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glibc: Define _IO_stdin_used in start code and reference it in stub asm.

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This is necessary to inform the real, non-stub glibc that a program built with
Zig is using a modern `FILE` structure, i.e. glibc 2.1+. This is particularly
important on lesser-used architectures where the legacy code is poorly tested;
for example, glibc 2.40 introduced a regression for the legacy case in the
libio cleanup code, causing all Zig-compiled MIPS binaries to crash on exit.
This commit is contained in:
Alex Rønne Petersen 2024-08-17 11:42:32 +02:00 committed by Andrew Kelley
parent df6907f601
commit 5dd2bb525d
2 changed files with 59 additions and 1 deletions
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23
lib/libc/glibc/csu/init.c Normal file
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@ -0,0 +1,23 @@
/* Special startup support.
Copyright (C) 1997-2024 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
/* Vestigial libio version number. Some code in libio checks whether
this symbol exists in the executable, but nothing looks at its
value anymore; the value it was historically set to has been
preserved out of an abundance of caution. */
const int _IO_stdin_used = 0x20001;

37
src/glibc.zig
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@ -286,7 +286,11 @@ pub fn buildCRTFile(comp: *Compilation, crt_file: CRTFile, prog_node: std.Progre
.owner = undefined,
};
};
var files = [_]Compilation.CSourceFile{ start_o, abi_note_o };
const init_o: Compilation.CSourceFile = .{
.src_path = try lib_path(comp, arena, lib_libc_glibc ++ "csu" ++ path.sep_str ++ "init.c"),
.owner = undefined,
};
var files = [_]Compilation.CSourceFile{ start_o, abi_note_o, init_o };
return comp.build_crt_file("Scrt1", .Obj, .@"glibc Scrt1.o", prog_node, &files);
},
.libc_nonshared_a => {
@ -682,6 +686,12 @@ pub const BuiltSharedObjects = struct {
const all_map_basename = "all.map";
fn wordDirective(target: 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";
}
pub fn buildSharedObjects(comp: *Compilation, prog_node: std.Progress.Node) !void {
const tracy = trace(@src());
defer tracy.end();
@ -923,6 +933,31 @@ pub fn buildSharedObjects(comp: *Compilation, prog_node: std.Progress.Node) !voi
try stubs_asm.appendSlice(".data\n");
// For some targets, the real `libc.so.6` will contain a weak reference to `_IO_stdin_used`,
// making the linker put the symbol in the dynamic symbol table. We likewise need to emit a
// reference to it here for that effect, or it will not show up, which in turn will cause
// the real glibc to think that the program was built against an ancient `FILE` structure
// (pre-glibc 2.1).
//
// Note that glibc only compiles in the legacy compatibility code for some targets; it
// depends on what is defined in the `shlib-versions` file for the particular architecture
// and ABI. Those files are preprocessed by 2 separate tools during the glibc build to get
// the final `abi-versions.h`, so it would be quite brittle to try to condition our emission
// of the `_IO_stdin_used` reference in the exact same way. The only downside of emitting
// the reference unconditionally is that it ends up being unused for newer targets; it
// otherwise has no negative effect.
//
// glibc uses a weak reference because it has to work with programs compiled against pre-2.1
// versions where the symbol didn't exist. We only care about modern glibc versions, so use
// a strong reference.
if (std.mem.eql(u8, lib.name, "c")) {
try stubs_asm.writer().print(
\\.globl _IO_stdin_used
\\{s} _IO_stdin_used
\\
, .{wordDirective(target)});
}
const obj_inclusions_len = mem.readInt(u16, metadata.inclusions[inc_i..][0..2], .little);
inc_i += 2;