[lxc-devel] [lxc/master] log: use thread-safe localtime_r()

[brauner on Github]

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From b4c424743aec447437a169837cfe3180a39f2331 Mon Sep 17 00:00:00 2001
From: Christian Brauner <christian.brauner at ubuntu.com>
Date: Fri, 25 Nov 2016 02:42:51 +0100
Subject: [PATCH] log: use thread-safe localtime_r()

This fixes a race in liblxc logging which can lead to deadlocks. The reproducer
for this issue before this is to simply compile with --enable-tests and then
run:

    lxc-test-concurrent -j 20 -m create,start,stop,destroy -D

which should deadlock.

Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
---
 src/lxc/log.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/lxc/log.c b/src/lxc/log.c
index 6775822..a08cc26 100644
--- a/src/lxc/log.c
+++ b/src/lxc/log.c
@@ -20,6 +20,8 @@
  * License along with this library; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
+
+#define _GNU_SOURCE
 #include <assert.h>
 #include <stdio.h>
 #include <errno.h>
@@ -31,8 +33,6 @@
 #include <pthread.h>
 #include <time.h>
 
-#define __USE_GNU /* for *_CLOEXEC */
-
 #include <syslog.h>
 #include <stdio.h>
 
@@ -139,7 +139,7 @@ static int log_append_logfile(const struct lxc_log_appender *appender,
 {
 	char date[LXC_LOG_DATEFOMAT_SIZE] = "20150427012246";
 	char buffer[LXC_LOG_BUFFER_SIZE];
-	const struct tm *t;
+	struct tm newtime;
 	int n;
 	int ms;
 	int fd_to_use = -1;
@@ -155,8 +155,10 @@ static int log_append_logfile(const struct lxc_log_appender *appender,
 	if (fd_to_use == -1)
 		return 0;
 
-	t = localtime(&event->timestamp.tv_sec);
-	strftime(date, sizeof(date), "%Y%m%d%H%M%S", t);
+	if (!localtime_r(&event->timestamp.tv_sec, &newtime))
+		return 0;
+
+	strftime(date, sizeof(date), "%Y%m%d%H%M%S", &newtime);
 	ms = event->timestamp.tv_usec / 1000;
 	n = snprintf(buffer, sizeof(buffer),
 		     "%15s%s%s %10s.%03d %-8s %s - %s:%s:%d - ",