diff options
| author | H. Peter Anvin <hpa@zytor.com> | 2026-02-26 17:03:07 -0800 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2026-03-04 17:06:08 +0100 |
| commit | b5ef09a77d0b5213268300eedd8a7d28b4e92d47 (patch) | |
| tree | 0aece112cb84509af6d345a84de1da8cc1267b50 | |
| parent | 59674fc9d0bfd96ce8a776680ee1cf22c28c9ac7 (diff) | |
x86/entry/vdso32: Work around libgcc unwinder bug
The unwinder code in libgcc has a long standing bug which causes it to
fail to pick up the signal frame CFI flag. This is a generic bug
across all platforms.
It affects the __kernel_sigreturn and __kernel_rt_sigreturn vdso entry
points on i386. The x86-64 kernel doesn't provide a sigreturn stub,
and so there is no kernel-provided code that is affected on x86-64.
libgcc does have a legacy fallback path which happens to work as long
as the bytes immediately before each of the sigreturn functions fall
outside any function. This patch adds a nop before the ALIGN to each
of the sigreturn stubs to ensure that this is, indeed, the case.
The rest of the patch is just a comment which documents the invariants
that need to be maintained for this legacy path to work correctly.
This is a manifest bug: in the current vdso, __kernel_vsyscall is a
multiple of 16 bytes long and thus __kernel_sigreturn does not have
any padding in front of it.
Closes: https://lore.kernel.org/lkml/f3412cc3e8f66d1853cc9d572c0f2fab076872b1.camel@xry111.site
Fixes: 884961618ee5 ("x86/entry/vdso32: Remove open-coded DWARF in sigreturn.S")
Reported-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124050
Link: https://patch.msgid.link/20260227010308.310342-1-hpa@zytor.com
| -rw-r--r-- | arch/x86/entry/vdso/vdso32/sigreturn.S | 30 |
1 files changed, 30 insertions, 0 deletions
diff --git a/arch/x86/entry/vdso/vdso32/sigreturn.S b/arch/x86/entry/vdso/vdso32/sigreturn.S index b433353bc8e3..b33fcc501ba3 100644 --- a/arch/x86/entry/vdso/vdso32/sigreturn.S +++ b/arch/x86/entry/vdso/vdso32/sigreturn.S @@ -35,9 +35,38 @@ #endif .endm +/* + * WARNING: + * + * A bug in the libgcc unwinder as of at least gcc 15.2 (2026) means that + * the unwinder fails to recognize the signal frame flag. + * + * There is a hacky legacy fallback path in libgcc which ends up + * getting invoked instead. It happens to work as long as BOTH of the + * following conditions are true: + * + * 1. There is at least one byte before the each of the sigreturn + * functions which falls outside any function. This is enforced by + * an explicit nop instruction before the ALIGN. + * 2. The code sequences between the entry point up to and including + * the int $0x80 below need to match EXACTLY. Do not change them + * in any way. The exact byte sequences are: + * + * __kernel_sigreturn: + * 0: 58 pop %eax + * 1: b8 77 00 00 00 mov $0x77,%eax + * 6: cd 80 int $0x80 + * + * __kernel_rt_sigreturn: + * 0: b8 ad 00 00 00 mov $0xad,%eax + * 5: cd 80 int $0x80 + * + * For details, see: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124050 + */ .text .globl __kernel_sigreturn .type __kernel_sigreturn,@function + nop /* libgcc hack: see comment above */ ALIGN __kernel_sigreturn: STARTPROC_SIGNAL_FRAME IA32_SIGFRAME_sigcontext @@ -52,6 +81,7 @@ SYM_INNER_LABEL(vdso32_sigreturn_landing_pad, SYM_L_GLOBAL) .globl __kernel_rt_sigreturn .type __kernel_rt_sigreturn,@function + nop /* libgcc hack: see comment above */ ALIGN __kernel_rt_sigreturn: STARTPROC_SIGNAL_FRAME IA32_RT_SIGFRAME_sigcontext |