x86: be careful about tailcall breakage for sys_open[at] too

Came up through a quick grep for other cases similar to the ftruncate()
one in commit 0a489cb3.

Also, add a comment, so that people who read the code understand why we
do what looks like a no-op.

(Again, this won't actually matter to any sane user, since libc will
save and restore the register gcc stomps on, but it's still wrong to
stomp on it)
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/open.c

@@ -332,6 +332,7 @@ static long do_sys_ftruncate(unsigned int fd, loff_t length, int small)
 asmlinkage long sys_ftruncate(unsigned int fd, unsigned long length)
 {
 	long ret = do_sys_ftruncate(fd, length, 1);
+	/* avoid REGPARM breakage on x86: */
 	prevent_tail_call(ret);
 	return ret;
 }
@@ -346,6 +347,7 @@ asmlinkage long sys_truncate64(const char __user * path, loff_t length)
 asmlinkage long sys_ftruncate64(unsigned int fd, loff_t length)
 {
 	long ret = do_sys_ftruncate(fd, length, 0);
+	/* avoid REGPARM breakage on x86: */
 	prevent_tail_call(ret);
 	return ret;
 }
@@ -1097,20 +1099,30 @@ long do_sys_open(int dfd, const char __user *filename, int flags, int mode)
 
 asmlinkage long sys_open(const char __user *filename, int flags, int mode)
 {
+	long ret;
+
 	if (force_o_largefile())
 		flags |= O_LARGEFILE;
 
-	return do_sys_open(AT_FDCWD, filename, flags, mode);
+	ret = do_sys_open(AT_FDCWD, filename, flags, mode);
+	/* avoid REGPARM breakage on x86: */
+	prevent_tail_call(ret);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(sys_open);
 
 asmlinkage long sys_openat(int dfd, const char __user *filename, int flags,
 			   int mode)
 {
+	long ret;
+
 	if (force_o_largefile())
 		flags |= O_LARGEFILE;
 
-	return do_sys_open(dfd, filename, flags, mode);
+	ret = do_sys_open(dfd, filename, flags, mode);
+	/* avoid REGPARM breakage on x86: */
+	prevent_tail_call(ret);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(sys_openat);