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MachO: fix dynamic lookup of undefined symbols at runtime #26042

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MachO: fix dynamic lookup of undefined symbols at runtime #26042

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8 changes: 7 additions & 1 deletion src/link/MachO.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -2937,7 +2937,13 @@ fn writeLoadCommands(self: *MachO) !struct { usize, usize, u64 } {

fn writeHeader(self: *MachO, ncmds: usize, sizeofcmds: usize) !void {
var header: macho.mach_header_64 = .{};
header.flags = macho.MH_NOUNDEFS | macho.MH_DYLDLINK;
header.flags = macho.MH_DYLDLINK;

// Only set MH_NOUNDEFS if we're not allowing undefined symbols via dynamic lookup.
// When dynamic_lookup is enabled, undefined symbols are resolved at runtime by dyld.
if (self.undefined_treatment != .dynamic_lookup) {
header.flags |= macho.MH_NOUNDEFS;
}

// TODO: if (self.options.namespace == .two_level) {
header.flags |= macho.MH_TWOLEVEL;
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24 changes: 24 additions & 0 deletions test/link/macho.zig
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Expand Up @@ -68,6 +68,7 @@ pub fn testAll(b: *Build, build_opts: BuildOptions) *Step {
macho_step.dependOn(testTlsLargeTbss(b, .{ .target = default_target }));
macho_step.dependOn(testTlsZig(b, .{ .target = default_target }));
macho_step.dependOn(testUndefinedFlag(b, .{ .target = default_target }));
macho_step.dependOn(testUndefinedDynamicLookup(b, .{ .target = default_target }));
macho_step.dependOn(testDiscardLocalSymbols(b, .{ .target = default_target }));
macho_step.dependOn(testUnresolvedError(b, .{ .target = default_target }));
macho_step.dependOn(testUnresolvedError2(b, .{ .target = default_target }));
Expand Down Expand Up @@ -2632,6 +2633,29 @@ fn testUndefinedFlag(b: *Build, opts: Options) *Step {
return test_step;
}

fn testUndefinedDynamicLookup(b: *Build, opts: Options) *Step {
const test_step = addTestStep(b, "undefined-dynamic-lookup", opts);

// Create a dylib with an undefined external symbol reference
const dylib = addSharedLibrary(b, opts, .{ .name = "a" });
addCSourceBytes(dylib,
\\extern int undefined_symbol(void);
\\int call_undefined(void) {
\\ return undefined_symbol();
\\}
, &.{});
dylib.linker_allow_shlib_undefined = true;

// Verify the Mach-O header does NOT contain NOUNDEFS flag
const check = dylib.checkObject();
check.checkInHeaders();
check.checkExact("header");
check.checkNotPresent("NOUNDEFS");
test_step.dependOn(&check.step);

return test_step;
}

fn testUnresolvedError(b: *Build, opts: Options) *Step {
const test_step = addTestStep(b, "unresolved-error", opts);

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