[PATCH] dup3 fix

Al Viro notice one cornercase that the new dup3() code.  The dup2()
function, as a special case, handles dup-ing to the same file
descriptor.  In this case the current dup3() code does nothing at
all.  I.e., it ingnores the flags parameter.  This shouldn't happen,
the close-on-exec flag should be set if requested.

In case the O_CLOEXEC bit in the flags parameter is not set the
dup3() function should behave in this respect identical to dup2().
This means dup3(fd, fd, 0) should not actively reset the c-o-e
flag.

The patch below implements this minor change.

[AV: credits to Artur Grabowski for bringing that up as potential subtle point
in dup2() behaviour]
Signed-off-by: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/fcntl.c

@@ -139,8 +139,13 @@ asmlinkage long sys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
 	if (!(file = fcheck(oldfd)))
 		goto out_unlock;
 	err = newfd;
-	if (newfd == oldfd)
+	if (unlikely(newfd == oldfd)) {
+		if (flags & O_CLOEXEC) {
+			fdt = files_fdtable(files);
+			FD_SET(newfd, fdt->close_on_exec);
+		}
 		goto out_unlock;
+	}
 	err = -EBADF;
 	if (newfd >= current->signal->rlim[RLIMIT_NOFILE].rlim_cur)
 		goto out_unlock;