Make _gcry_burn_stack use variable length array

* configure.ac (HAVE_VLA): Add check.
* src/misc.c (_gcry_burn_stack) [HAVE_VLA]: Add VLA code.
--

Some gcc versions convert _gcry_burn_stack into loop that overwrites the same
64-byte stack buffer instead of burn stack deeper. It's argued at GCC bugzilla
that _gcry_burn_stack is doing wrong thing here [1] and that this kind of
optimization is allowed.

So lets fix _gcry_burn_stack by using variable length array when VLAs are
supported by compiler. This should ensure proper stack burning to the requested
depth and avoid GCC loop optimizations.

[1] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52285

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>

1 files changed, 8 insertions, 1 deletions

diff --git a/src/misc.c b/src/misc.c
index 2d9c73aa..135aeb4c 100644
--- a/src/misc.c
+++ b/src/misc.c
@@ -290,13 +290,20 @@ _gcry_log_printhex (const char *text, const void *buffer, size_t length)
 void
 _gcry_burn_stack (int bytes)
 {
-    char buf[64];
+#ifdef HAVE_VLA
+    int buflen = (((bytes <= 0) ? 1 : bytes) + 63) & ~63;
+    volatile char buf[buflen];
+
+    wipememory (buf, sizeof buf);
+#else
+    volatile char buf[64];
 
     wipememory (buf, sizeof buf);
 
     bytes -= sizeof buf;
     if (bytes > 0)
         _gcry_burn_stack (bytes);
+#endif
 }
 
 void