Patch "uml: flush stdout before forking" has been added to the 3.14-stable tree

Patch "uml: flush stdout before forking" has been added to the 3.14-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    uml: flush stdout before forking

to the 3.14-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     uml-flush-stdout-before-forking.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 0754fb298f2f2719f0393491d010d46cfb25d043 Mon Sep 17 00:00:00 2001
From: Vegard Nossum <vegard.nossum@oracle.com>
Date: Fri, 18 Dec 2015 21:28:53 +0100
Subject: uml: flush stdout before forking

From: Vegard Nossum <vegard.nossum@oracle.com>

commit 0754fb298f2f2719f0393491d010d46cfb25d043 upstream.

I was seeing some really weird behaviour where piping UML's output
somewhere would cause output to get duplicated:

  $ ./vmlinux | head -n 40
  Checking that ptrace can change system call numbers...Core dump limits :
          soft - 0
          hard - NONE
  OK
  Checking syscall emulation patch for ptrace...Core dump limits :
          soft - 0
          hard - NONE
  OK
  Checking advanced syscall emulation patch for ptrace...Core dump limits :
          soft - 0
          hard - NONE
  OK
  Core dump limits :
          soft - 0
          hard - NONE

This is because these tests do a fork() which duplicates the non-empty
stdout buffer, then glibc flushes the duplicated buffer as each child
exits.

A simple workaround is to flush before forking.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/um/os-Linux/start_up.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/arch/um/os-Linux/start_up.c
+++ b/arch/um/os-Linux/start_up.c
@@ -95,6 +95,8 @@ static int start_ptraced_child(void)
 {
 	int pid, n, status;
 
+	fflush(stdout);
+
 	pid = fork();
 	if (pid == 0)
 		ptrace_child();


Patches currently in stable-queue which might be from vegard.nossum@oracle.com are

queue-3.14/uml-flush-stdout-before-forking.patch
queue-3.14/rds-fix-race-condition-when-sending-a-message-on-unbound-socket.patch
queue-3.14/uml-fix-hostfs-mknod.patch