| From | Jiri Slaby <> | Subject | [PATCH 3.12 080/116] uml: flush stdout before forking | Date | Fri, 4 Mar 2016 10:02:05 +0100 |
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From: Vegard Nossum <vegard.nossum@oracle.com>
3.12-stable review patch. If anyone has any objections, please let me know.
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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: Jiri Slaby <jslaby@suse.cz> --- 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(); -- 2.7.2
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