From 9cbb43b99bf138e44deef9957678bc464f3bfd82 Mon Sep 17 00:00:00 2001 From: Vegard Nossum Date: Fri, 18 Dec 2015 21:28:53 +0100 Subject: [PATCH] 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 Signed-off-by: Richard Weinberger Signed-off-by: Greg Kroah-Hartman --- 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();