[PATCH] tailcall prevention in sys_wait4() and sys_waitid()

A hack to prevent the compiler from generatin tailcalls in these two
functions.

With CONFIG_REGPARM=y, the tailcalled code ends up stomping on the
syscall's argument frame which corrupts userspace's registers.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/asm-i386/linkage.h

@@ -5,6 +5,10 @@
 #define FASTCALL(x)	x __attribute__((regparm(3)))
 #define fastcall	__attribute__((regparm(3)))
 
+#ifdef CONFIG_REGPARM
+# define prevent_tail_call(ret) __asm__ ("" : "=r" (ret) : "0" (ret))
+#endif
+
 #ifdef CONFIG_X86_ALIGNMENT_16
 #define __ALIGN .align 16,0x90
 #define __ALIGN_STR ".align 16,0x90"

include/linux/linkage.h

@@ -14,6 +14,10 @@
 #define asmlinkage CPP_ASMLINKAGE
 #endif
 
+#ifndef prevent_tail_call
+# define prevent_tail_call(ret) do { } while (0)
+#endif
+
 #ifndef __ALIGN
 #define __ALIGN		.align 4,0x90
 #define __ALIGN_STR	".align 4,0x90"

kernel/exit.c

@@ -1376,6 +1376,8 @@ asmlinkage long sys_waitid(int which, pid_t pid,
 			   struct siginfo __user *infop, int options,
 			   struct rusage __user *ru)
 {
+	long ret;
+
 	if (options & ~(WNOHANG|WNOWAIT|WEXITED|WSTOPPED|WCONTINUED))
 		return -EINVAL;
 	if (!(options & (WEXITED|WSTOPPED|WCONTINUED)))
@@ -1398,15 +1400,25 @@ asmlinkage long sys_waitid(int which, pid_t pid,
 		return -EINVAL;
 	}
 
-	return do_wait(pid, options, infop, NULL, ru);
+	ret = do_wait(pid, options, infop, NULL, ru);
+
+	/* avoid REGPARM breakage on x86: */
+	prevent_tail_call(ret);
+	return ret;
 }
 
 asmlinkage long sys_wait4(pid_t pid, int __user *stat_addr,
 			  int options, struct rusage __user *ru)
 {
+	long ret;
+
 	if (options & ~(WNOHANG|WUNTRACED|__WNOTHREAD|__WCLONE|__WALL))
 		return -EINVAL;
-	return do_wait(pid, options | WEXITED, NULL, stat_addr, ru);
+	ret = do_wait(pid, options | WEXITED, NULL, stat_addr, ru);
+
+	/* avoid REGPARM breakage on x86: */
+	prevent_tail_call(ret);
+	return ret;
 }
 
 #ifdef __ARCH_WANT_SYS_WAITPID