linux-kernel - [PATCH 3.2 38/87] uml: flush stdout before forking

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Message-ID: <lsq.1454975631.240976283@decadent.org.uk>
Date:	Mon, 08 Feb 2016 23:53:51 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Richard Weinberger" <richard@....at>,
	"Vegard Nossum" <vegard.nossum@...cle.com>
Subject: [PATCH 3.2 38/87] uml: flush stdout before forking

3.2.77-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Vegard Nossum <vegard.nossum@...cle.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@...cle.com>
Signed-off-by: Richard Weinberger <richard@....at>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 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();