Improve strncpy_s src/dst overlap check

Let m = user estimate of the (max) src string length, low = smaller
address of (src, dst), hi = larger address (src, dst).

if (low + (m - 1) >= hi), we have a *potential* overlapping copy which
is not allowed. Before we declare overlap - and return an error -
retry the check with m = actual src string length.

The resulting "test string" failure affected aarch64 (only) because of
differences in test code stack variable placement / alignment.

Change-Id: I2931d1ce2c61af3d3880075b033d2a4c4e421f09
Signed-off-by: Dave Barach <dave@barachs.net>

1 files changed, 12 insertions, 2 deletions

diff --git a/src/vppinfra/string.h b/src/vppinfra/string.h
index a25d461868b..d5686704c22 100644
--- a/src/vppinfra/string.h
+++ b/src/vppinfra/string.h
@@ -1031,10 +1031,20 @@ strncpy_s_inline (char *__restrict__ dest, rsize_t dmax,
   low = (uword) (src < dest ? src : dest);
   hi = (uword) (src < dest ? dest : src);
 
+  /*
+   * This check may fail innocently if src + dmax >= dst, but
+   * src + strlen(src) < dst. If it fails, check more carefully before
+   * blowing the whistle.
+   */
   if (PREDICT_FALSE (low + (m - 1) >= hi))
     {
-      clib_c11_violation ("src/dest overlap");
-      return EINVAL;
+      m = clib_strnlen (src, m);
+
+      if (low + (m - 1) >= hi)
+	{
+	  clib_c11_violation ("src/dest overlap");
+	  return EINVAL;
+	}
     }
 
   clib_memcpy_fast (dest, src, m);