sparc: Move sigreturn stub to assembly

It seems that some gcc versions might generates a stack frame for the
sigreturn stub requires on sparc signal handling.  For instance:

  $ cat test.c
  #define _GNU_SOURCE
  #include <sys/syscall.h>

  __attribute__ ((__optimize__ ("-fno-stack-protector")))
  void
  __sigreturn_stub (void)
  {
    __asm__ ("mov %0, %%g1\n\t"
            "ta  0x10\n\t"
            : /* no outputs */
            : "i" (SYS_rt_sigreturn));
  }
  $ gcc -v
  [...]
  gcc version 9.2.1 20200224 (Debian 9.2.1-30)
  $ gcc -O2 -m64 test.c -S -o -
  [...]
    __sigreturn_stub:
          save    %sp, -176, %sp
  #APP
  ! 9 "t.c" 1
          mov 101, %g1
          ta  0x10

  ! 0 "" 2
  #NO_APP
          .size   __sigreturn_stub, .-__sigreturn_stub

As indicated by kernel developers [1], the sigreturn stub can not change
the register window or the stack pointer since the kernel has setup the
restore frame at a precise location relative to the stack pointer when
the stub is invoked.

I tried to play with some compiler flags and even with _Noreturn and
__builtin_unreachable after the asm does not help (and Sparc does not
support naked functions).

To avoid similar issues, as the stack-protector support also have
stumbled, this patch moves the implementation of the sigreturn stubs to
assembly.

Checked on sparcv9-linux-gnu and sparc64-linux-gnu with gcc 9.2.1
and gcc 7.5.0.

[1] https://lkml.org/lkml/2016/5/27/465

0 files changed, 0 insertions, 0 deletions