Patch "uml: flush stdout before forking" has been added to the 4.4-stable tree

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

uml: flush stdout before forking

to the 4.4-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-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Vegard Nossum <[email protected]>
Date: Fri, 18 Dec 2015 21:28:53 +0100
Subject: uml: flush stdout before forking

From: Vegard Nossum <[email protected]>

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.

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

@@ -94,6 +94,8 @@ 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 [email protected] are

queue-4.4/uml-flush-stdout-before-forking.patch
queue-4.4/uml-fix-hostfs-mknod.patch