signal: Add __libc_sigaction

The generic implementation basically handle the system agnostic logic
(filtering out the invalid signals) while the __libc_sigaction is
the function with implements the system and architecture bits.

Checked on x86_64-linux-gnu and i686-linux-gnu.

sysdeps/unix/sysv/linux/i386/libc_sigaction.c

@@ -0,0 +1,79 @@
+/* POSIX.1 `sigaction' call for Linux/i386.
+   Copyright (C) 1991-2021 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 <signal.h>
+#include <ldsodefs.h>
+
+#define SA_RESTORER 0x04000000
+
+extern void restore_rt (void) asm ("__restore_rt") attribute_hidden;
+extern void restore (void) asm ("__restore") attribute_hidden;
+
+#define SET_SA_RESTORER(kact, act)				\
+  ({								\
+     if (GLRO(dl_sysinfo_dso) == NULL)				\
+       {							\
+	 (kact)->sa_flags |= SA_RESTORER;			\
+         (kact)->sa_restorer = (((act)->sa_flags & SA_SIGINFO)	\
+			       ? &restore_rt : &restore);	\
+       }							\
+     else							\
+       (kact)->sa_restorer = NULL;				\
+  })
+
+#define RESET_SA_RESTORER(act, kact) \
+  (act)->sa_restorer = (kact)->sa_restorer
+
+#include <sysdeps/unix/sysv/linux/libc_sigaction.c>
+
+/* NOTE: Please think twice before making any changes to the bits of
+   code below.  GDB needs some intimate knowledge about it to
+   recognize them as signal trampolines, and make backtraces through
+   signal handlers work right.  Important are both the names
+   (__restore and __restore_rt) and the exact instruction sequence.
+   If you ever feel the need to make any changes, please notify the
+   appropriate GDB maintainer.  */
+
+#define RESTORE(name, syscall) RESTORE2 (name, syscall)
+#define RESTORE2(name, syscall) \
+asm						\
+  (						\
+   ".text\n"					\
+   "	.align 16\n"				\
+   "__" #name ":\n"				\
+   "	movl $" #syscall ", %eax\n"		\
+   "	int  $0x80"				\
+   );
+
+/* The return code for realtime-signals.  */
+RESTORE (restore_rt, __NR_rt_sigreturn)
+
+/* For the boring old signals.  */
+#undef RESTORE2
+#define RESTORE2(name, syscall) \
+asm						\
+  (						\
+   ".text\n"					\
+   "	.align 8\n"				\
+   "__" #name ":\n"				\
+   "	popl %eax\n"				\
+   "	movl $" #syscall ", %eax\n"		\
+   "	int  $0x80"				\
+   );
+
+RESTORE (restore, __NR_sigreturn)