[3.16.y-ckt stable] Patch "kernel/signal.c: unexport sigsuspend()" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    kernel/signal.c: unexport sigsuspend()

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt22.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From df313e158181ee0425d0c40c0ca6fb5fa887fbbe Mon Sep 17 00:00:00 2001
From: Richard Weinberger <richard at nod.at>
Date: Fri, 20 Nov 2015 15:57:21 -0800
Subject: kernel/signal.c: unexport sigsuspend()

commit 9d8a765211335cfdad464b90fb19f546af5706ae upstream.

sigsuspend() is nowhere used except in signal.c itself, so we can mark it
static do not pollute the global namespace.

But this patch is more than a boring cleanup patch, it fixes a real issue
on UserModeLinux.  UML has a special console driver to display ttys using
xterm, or other terminal emulators, on the host side.  Vegard reported
that sometimes UML is unable to spawn a xterm and he's facing the
following warning:

  WARNING: CPU: 0 PID: 908 at include/linux/thread_info.h:128 sigsuspend+0xab/0xc0()

It turned out that this warning makes absolutely no sense as the UML
xterm code calls sigsuspend() on the host side, at least it tries.  But
as the kernel itself offers a sigsuspend() symbol the linker choose this
one instead of the glibc wrapper.  Interestingly this code used to work
since ever but always blocked signals on the wrong side.  Some recent
kernel change made the WARN_ON() trigger and uncovered the bug.

It is a wonderful example of how much works by chance on computers. :-)

Fixes: 68f3f16d9ad0f1 ("new helper: sigsuspend()")
Signed-off-by: Richard Weinberger <richard at nod.at>
Reported-by: Vegard Nossum <vegard.nossum at oracle.com>
Tested-by: Vegard Nossum <vegard.nossum at oracle.com>
Acked-by: Oleg Nesterov <oleg at redhat.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 include/linux/signal.h | 1 -
 kernel/signal.c        | 2 +-
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/include/linux/signal.h b/include/linux/signal.h
index c9e65360c49a..aa2625788594 100644
--- a/include/linux/signal.h
+++ b/include/linux/signal.h
@@ -242,7 +242,6 @@ extern int sigprocmask(int, sigset_t *, sigset_t *);
 extern void set_current_blocked(sigset_t *);
 extern void __set_current_blocked(const sigset_t *);
 extern int show_unhandled_signals;
-extern int sigsuspend(sigset_t *);

 struct sigaction {
 #ifndef __ARCH_HAS_IRIX_SIGACTION
diff --git a/kernel/signal.c b/kernel/signal.c
index 720fad0817e4..6eb8466819b7 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3547,7 +3547,7 @@ SYSCALL_DEFINE0(pause)

 #endif

-int sigsuspend(sigset_t *set)
+static int sigsuspend(sigset_t *set)
 {
 	current->saved_sigmask = current->blocked;
 	set_current_blocked(set);