1 files changed, 45 insertions, 2 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 4c5094b8f48..ead5c8a8ca4 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -482,10 +482,53 @@ buffer_memory_full (EMACS_INT nbytes)
   xsignal (Qnil, Vmemory_signal_data);
 }
 
-#ifdef XMALLOC_OVERRUN_CHECK
+
+#ifndef XMALLOC_OVERRUN_CHECK
+#define XMALLOC_OVERRUN_CHECK_OVERHEAD 0
+#else
 
 /* Check for overrun in malloc'ed buffers by wrapping a header and trailer
-   around each block.  */
+   around each block.
+
+   The header consists of XMALLOC_OVERRUN_CHECK_SIZE fixed bytes
+   followed by XMALLOC_OVERRUN_SIZE_SIZE bytes containing the original
+   block size in little-endian order.  The trailer consists of
+   XMALLOC_OVERRUN_CHECK_SIZE fixed bytes.
+
+   The header is used to detect whether this block has been allocated
+   through these functions, as some low-level libc functions may
+   bypass the malloc hooks.  */
+
+#define XMALLOC_OVERRUN_CHECK_SIZE 16
+#define XMALLOC_OVERRUN_CHECK_OVERHEAD \
+  (2 * XMALLOC_OVERRUN_CHECK_SIZE + XMALLOC_OVERRUN_SIZE_SIZE)
+
+/* Define XMALLOC_OVERRUN_SIZE_SIZE so that (1) it's large enough to
+   hold a size_t value and (2) the header size is a multiple of the
+   alignment that Emacs needs for C types and for USE_LSB_TAG.  */
+#define XMALLOC_BASE_ALIGNMENT                          \
+  offsetof (                                            \
+    struct {                                            \
+      union { long double d; intmax_t i; void *p; } u;  \
+      char c;                                           \
+    },                                                  \
+    c)
+#ifdef USE_LSB_TAG
+/* A common multiple of the positive integers A and B.  Ideally this
+   would be the least common multiple, but there's no way to do that
+   as a constant expression in C, so do the best that we can easily do.  */
+# define COMMON_MULTIPLE(a, b) \
+    ((a) % (b) == 0 ? (a) : (b) % (a) == 0 ? (b) : (a) * (b))
+# define XMALLOC_HEADER_ALIGNMENT \
+    COMMON_MULTIPLE (1 << GCTYPEBITS, XMALLOC_BASE_ALIGNMENT)
+#else
+# define XMALLOC_HEADER_ALIGNMENT XMALLOC_BASE_ALIGNMENT
+#endif
+#define XMALLOC_OVERRUN_SIZE_SIZE                               \
+   (((XMALLOC_OVERRUN_CHECK_SIZE + sizeof (size_t)              \
+      + XMALLOC_HEADER_ALIGNMENT - 1)                           \
+     / XMALLOC_HEADER_ALIGNMENT * XMALLOC_HEADER_ALIGNMENT)     \
+    - XMALLOC_OVERRUN_CHECK_SIZE)
 
 static char const xmalloc_overrun_check_header[XMALLOC_OVERRUN_CHECK_SIZE] =
   { '\x9a', '\x9b', '\xae', '\xaf',