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uml: flush stdout before forking 

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>
This commit is contained in:
Vegard Nossum 2015-12-18 21:28:53 +01:00 committed by Greg Kroah-Hartman
parent 509a4bccfe
commit 9cbb43b99b
1 changed files with 2 additions and 0 deletions
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2
arch/um/os-Linux/start_up.c
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@ -95,6 +95,8 @@ static int start_ptraced_child(void)
{
int pid, n, status;
fflush(stdout);
pid = fork();
if (pid == 0)
ptrace_child();