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linux-user/ppc: Fix ppc64 rt_sigframe stack offset

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The kernel's 64-bit signal delivery (signal_64.c) uses:

    newsp = frame - __SIGNAL_FRAMESIZE

while the 32-bit path (signal_32.c) uses:

    newsp = frame - (__SIGNAL_FRAMESIZE + 16)

The extra 16 bytes in the 32-bit case is to place siginfo and ucontext
at the same offsets as older kernels (see the comment in signal_32.c).
The 64-bit rt_sigframe starts with ucontext directly and does not need
this adjustment.

QEMU's setup_rt_frame() unconditionally used (SIGNAL_FRAMESIZE + 16)
for both 32-bit and 64-bit, placing the handler's SP 16 bytes too low
on ppc64. Signal delivery and return still worked because do_rt_sigreturn
had the matching wrong offset, but the vDSO DWARF unwind info encodes
the correct kernel offset. This caused any DWARF unwinder (libunwind,
libgcc, etc.) to compute a CFA that is 16 bytes off, reading garbage
register values from the signal frame.

Define RT_SIGFRAME_ADJUST (0 on ppc64, 16 on ppc32) and use it in both
setup_rt_frame and do_rt_sigreturn to match the kernel.

This was verified by A/B testing with libunwind's test suite:

  ppc64le: Gtest-bt, Ltest-bt, Gtest-concurrent, Ltest-concurrent,
           and Ltest-sig-context all change from FAIL to PASS.
  ppc64be: Gtest-bt, Ltest-bt, and Ltest-sig-context all change
           from FAIL to PASS.

Signed-off-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: qemu-stable@nongnu.org
This commit is contained in:
Matt Turner
2026-04-16 14:25:27 -04:00
committed by Helge Deller
parent 3201105f46
commit 654dce6c52
1 changed files with 14 additions and 2 deletions
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16
linux-user/ppc/signal.c
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@@ -210,6 +210,18 @@ QEMU_BUILD_BUG_ON(offsetof(struct target_rt_sigframe, uc.tuc_mcontext)
#endif
#ifdef TARGET_PPC64
#define RT_SIGFRAME_ADJUST 0
#else
/*
* For 32-bit rt sigframes we have an extra 16 bytes of gap
* on top of __SIGNAL_FRAMESIZE; this is to get the siginfo
* and ucontext in the same positions as in older kernels.
* See Linux's arch/powerpc/kernel/signal_32.c.
*/
#define RT_SIGFRAME_ADJUST 16
#endif
#if defined(TARGET_PPC64)
struct target_func_ptr {
@@ -525,7 +537,7 @@ void setup_rt_frame(int sig, struct target_sigaction *ka,
env->fpscr = 0;
/* Create a stack frame for the caller of the handler. */
newsp = rt_sf_addr - (SIGNAL_FRAMESIZE + 16);
newsp = rt_sf_addr - (SIGNAL_FRAMESIZE + RT_SIGFRAME_ADJUST);
err |= put_user(env->gpr[1], newsp, target_ulong);
if (err)
@@ -641,7 +653,7 @@ long do_rt_sigreturn(CPUPPCState *env)
struct target_rt_sigframe *rt_sf = NULL;
target_ulong rt_sf_addr;
rt_sf_addr = env->gpr[1] + SIGNAL_FRAMESIZE + 16;
rt_sf_addr = env->gpr[1] + SIGNAL_FRAMESIZE + RT_SIGFRAME_ADJUST;
if (!lock_user_struct(VERIFY_READ, rt_sf, rt_sf_addr, 1))
goto sigsegv;