[PATCH 3.16.y-ckt 081/180] uml: flush stdout before forking

From: Luis Henriques
Date: Wed Feb 03 2016 - 18:32:49 EST


3.16.7-ckt24 -stable review patch. If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Vegard Nossum <vegard.nossum@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Richard Weinberger <richard@xxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
arch/um/os-Linux/start_up.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/arch/um/os-Linux/start_up.c b/arch/um/os-Linux/start_up.c
index 337518c5042a..b412c62486f0 100644
--- 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();