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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <1454538786-12215-82-git-send-email-luis.henriques@canonical.com>
Date:	Wed,  3 Feb 2016 22:31:27 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Vegard Nossum <vegard.nossum@...cle.com>,
	Richard Weinberger <richard@....at>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.16.y-ckt 081/180] uml: flush stdout before forking

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

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

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: Luis Henriques <luis.henriques@...onical.com>
---
 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();