From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Thu, 14 Feb 2019 21:14:42 +0000 (-0500)
Subject: Fix: procname context async-signal safety
X-Git-Tag: v2.12.0-rc1~56
X-Git-Url: http://git.lttng.org/?p=lttng-ust.git;a=commitdiff_plain;h=74011e88983b12958dacd6621d60c18f6a84f59b

Fix: procname context async-signal safety

Ensure an instrumented signal handler interrupting the procname
context code will not write a partial process name string into the
trace context.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/liblttng-ust/lttng-context-procname.c b/liblttng-ust/lttng-context-procname.c
index b6e69501..95d5f28d 100644
--- a/liblttng-ust/lttng-context-procname.c
+++ b/liblttng-ust/lttng-context-procname.c
@@ -28,6 +28,9 @@
 #include <assert.h>
 #include "compat.h"
 
+/* Maximum number of nesting levels for the procname cache. */
+#define PROCNAME_NESTING_MAX	2
+
 /*
  * We cache the result to ensure we don't trigger a system call for
  * each event.
@@ -37,22 +40,34 @@
  * be set for a thread before the first event is logged within this
  * thread.
  */
-typedef char procname_array[17];
+typedef char procname_array[PROCNAME_NESTING_MAX][17];
+
 static DEFINE_URCU_TLS(procname_array, cached_procname);
 
+static DEFINE_URCU_TLS(int, procname_nesting);
+
 static inline
 char *wrapper_getprocname(void)
 {
-	if (caa_unlikely(!URCU_TLS(cached_procname)[0])) {
-		lttng_ust_getprocname(URCU_TLS(cached_procname));
-		URCU_TLS(cached_procname)[LTTNG_UST_PROCNAME_LEN - 1] = '\0';
+	int nesting = URCU_TLS(procname_nesting);
+
+	if (caa_unlikely(nesting >= PROCNAME_NESTING_MAX))
+		return "<unknown>";
+	if (caa_unlikely(!URCU_TLS(cached_procname)[nesting][0])) {
+		CMM_STORE_SHARED(URCU_TLS(procname_nesting), nesting + 1);
+		lttng_ust_getprocname(URCU_TLS(cached_procname)[nesting]);
+		URCU_TLS(cached_procname)[nesting][LTTNG_UST_PROCNAME_LEN - 1] = '\0';
+		CMM_STORE_SHARED(URCU_TLS(procname_nesting), nesting);
 	}
-	return URCU_TLS(cached_procname);
+	return URCU_TLS(cached_procname)[nesting];
 }
 
 void lttng_context_procname_reset(void)
 {
-	URCU_TLS(cached_procname)[0] = '\0';
+	URCU_TLS(cached_procname)[1][0] = '\0';
+	CMM_STORE_SHARED(URCU_TLS(procname_nesting), 1);
+	URCU_TLS(cached_procname)[0][0] = '\0';
+	CMM_STORE_SHARED(URCU_TLS(procname_nesting), 0);
 }
 
 static