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

sysdeps/unix/sysv/linux/sparc/Makefile

@@ -11,8 +11,12 @@ ifeq ($(subdir),sysvipc)
 sysdep_routines += getshmlba
 endif
 
+ifeq ($(subdir),signal)
+sysdep_routines += sigreturn_stub
+endif
+
 ifeq ($(subdir),nptl)
 # pull in __syscall_error routine
-libpthread-routines += sysdep
+libpthread-routines += sysdep sigreturn_stub
-libpthread-shared-only-routines += sysdep
+libpthread-shared-only-routines += sysdep sigreturn_stub
 endif

sysdeps/unix/sysv/linux/sparc/sparc32/sigaction.c

@@ -24,8 +24,8 @@
 #include <kernel_sigaction.h>
 #include <sysdep.h>
 
-static void __rt_sigreturn_stub (void);
+void __rt_sigreturn_stub (void);
-static void __sigreturn_stub (void);
+void __sigreturn_stub (void);
 
 #define STUB(act, sigsetsize) \
   (act) ? ((unsigned long)((act->sa_flags & SA_SIGINFO)	\
@@ -35,25 +35,3 @@ static void __sigreturn_stub (void);
   (sigsetsize)
 
 #include <sysdeps/unix/sysv/linux/sigaction.c>
-
-static
-inhibit_stack_protector
-void
-__rt_sigreturn_stub (void)
-{
-  __asm__ ("mov %0, %%g1\n\t"
-	   "ta	0x10\n\t"
-	   : /* no outputs */
-	   : "i" (__NR_rt_sigreturn));
-}
-
-static
-inhibit_stack_protector
-void
-__sigreturn_stub (void)
-{
-  __asm__ ("mov %0, %%g1\n\t"
-	   "ta	0x10\n\t"
-	   : /* no outputs */
-	   : "i" (__NR_sigreturn));
-}

sysdeps/unix/sysv/linux/sparc/sparc32/sigreturn_stub.S

@@ -0,0 +1,34 @@
+/* Sigreturn stub function used on sa_restore field.
+   Copyright (C) 2020 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/>.  */
+
+#include <sysdep.h>
+
+/* These functions must not change the register window or the stack
+   pointer [1].
+
+   [1] https://lkml.org/lkml/2016/5/27/465  */
+
+ENTRY (__rt_sigreturn_stub)
+	mov	__NR_rt_sigreturn, %g1
+	ta	0x10
+END (__rt_sigreturn_stub)
+
+ENTRY (__sigreturn_stub)
+	mov	__NR_sigreturn, %g1
+	ta	0x10
+END (__sigreturn_stub)

sysdeps/unix/sysv/linux/sparc/sparc64/sigaction.c

@@ -22,21 +22,11 @@
 #include <syscall.h>
 #include <sysdep.h>
 
-static void __rt_sigreturn_stub (void);
+/* Defined on sigreturn_stub.S.  */
+void __rt_sigreturn_stub (void);
 
 #define STUB(act, sigsetsize) \
   (((unsigned long) &__rt_sigreturn_stub) - 8),	\
   (sigsetsize)
 
 #include <sysdeps/unix/sysv/linux/sigaction.c>
-
-static
-inhibit_stack_protector
-void
-__rt_sigreturn_stub (void)
-{
-  __asm__ ("mov %0, %%g1\n\t"
-	   "ta	0x6d\n\t"
-	   : /* no outputs */
-	   : "i" (__NR_rt_sigreturn));
-}

sysdeps/unix/sysv/linux/sparc/sparc64/sigreturn_stub.S

@@ -0,0 +1,29 @@
+/* Sigreturn stub function used on sa_restore field.
+   Copyright (C) 2020 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/>.  */
+
+#include <sysdep.h>
+
+/* This function must not change the register window or the stack
+   pointer [1].
+
+   [1] https://lkml.org/lkml/2016/5/27/465  */
+
+ENTRY (__rt_sigreturn_stub)
+	mov	__NR_rt_sigreturn, %g1
+	ta	0x6d
+END (__rt_sigreturn_stub)