[PATCH] powerpc64: Workaround sigtramp vdso return call

[Raoni Fassina Firmino <raoni@linux.ibm.com>]

Hi, this is a backport from the commit already on 2.33, I would like
to backport it to 2.31 and 2.32. The reason is the same as the
original patch, there is already released kernels with the regression,
for example, Debian 10 ppc64 has glibc 2.31 and kernel 5.10.

I "cherry-picked -x" the commit as-is, I am not sure if I have to
change anything in the commit message.

I tested on top of release/2.31/master and release/2.32/master in the
following configurations:

- powerpc64le-linux-gnu (power8) kernel 5.11.0-rc5
- powerpc64-linux-gnu (power8) kernel 5.11.0-rc5
  (this kernels where patched with the fix upstream from 5.11-rc7)

- powerpc64-linux-gnu (power9) kernel 5.10.0-1
  (this is a kernel with the regression in it)

- powerpc64le-linux-gnu (power9) kernel 5.4.0-59
  (this is a kerenel version prior to the regression)

And tested only on top of release/2.32/master on:

- powerpc64le-linux-gnu (power9) kernel 5.9.14-1
  (this is a kernel with the regression in it)

o/
Raoni

PS: I hope I tagged the email correctly.

---- %< ----

A not so recent kernel change[1] changed how the trampoline
`__kernel_sigtramp_rt64` is used to call signal handlers.

This was exposed on the test misc/tst-sigcontext-get_pc

Before kernel 5.9, the kernel set LR to the trampoline address and
jumped directly to the signal handler, and at the end the signal
handler, as any other function, would `blr` to the address set.  In
other words, the trampoline was executed just at the end of the signal
handler and the only thing it did was call sigreturn.  But since
kernel 5.9 the kernel set CTRL to the signal handler and calls to the
trampoline code, the trampoline then `bctrl` to the address in CTRL,
setting the LR to the next instruction in the middle of the
trampoline, when the signal handler returns, the rest of the
trampoline code executes the same code as before.

Here is the full trampoline code as of kernel 5.11.0-rc5 for
reference:

    V_FUNCTION_BEGIN(__kernel_sigtramp_rt64)
    .Lsigrt_start:
            bctrl   /* call the handler */
            addi    r1, r1, __SIGNAL_FRAMESIZE
            li      r0,__NR_rt_sigreturn
            sc
    .Lsigrt_end:
    V_FUNCTION_END(__kernel_sigtramp_rt64)

This new behavior breaks how `backtrace()` uses to detect the
trampoline frame to correctly reconstruct the stack frame when it is
called from inside a signal handling.

This workaround rely on the fact that the trampoline code is at very
least two (maybe 3?) instructions in size (as it is in the 32 bits
version, only on `li` and `sc`), so it is safe to check the return
address be in the range __kernel_sigtramp_rt64 .. + 4.

[1] subject: powerpc/64/signal: Balance return predictor stack in signal trampoline
    commit: 0138ba5783ae0dcc799ad401a1e8ac8333790df9
    url: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0138ba5783ae0dcc799ad401a1e8ac8333790df9

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
(cherry picked from commit 5ee506ed35a2c9184bcb1fb5e79b6cceb9bb0dd1)
---
 sysdeps/powerpc/powerpc64/backtrace.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/sysdeps/powerpc/powerpc64/backtrace.c b/sysdeps/powerpc/powerpc64/backtrace.c
index 8a53a1088f27..362a2b713cf9 100644
--- a/sysdeps/powerpc/powerpc64/backtrace.c
+++ b/sysdeps/powerpc/powerpc64/backtrace.c
@@ -54,11 +54,22 @@ struct signal_frame_64 {
   /* We don't care about the rest, since the IP value is at 'uc' field.  */
 };
 
+/* Test if the address match to the inside the trampoline code.
+   Up to and including kernel 5.8, returning from an interrupt or syscall to a
+   signal handler starts execution directly at the handler's entry point, with
+   LR set to address of the sigreturn trampoline (the vDSO symbol).
+   Newer kernels will branch to signal handler from the trampoline instead, so
+   checking the stacktrace against the vDSO entrypoint does not work in such
+   case.
+   The vDSO branches with a 'bctrl' instruction, so checking either the
+   vDSO address itself and the next instruction should cover all kernel
+   versions.  */
 static inline bool
 is_sigtramp_address (void *nip)
 {
 #ifdef HAVE_SIGTRAMP_RT64
-  if (nip == GLRO (dl_vdso_sigtramp_rt64))
+  if (nip == GLRO (dl_vdso_sigtramp_rt64) ||
+      nip == GLRO (dl_vdso_sigtramp_rt64) + 4)
     return true;
 #endif
   return false;
-- 
2.26.2