syscalls/x86: Use COMPAT_SYSCALL_DEFINE0 for IA32 (rt_)sigreturn

Use COMPAT_SYSCALL_DEFINE0 to define (rt_)sigreturn() syscalls to
replace sys32_sigreturn() and sys32_rt_sigreturn(). This fixes indirect
call mismatches with Control-Flow Integrity (CFI) checking.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H . Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191008224049.115427-4-samitolvanen@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/syscalls/syscall_32.tbl

@@ -130,7 +130,7 @@
 116	i386	sysinfo			sys_sysinfo			__ia32_compat_sys_sysinfo
 117	i386	ipc			sys_ipc				__ia32_compat_sys_ipc
 118	i386	fsync			sys_fsync			__ia32_sys_fsync
-119	i386	sigreturn		sys_sigreturn			sys32_sigreturn
+119	i386	sigreturn		sys_sigreturn			__ia32_compat_sys_sigreturn
 120	i386	clone			sys_clone			__ia32_compat_sys_x86_clone
 121	i386	setdomainname		sys_setdomainname		__ia32_sys_setdomainname
 122	i386	uname			sys_newuname			__ia32_sys_newuname
@@ -184,7 +184,7 @@
 170	i386	setresgid		sys_setresgid16			__ia32_sys_setresgid16
 171	i386	getresgid		sys_getresgid16			__ia32_sys_getresgid16
 172	i386	prctl			sys_prctl			__ia32_sys_prctl
-173	i386	rt_sigreturn		sys_rt_sigreturn		sys32_rt_sigreturn
+173	i386	rt_sigreturn		sys_rt_sigreturn		__ia32_compat_sys_rt_sigreturn
 174	i386	rt_sigaction		sys_rt_sigaction		__ia32_compat_sys_rt_sigaction
 175	i386	rt_sigprocmask		sys_rt_sigprocmask		__ia32_compat_sys_rt_sigprocmask
 176	i386	rt_sigpending		sys_rt_sigpending		__ia32_compat_sys_rt_sigpending