Log a backtrace when the sandbox finds a failure

This involves some duplicate code between backtrace.c and sandbox.c,
but I don't see a way around it: calling more functions would mean
adding more steps to our call stack, and running clean_backtrace()
against the wrong point on the stack.

1 files changed, 16 insertions, 2 deletions

diff --git a/src/common/backtrace.c b/src/common/backtrace.c
index 239c65782b..3a073a8ff5 100644
--- a/src/common/backtrace.c
+++ b/src/common/backtrace.c
@@ -5,7 +5,6 @@
 #define _GNU_SOURCE 1
 
 #include "orconfig.h"
-#include "backtrace.h"
 #include "compat.h"
 #include "util.h"
 #include "torlog.h"
@@ -31,6 +30,9 @@
 #include <ucontext.h>
 #endif
 
+#define EXPOSE_CLEAN_BACKTRACE
+#include "backtrace.h"
+
 #if defined(HAVE_EXECINFO_H) && defined(HAVE_BACKTRACE) && \
   defined(HAVE_BACKTRACE_SYMBOLS_FD) && defined(HAVE_SIGACTION)
 #define USE_BACKTRACE
@@ -59,7 +61,7 @@ static tor_mutex_t cb_buf_mutex;
  * onto the stack.  Fortunately, we usually have the program counter in the
  * ucontext_t structure.
  */
-static void
+void
 clean_backtrace(void **stack, int depth, const ucontext_t *ctx)
 {
 #ifdef PC_FROM_UCONTEXT
@@ -165,6 +167,18 @@ install_bt_handler(void)
       rv = -1;
     }
   }
+
+  {
+    /* Now, generate (but do not log) a backtrace.  This ensures that
+     * libc has pre-loaded the symbols we need to dump things, so that later
+     * reads won't be denied by the sandbox code */
+    char **symbols;
+    int depth = backtrace(cb_buf, MAX_DEPTH);
+    symbols = backtrace_symbols(cb_buf, depth);
+    if (symbols)
+      free(symbols);
+  }
+
   return rv;
 }