_path = try csuPath(comp, arena, file.path),
+                    .cache_exempt_flags = file.flags,
+                    .owner = undefined,
+                };
+                files_index += 1;
+            }
+            const files = files_buf[0..files_index];
+
+            return comp.build_crt_file("crt0", .Obj, .@"openbsd libc Scrt0.o", prog_node, files, .{
+                // Unclear why OpenBSD does this, but we'll do the same.
+                .omit_frame_pointer = if (target.cpu.arch.isX86()) false else null,
+                .pic = true,
+            });
+        },
+    }
+}
+
+pub const Lib = struct {
+    name: []const u8,
+};
+
+// Library versions are bumped frequently on OpenBSD. Fortunately, by linking to
+// just libc.so, the dynamic linker will happily bind to e.g. libc.so.102.0.
+pub const libs = [_]Lib{
+    .{ .name = "m" },
+    .{ .name = "pthread" },
+    .{ .name = "c" },
+    .{ .name = "ld" },
+    .{ .name = "util" },
+    .{ .name = "execinfo" },
+};
+
+pub const ABI = struct {
+    all_versions: []const Version, // all defined versions (one abilist from v2.0.0 up to current)
+    all_targets: []const std.zig.target.ArchOsAbi,
+    /// The bytes from the file verbatim, starting from the u16 number
+    /// of function inclusions.
+    inclusions: []const u8,
+    arena_state: std.heap.ArenaAllocator.State,
+
+    pub fn destroy(abi: *ABI, gpa: Allocator) void {
+        abi.arena_state.promote(gpa).deinit();
+    }
+};
+
+pub const LoadMetaDataError = error{
+    /// The files that ship with the Zig compiler were unable to be read, or otherwise had malformed data.
+    ZigInstallationCorrupt,
+    OutOfMemory,
+};
+
+pub const abilists_path = "libc" ++ path.sep_str ++ "openbsd" ++ path.sep_str ++ "abilists";
+pub const abilists_max_size = 300 * 1024; // Bigger than this and something is definitely borked.
+
+/// This function will emit a log error when there is a problem with the zig
+/// installation and then return `error.ZigInstallationCorrupt`.
+pub fn loadMetaData(gpa: Allocator, contents: []const u8) LoadMetaDataError!*ABI {
+    const tracy = trace(@src());
+    defer tracy.end();
+
+    var arena_allocator = std.heap.ArenaAllocator.init(gpa);
+    errdefer arena_allocator.deinit();
+    const arena = arena_allocator.allocator();
+
+    var index: usize = 0;
+
+    {
+        const libs_len = contents[index];
+        index += 1;
+
+        var i: u8 = 0;
+        while (i < libs_len) : (i += 1) {
+            const lib_name = mem.sliceTo(contents[index..], 0);
+            index += lib_name.len + 1;
+
+            if (i >= libs.len or !mem.eql(u8, libs[i].name, lib_name)) {
+                log.err("libc" ++ path.sep_str ++ "openbsd" ++ path.sep_str ++
+                    "abilists: invalid library name or index ({d}): '{s}'", .{ i, lib_name });
+                return error.ZigInstallationCorrupt;
+            }
+        }
+    }
+
+    const versions = b: {
+        const versions_len = contents[index];
+        index += 1;
+
+        const versions = try arena.alloc(Version, versions_len);
+        var i: u8 = 0;
+        while (i < versions.len) : (i += 1) {
+            versions[i] = .{
+                .major = contents[index + 0],
+                .minor = contents[index + 1],
+                .patch = contents[index + 2],
+            };
+            index += 3;
+        }
+        break :b versions;
+    };
+
+    const targets = b: {
+        const targets_len = contents[index];
+        index += 1;
+
+        const targets = try arena.alloc(std.zig.target.ArchOsAbi, targets_len);
+        var i: u8 = 0;
+        while (i < targets.len) : (i += 1) {
+            const target_name = mem.sliceTo(contents[index..], 0);
+            index += target_name.len + 1;
+
+            var component_it = mem.tokenizeScalar(u8, target_name, '-');
+            const arch_name = component_it.next() orelse {
+                log.err("abilists: expected arch name", .{});
+                return error.ZigInstallationCorrupt;
+            };
+            const os_name = component_it.next() orelse {
+                log.err("abilists: expected OS name", .{});
+                return error.ZigInstallationCorrupt;
+            };
+            const abi_name = component_it.next() orelse {
+                log.err("abilists: expected ABI name", .{});
+                return error.ZigInstallationCorrupt;
+            };
+            const arch_tag = std.meta.stringToEnum(std.Target.Cpu.Arch, arch_name) orelse {
+                log.err("abilists: unrecognized arch: '{s}'", .{arch_name});
+                return error.ZigInstallationCorrupt;
+            };
+            if (!mem.eql(u8, os_name, "openbsd")) {
+                log.err("abilists: expected OS 'openbsd', found '{s}'", .{os_name});
+                return error.ZigInstallationCorrupt;
+            }
+            const abi_tag = std.meta.stringToEnum(std.Target.Abi, abi_name) orelse {
+                log.err("abilists: unrecognized ABI: '{s}'", .{abi_name});
+                return error.ZigInstallationCorrupt;
+            };
+
+            targets[i] = .{
+                .arch = arch_tag,
+                .os = .openbsd,
+                .abi = abi_tag,
+            };
+        }
+        break :b targets;
+    };
+
+    const abi = try arena.create(ABI);
+    abi.* = .{
+        .all_versions = versions,
+        .all_targets = targets,
+        .inclusions = contents[index..],
+        .arena_state = arena_allocator.state,
+    };
+    return abi;
+}
+
+pub const BuiltSharedObjects = struct {
+    lock: Cache.Lock,
+    dir_path: Path,
+
+    pub fn deinit(self: *BuiltSharedObjects, gpa: Allocator) void {
+        self.lock.release();
+        gpa.free(self.dir_path.sub_path);
+        self.* = undefined;
+    }
+};
+
+fn wordDirective(target: *const std.Target) []const u8 {
+    // Based on its description in the GNU `as` manual, you might assume that `.word` is sized
+    // according to the target word size. But no; that would just make too much sense.
+    return if (target.ptrBitWidth() == 64) ".quad" else ".long";
+}
+
+/// TODO replace anyerror with explicit error set, recording user-friendly errors with
+/// lockAndSetMiscFailure and returning error.AlreadyReported. see libcxx.zig for example.
+pub fn buildSharedObjects(comp: *Compilation, prog_node: std.Progress.Node) anyerror!void {
+    // See also glibc.zig which this code is based on.
+
+    const tracy = trace(@src());
+    defer tracy.end();
+
+    if (!build_options.have_llvm) {
+        return error.ZigCompilerNotBuiltWithLLVMExtensions;
+    }
+
+    const gpa = comp.gpa;
+    const io = comp.io;
+
+    var arena_allocator = std.heap.ArenaAllocator.init(gpa);
+    defer arena_allocator.deinit();
+    const arena = arena_allocator.allocator();
+
+    const target = comp.getTarget();
+    const target_version = target.os.version_range.semver.min;
+
+    // Use the global cache directory.
+    var cache: Cache = .{
+        .gpa = gpa,
+        .io = io,
+        .manifest_dir = try comp.dirs.global_cache.handle.makeOpenPath("h", .{}),
+    };
+    cache.addPrefix(.{ .path = null, .handle = fs.cwd() });
+    cache.addPrefix(comp.dirs.zig_lib);
+    cache.addPrefix(comp.dirs.global_cache);
+    defer cache.manifest_dir.close();
+
+    var man = cache.obtain();
+    defer man.deinit();
+    man.hash.addBytes(build_options.version);
+    man.hash.add(target.cpu.arch);
+    man.hash.add(target.abi);
+    man.hash.add(target_version);
+
+    const full_abilists_path = try comp.dirs.zig_lib.join(arena, &.{abilists_path});
+    const abilists_index = try man.addFile(full_abilists_path, abilists_max_size);
+
+    if (try man.hit()) {
+        const digest = man.final();
+
+        return queueSharedObjects(comp, .{
+            .lock = man.toOwnedLock(),
+            .dir_path = .{
+                .root_dir = comp.dirs.global_cache,
+                .sub_path = try gpa.dupe(u8, "o" ++ fs.path.sep_str ++ digest),
+            },
+        });
+    }
+
+    const digest = man.final();
+    const o_sub_path = try path.join(arena, &[_][]const u8{ "o", &digest });
+
+    var o_directory: Cache.Directory = .{
+        .handle = try comp.dirs.global_cache.handle.makeOpenPath(o_sub_path, .{}),
+        .path = try comp.dirs.global_cache.join(arena, &.{o_sub_path}),
+    };
+    defer o_directory.handle.close();
+
+    const abilists_contents = man.files.keys()[abilists_index].contents.?;
+    const metadata = try loadMetaData(gpa, abilists_contents);
+    defer metadata.destroy(gpa);
+
+    const target_targ_index = for (metadata.all_targets, 0..) |targ, i| {
+        if (targ.arch == target.cpu.arch and
+            targ.os == target.os.tag and
+            targ.abi == target.abi)
+        {
+            break i;
+        }
+    } else {
+        unreachable; // std.zig.target.available_libcs prevents us from getting here
+    };
+
+    const target_ver_index = for (metadata.all_versions, 0..) |ver, i| {
+        switch (ver.order(target_version)) {
+            .eq => break i,
+            .lt => continue,
+            .gt => {
+                // TODO Expose via compile error mechanism instead of log.
+                log.warn("invalid target OpenBSD libc version: {f}", .{target_version});
+                return error.InvalidTargetLibCVersion;
+            },
+        }
+    } else blk: {
+        const latest_index = metadata.all_versions.len - 1;
+        log.warn("zig cannot build new OpenBSD libc version {f}; providing instead {f}", .{
+            target_version, metadata.all_versions[latest_index],
+        });
+        break :blk latest_index;
+    };
+
+    var stubs_asm = std.array_list.Managed(u8).init(gpa);
+    defer stubs_asm.deinit();
+
+    for (libs, 0..) |lib, lib_i| {
+        stubs_asm.shrinkRetainingCapacity(0);
+
+        try stubs_asm.appendSlice(".text\n");
+
+        var sym_i: usize = 0;
+        var sym_name_buf: std.Io.Writer.Allocating = .init(arena);
+        var opt_symbol_name: ?[]const u8 = null;
+
+        var inc_reader: std.Io.Reader = .fixed(metadata.inclusions);
+
+        const fn_inclusions_len = try inc_reader.takeInt(u16, .little);
+
+        var chosen_ver_index: usize = 255;
+        var chosen_is_weak: bool = undefined;
+
+        while (sym_i < fn_inclusions_len) : (sym_i += 1) {
+            const sym_name = opt_symbol_name orelse n: {
+                sym_name_buf.clearRetainingCapacity();
+                _ = try inc_reader.streamDelimiter(&sym_name_buf.writer, 0);
+                assert(inc_reader.buffered()[0] == 0); // TODO change streamDelimiter API
+                inc_reader.toss(1);
+
+                opt_symbol_name = sym_name_buf.written();
+                chosen_ver_index = 255;
+
+                break :n sym_name_buf.written();
+            };
+
+            {
+                const targets = try inc_reader.takeLeb128(u64);
+                var lib_index = try inc_reader.takeByte();
+
+                const is_weak = (lib_index & (1 << 6)) != 0;
+                const is_terminal = (lib_index & (1 << 7)) != 0;
+
+                lib_index = @as(u5, @truncate(lib_index));
+
+                // Test whether the inclusion applies to our current library and target.
+                const ok_lib_and_target =
+                    (lib_index == lib_i) and
+                    ((targets & (@as(u64, 1) << @as(u6, @intCast(target_targ_index)))) != 0);
+
+                while (true) {
+                    const byte = try inc_reader.takeByte();
+                    const last = (byte & 0b1000_0000) != 0;
+                    const ver_i = @as(u7, @truncate(byte));
+                    if (ok_lib_and_target and ver_i <= target_ver_index and
+                        (chosen_ver_index == 255 or ver_i > chosen_ver_index))
+                    {
+                        chosen_ver_index = ver_i;
+                        chosen_is_weak = is_weak;
+                    }
+                    if (last) break;
+                }
+
+                if (is_terminal) {
+                    opt_symbol_name = null;
+                } else continue;
+            }
+
+            if (chosen_ver_index != 255) {
+                // Example:
+                // .balign 4
+                // .globl _Exit
+                // .type _Exit, %function
+                // _Exit: .long 0
+                try stubs_asm.print(
+                    \\.balign {d}
+                    \\.{s} {s}
+                    \\.type {s}, %function
+                    \\{s}: {s} 0
+                    \\
+                , .{
+                    target.ptrBitWidth() / 8,
+                    if (chosen_is_weak) "weak" else "globl",
+                    sym_name,
+                    sym_name,
+                    sym_name,
+                    wordDirective(target),
+                });
+            }
+        }
+
+        try stubs_asm.appendSlice(".data\n");
+
+        const obj_inclusions_len = try inc_reader.takeInt(u16, .little);
+
+        sym_i = 0;
+        opt_symbol_name = null;
+
+        var chosen_size: u16 = undefined;
+
+        while (sym_i < obj_inclusions_len) : (sym_i += 1) {
+            const sym_name = opt_symbol_name orelse n: {
+                sym_name_buf.clearRetainingCapacity();
+                _ = try inc_reader.streamDelimiter(&sym_name_buf.writer, 0);
+                assert(inc_reader.buffered()[0] == 0); // TODO change streamDelimiter API
+                inc_reader.toss(1);
+
+                opt_symbol_name = sym_name_buf.written();
+                chosen_ver_index = 255;
+
+                break :n sym_name_buf.written();
+            };
+
+            {
+                const targets = try inc_reader.takeLeb128(u64);
+                const size = try inc_reader.takeLeb128(u16);
+                var lib_index = try inc_reader.takeByte();
+
+                const is_weak = (lib_index & (1 << 6)) != 0;
+                const is_terminal = (lib_index & (1 << 7)) != 0;
+
+                lib_index = @as(u5, @truncate(lib_index));
+
+                // Test whether the inclusion applies to our current library and target.
+                const ok_lib_and_target =
+                    (lib_index == lib_i) and
+                    ((targets & (@as(u64, 1) << @as(u6, @intCast(target_targ_index)))) != 0);
+
+                while (true) {
+                    const byte = try inc_reader.takeByte();
+                    const last = (byte & 0b1000_0000) != 0;
+                    const ver_i = @as(u7, @truncate(byte));
+                    if (ok_lib_and_target and ver_i <= target_ver_index and
+                        (chosen_ver_index == 255 or ver_i > chosen_ver_index))
+                    {
+                        chosen_ver_index = ver_i;
+                        chosen_size = size;
+                        chosen_is_weak = is_weak;
+                    }
+                    if (last) break;
+                }
+
+                if (is_terminal) {
+                    opt_symbol_name = null;
+                } else continue;
+            }
+
+            if (chosen_ver_index != 255) {
+                // Example:
+                // .balign 4
+                // .globl malloc_conf
+                // .type malloc_conf, %object
+                // .size malloc_conf, 4
+                // malloc_conf: .fill 4, 1, 0
+                try stubs_asm.print(
+                    \\.balign {d}
+                    \\.{s} {s}
+                    \\.type {s}, %object
+                    \\.size {s}, {d}
+                    \\{s}: {s} 0
+                    \\
+                , .{
+                    target.ptrBitWidth() / 8,
+                    if (chosen_is_weak) "weak" else "globl",
+                    sym_name,
+                    sym_name,
+                    sym_name,
+                    chosen_size,
+                    sym_name,
+                    wordDirective(target),
+                });
+            }
+        }
+
+        var lib_name_buf: [32]u8 = undefined; // Larger than each of the names "c", "pthread", etc.
+        const asm_file_basename = std.fmt.bufPrint(&lib_name_buf, "{s}.s", .{lib.name}) catch unreachable;
+        try o_directory.handle.writeFile(.{ .sub_path = asm_file_basename, .data = stubs_asm.items });
+        try buildSharedLib(comp, arena, o_directory, asm_file_basename, lib, prog_node);
+    }
+
+    man.writeManifest() catch |err| {
+        log.warn("failed to write cache manifest for OpenBSD libc stubs: {s}", .{@errorName(err)});
+    };
+
+    return queueSharedObjects(comp, .{
+        .lock = man.toOwnedLock(),
+        .dir_path = .{
+            .root_dir = comp.dirs.global_cache,
+            .sub_path = try gpa.dupe(u8, "o" ++ fs.path.sep_str ++ digest),
+        },
+    });
+}
+
+fn queueSharedObjects(comp: *Compilation, so_files: BuiltSharedObjects) void {
+    assert(comp.openbsd_so_files == null);
+    comp.openbsd_so_files = so_files;
+
+    var task_buffer: [libs.len]link.PrelinkTask = undefined;
+    var task_buffer_i: usize = 0;
+
+    {
+        comp.mutex.lock(); // protect comp.arena
+        defer comp.mutex.unlock();
+
+        for (libs) |lib| {
+            const so_path: Path = .{
+                .root_dir = so_files.dir_path.root_dir,
+                .sub_path = std.fmt.allocPrint(comp.arena, "{s}{c}lib{s}.so", .{
+                    so_files.dir_path.sub_path, fs.path.sep, lib.name,
+                }) catch return comp.setAllocFailure(),
+            };
+            task_buffer[task_buffer_i] = .{ .load_dso = so_path };
+            task_buffer_i += 1;
+        }
+    }
+
+    comp.queuePrelinkTasks(task_buffer[0..task_buffer_i]);
+}
+
+fn buildSharedLib(
+    comp: *Compilation,
+    arena: Allocator,
+    bin_directory: Cache.Directory,
+    asm_file_basename: []const u8,
+    lib: Lib,
+    prog_node: std.Progress.Node,
+) !void {
+    const tracy = trace(@src());
+    defer tracy.end();
+
+    const io = comp.io;
+    const basename = try std.fmt.allocPrint(arena, "lib{s}.so", .{lib.name});
+    const ld_basename = path.basename(comp.getTarget().standardDynamicLinkerPath().get().?);
+    const soname = if (mem.eql(u8, lib.name, "ld")) ld_basename else basename;
+
+    const optimize_mode = comp.compilerRtOptMode();
+    const strip = comp.compilerRtStrip();
+    const config = try Compilation.Config.resolve(.{
+        .output_mode = .Lib,
+        .link_mode = .dynamic,
+        .resolved_target = comp.root_mod.resolved_target,
+        .is_test = false,
+        .have_zcu = false,
+        .emit_bin = true,
+        .root_optimize_mode = optimize_mode,
+        .root_strip = strip,
+        .link_libc = false,
+    });
+
+    const root_mod = try Module.create(arena, .{
+        .paths = .{
+            .root = .zig_lib_root,
+            .root_src_path = "",
+        },
+        .fully_qualified_name = "root",
+        .inherited = .{
+            .resolved_target = comp.root_mod.resolved_target,
+            .strip = strip,
+            .stack_check = false,
+            .stack_protector = 0,
+            .sanitize_c = .off,
+            .sanitize_thread = false,
+            .red_zone = comp.root_mod.red_zone,
+            .omit_frame_pointer = comp.root_mod.omit_frame_pointer,
+            .valgrind = false,
+            .optimize_mode = optimize_mode,
+            .structured_cfg = comp.root_mod.structured_cfg,
+        },
+        .global = config,
+        .cc_argv = &.{},
+        .parent = null,
+    });
+
+    const c_source_files = [1]Compilation.CSourceFile{
+        .{
+            .src_path = try path.join(arena, &.{ bin_directory.path.?, asm_file_basename }),
+            .owner = root_mod,
+        },
+    };
+
+    const misc_task: Compilation.MiscTask = .@"openbsd libc shared object";
+
+    var sub_create_diag: Compilation.CreateDiagnostic = undefined;
+    const sub_compilation = Compilation.create(comp.gpa, arena, io, &sub_create_diag, .{
+        .dirs = comp.dirs.withoutLocalCache(),
+        .thread_pool = comp.thread_pool,
+        .self_exe_path = comp.self_exe_path,
+        // Because we manually cache the whole set of objects, we don't cache the individual objects
+        // within it. In fact, we *can't* do that, because we need `emit_bin` to specify the path.
+        .cache_mode = .none,
+        .config = config,
+        .root_mod = root_mod,
+        .root_name = lib.name,
+        .libc_installation = comp.libc_installation,
+        .emit_bin = .{ .yes_path = try bin_directory.join(arena, &.{basename}) },
+        .verbose_cc = comp.verbose_cc,
+        .verbose_link = comp.verbose_link,
+        .verbose_air = comp.verbose_air,
+        .verbose_llvm_ir = comp.verbose_llvm_ir,
+        .verbose_llvm_bc = comp.verbose_llvm_bc,
+        .verbose_cimport = comp.verbose_cimport,
+        .verbose_llvm_cpu_features = comp.verbose_llvm_cpu_features,
+        .clang_passthrough_mode = comp.clang_passthrough_mode,
+        .soname = soname,
+        .c_source_files = &c_source_files,
+        .skip_linker_dependencies = true,
+    }) catch |err| switch (err) {
+        error.CreateFail => {
+            comp.lockAndSetMiscFailure(misc_task, "sub-compilation of {t} failed: {f}", .{ misc_task, sub_create_diag });
+            return error.AlreadyReported;
+        },
+        else => |e| return e,
+    };
+    defer sub_compilation.destroy();
+
+    try comp.updateSubCompilation(sub_compilation, misc_task, prog_node);
+}

src/link/Lld.zig

@@ -1202,6 +1202,13 @@ fn elfLink(lld: *Lld, arena: Allocator) !void {
                         });
                         try argv.append(lib_path);
                     }
+                } else if (target.isOpenBSDLibC()) {
+                    for (openbsd.libs) |lib| {
+                        const lib_path = try std.fmt.allocPrint(arena, "{f}{c}lib{s}.so", .{
+                            comp.openbsd_so_files.?.dir_path, fs.path.sep, lib.name,
+                        });
+                        try argv.append(lib_path);
+                    }
                 } else {
                     diags.flags.missing_libc = true;
                 }
@@ -1702,6 +1709,7 @@ const dev = @import("../dev.zig");
 const freebsd = @import("../libs/freebsd.zig");
 const glibc = @import("../libs/glibc.zig");
 const netbsd = @import("../libs/netbsd.zig");
+const openbsd = @import("../libs/openbsd.zig");
 const wasi_libc = @import("../libs/wasi_libc.zig");
 const link = @import("../link.zig");
 const lldMain = @import("../main.zig").lldMain;