[ovs-dev] [PATCHv2] fatal-signal: Remove snprintf.

William Tu u9012063 at gmail.com
Wed Mar 25 18:49:36 UTC 2020


Function snprintf is not async-signal-safe.  Replace it with
our own implementation.  Example ovs-vswitchd.log output:
  2020-03-25T01:08:19.673Z|00050|memory|INFO|handlers:2 ports:3
  SIGSEGV detected, backtrace:
  0x4872d9         <fatal_signal_handler+0x49>
  0x7f4e2ab974b0   <killpg+0x40>
  0x7f4e2ac5d74d   <__poll+0x2d>
  0x531098         <time_poll+0x108>
  0x51aefc         <poll_block+0x8c>
  0x445ca9         <udpif_revalidator+0x289>
  0x5056fd         <ovsthread_wrapper+0x7d>
  0x7f4e2b65f6ba   <start_thread+0xca>
  0x7f4e2ac6941d   <clone+0x6d>
  0x0              <+0x0>

Tested-at: https://travis-ci.org/github/williamtu/ovs-travis/builds/666875084 
Signed-off-by: William Tu <u9012063 at gmail.com>
---
v2:
  - avoid strcat overflow buffer, switch to use strncat
  - some code refactor

---
 lib/fatal-signal.c | 45 +++++++++++++++++++++++++++++++++++++--------
 1 file changed, 37 insertions(+), 8 deletions(-)

diff --git a/lib/fatal-signal.c b/lib/fatal-signal.c
index 51cf628d994e..e033f1ec59ec 100644
--- a/lib/fatal-signal.c
+++ b/lib/fatal-signal.c
@@ -158,6 +158,23 @@ fatal_signal_add_hook(void (*hook_cb)(void *aux), void (*cancel_cb)(void *aux),
 }
 
 #ifdef HAVE_UNWIND
+/* Convert unsigned long long to string.  This is needed because
+ * using snprintf() is not async signal safe. */
+static inline int
+llong_to_hex_str(unsigned long long value, char *str)
+{
+    int i = 0, res;
+
+    if (value / 16 > 0) {
+        i = llong_to_hex_str(value / 16, str);
+    }
+
+    res = value % 16;
+    str[i] = "0123456789abcdef"[res];
+
+    return i + 1;
+}
+
 /* Send the backtrace buffer to monitor thread.
  *
  * Note that this runs in the signal handling context, any system
@@ -192,20 +209,32 @@ send_backtrace_to_monitor(void) {
                      dep * sizeof(struct unw_backtrace)));
     } else {
         /* Since there is no monitor daemon running, write backtrace
-         * in current process.  This is not asyn-signal-safe due to
-         * use of snprintf().
+         * in current process.
          */
         char str[] = "SIGSEGV detected, backtrace:\n";
+        char ip_str[16], offset_str[6];
+        char _line[64];
+        char *line = (char *)_line;
 
         vlog_direct_write_to_log_file_unsafe(str);
 
         for (int i = 0; i < dep; i++) {
-            char line[64];
-
-            snprintf(line, 64, "0x%016"PRIxPTR" <%s+0x%"PRIxPTR">\n",
-                     unw_bt[i].ip,
-                     unw_bt[i].func,
-                     unw_bt[i].offset);
+            memset(line, 0, sizeof _line);
+            memset(ip_str, ' ', sizeof ip_str);
+            memset(offset_str, 0, sizeof offset_str);
+            ip_str[sizeof(ip_str) - 1] = 0;
+            offset_str[sizeof(offset_str) - 1] = 0;
+
+            llong_to_hex_str(unw_bt[i].ip, ip_str);
+            llong_to_hex_str(unw_bt[i].offset, offset_str);
+
+            strcat(line, "0x");
+            strcat(line, ip_str);
+            strcat(line, "<");
+            strncat(line, unw_bt[i].func, UNW_MAX_FUNCN);
+            strcat(line, "+0x");
+            strcat(line, offset_str);
+            strcat(line, ">\n");
             vlog_direct_write_to_log_file_unsafe(line);
         }
     }
-- 
2.7.4



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