libbacktrace patch committed: Call munmap after memory test

[Ian Lance Taylor]

The bug report https://github.com/ianlancetaylor/libbacktrace/issues/13
points out that when backtrace_full checks whether memory is
available, it doesn't necessarily release that memory.  It will stay
on the free list, so libbacktrace will use more and more memory over
time.  This patch fixes that problem by explicitly calling munmap.
Bootstrapped and ran libbacktrace and Go tests on x86_64-pc-linux-gnu.
Committed to mainline.

Ian


2018-04-17  Ian Lance Taylor  <i...@golang.org>

* backtrace.c (backtrace_full): When testing whether we can
allocate memory, call mmap directly, and munmap the memory.

Index: backtrace.c
===================================================================
--- backtrace.c (revision 259359)
+++ backtrace.c (working copy)
@@ -32,12 +32,26 @@ POSSIBILITY OF SUCH DAMAGE.  */
 
 #include "config.h"
 
+#include <unistd.h>
 #include <sys/types.h>
 
+#if !BACKTRACE_USES_MALLOC
+#include <sys/mman.h>
+#endif
+
 #include "unwind.h"
 #include "backtrace.h"
+#include "backtrace-supported.h"
 #include "internal.h"
 
+#ifndef MAP_ANONYMOUS
+#define MAP_ANONYMOUS MAP_ANON
+#endif
+
+#ifndef MAP_FAILED
+#define MAP_FAILED ((void *)-1)
+#endif
+
 /* The main backtrace_full routine.  */
 
 /* Data passed through _Unwind_Backtrace.  */
@@ -104,7 +118,6 @@ backtrace_full (struct backtrace_state *
                backtrace_error_callback error_callback, void *data)
 {
   struct backtrace_data bdata;
-  void *p;
 
   bdata.skip = skip + 1;
   bdata.state = state;
@@ -113,16 +126,25 @@ backtrace_full (struct backtrace_state *
   bdata.data = data;
   bdata.ret = 0;
 
-  /* If we can't allocate any memory at all, don't try to produce
-     file/line information.  */
-  p = backtrace_alloc (state, 4096, NULL, NULL);
-  if (p == NULL)
-    bdata.can_alloc = 0;
-  else
-    {
-      backtrace_free (state, p, 4096, NULL, NULL);
-      bdata.can_alloc = 1;
-    }
+#if !BACKTRACE_USES_MALLOC
+  {
+    size_t pagesize;
+    void *page;
+
+    /* If we can't allocate any memory at all, don't try to produce
+       file/line information.  */
+    pagesize = getpagesize ();
+    page = mmap (NULL, pagesize, PROT_READ | PROT_WRITE, 
+                MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+    if (page == MAP_FAILED)
+      bdata.can_alloc = 0;
+    else
+      {
+       munmap (page, pagesize);
+       bdata.can_alloc = 1;
+      }
+  }
+#endif
 
   _Unwind_Backtrace (unwind, &bdata);
   return bdata.ret;