perf stat: Avoid sending SIGTERM to random processes

This patch fixes a problem with perf stat whereby on termination it may
send a SIGTERM signal to random processes on systems with high PID
recycling. I got some actual bug reports on this.

There is race between the SIGCHLD and sig_atexit() handlers.  This patch
addresses this problem by clearing child_pid in the SIGCHLD handler.

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20130604154426.GA2928@quad
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 22 insertions, 1 deletions

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index 7e910bab1097..95768afaae3e 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -87,7 +87,7 @@ static int			run_count			=  1;
 static bool                     no_inherit                      = false;
 static bool                     scale                           =  true;
 static enum aggr_mode           aggr_mode                       = AGGR_GLOBAL;
-static pid_t                    child_pid                       = -1;
+static volatile pid_t           child_pid                       = -1;
 static bool                     null_run                        =  false;
 static int                      detailed_run                    =  0;
 static bool                     big_num                         =  true;
@@ -1148,13 +1148,34 @@ static void skip_signal(int signo)
                 done = 1;
 
         signr = signo;
+        /*
+         * render child_pid harmless
+         * won't send SIGTERM to a random
+         * process in case of race condition
+         * and fast PID recycling
+         */
+        child_pid = -1;
 }
 
 static void sig_atexit(void)
 {
+        sigset_t set, oset;
+
+        /*
+         * avoid race condition with SIGCHLD handler
+         * in skip_signal() which is modifying child_pid
+         * goal is to avoid send SIGTERM to a random
+         * process
+         */
+        sigemptyset(&set);
+        sigaddset(&set, SIGCHLD);
+        sigprocmask(SIG_BLOCK, &set, &oset);
+
         if (child_pid != -1)
                 kill(child_pid, SIGTERM);
 
+        sigprocmask(SIG_SETMASK, &oset, NULL);
+
         if (signr == -1)
                 return;