bpo-31626: Mark ends of the reallocated block in debug build. (#4210)

Few bytes at the begin and at the end of the reallocated blocks, as well
as the header and the trailer, now are erased before calling realloc()
in debug build.  This will help to detect using or double freeing the
reallocated block.

Objects/obmalloc.c

@@ -1418,7 +1418,7 @@ static void *
 _PyMem_DebugRawAlloc(int use_calloc, void *ctx, size_t nbytes)
 {
     debug_alloc_api_t *api = (debug_alloc_api_t *)ctx;
-    uint8_t *p;           /* base address of malloc'epad d block */
+    uint8_t *p;           /* base address of malloc'ed pad block */
     uint8_t *data;        /* p + 2*SST == pointer to data bytes */
     uint8_t *tail;        /* data + nbytes == pointer to tail pad bytes */
     size_t total;         /* 2 * SST + nbytes + 2 * SST */
@@ -1516,45 +1516,83 @@ _PyMem_DebugRawRealloc(void *ctx, void *p, size_t nbytes)
     }
 
     debug_alloc_api_t *api = (debug_alloc_api_t *)ctx;
-    uint8_t *q;           /* base address of malloc'epad d block */
-    uint8_t *data;        /* p + 2*SST == pointer to data bytes */
+    uint8_t *head;        /* base address of malloc'ed pad block */
+    uint8_t *data;        /* pointer to data bytes */
+    uint8_t *r;
     uint8_t *tail;        /* data + nbytes == pointer to tail pad bytes */
     size_t total;         /* 2 * SST + nbytes + 2 * SST */
     size_t original_nbytes;
-    int i;
+    size_t serialno;
+#define ERASED_SIZE 64
+    uint8_t save[2*ERASED_SIZE];  /* A copy of erased bytes. */
 
     _PyMem_DebugCheckAddress(api->api_id, p);
 
-    q = (uint8_t *)p;
-    original_nbytes = read_size_t(q - 2*SST);
+    data = (uint8_t *)p;
+    head = data - 2*SST;
+    original_nbytes = read_size_t(head);
     if (nbytes > (size_t)PY_SSIZE_T_MAX - 4*SST) {
         /* integer overflow: can't represent total as a Py_ssize_t */
         return NULL;
     }
     total = nbytes + 4*SST;
 
-    /* Resize and add decorations. */
-    q = (uint8_t *)api->alloc.realloc(api->alloc.ctx, q - 2*SST, total);
-    if (q == NULL) {
-        return NULL;
+    tail = data + original_nbytes;
+    serialno = read_size_t(tail + SST);
+    /* Mark the header, the trailer, ERASED_SIZE bytes at the begin and
+       ERASED_SIZE bytes at the end as dead and save the copy of erased bytes.
+     */
+    if (original_nbytes <= sizeof(save)) {
+        memcpy(save, data, original_nbytes);
+        memset(data - 2*SST, DEADBYTE, original_nbytes + 4*SST);
+    }
+    else {
+        memcpy(save, data, ERASED_SIZE);
+        memset(head, DEADBYTE, ERASED_SIZE + 2*SST);
+        memcpy(&save[ERASED_SIZE], tail - ERASED_SIZE, ERASED_SIZE);
+        memset(tail - ERASED_SIZE, DEADBYTE, ERASED_SIZE + 2*SST);
     }
 
-    bumpserialno();
-    write_size_t(q, nbytes);
-    assert(q[SST] == (uint8_t)api->api_id);
-    for (i = 1; i < SST; ++i) {
-        assert(q[SST + i] == FORBIDDENBYTE);
+    /* Resize and add decorations. */
+    r = (uint8_t *)api->alloc.realloc(api->alloc.ctx, head, total);
+    if (r == NULL) {
+        nbytes = original_nbytes;
+    }
+    else {
+        head = r;
+        bumpserialno();
+        serialno = _PyRuntime.mem.serialno;
     }
-    data = q + 2*SST;
+
+    write_size_t(head, nbytes);
+    head[SST] = (uint8_t)api->api_id;
+    memset(head + SST + 1, FORBIDDENBYTE, SST-1);
+    data = head + 2*SST;
 
     tail = data + nbytes;
     memset(tail, FORBIDDENBYTE, SST);
-    write_size_t(tail + SST, _PyRuntime.mem.serialno);
+    write_size_t(tail + SST, serialno);
+
+    /* Restore saved bytes. */
+    if (original_nbytes <= sizeof(save)) {
+        memcpy(data, save, Py_MIN(nbytes, original_nbytes));
+    }
+    else {
+        size_t i = original_nbytes - ERASED_SIZE;
+        memcpy(data, save, Py_MIN(nbytes, ERASED_SIZE));
+        if (nbytes > i) {
+            memcpy(data + i, &save[ERASED_SIZE],
+                   Py_MIN(nbytes - i, ERASED_SIZE));
+        }
+    }
+
+    if (r == NULL) {
+        return NULL;
+    }
 
     if (nbytes > original_nbytes) {
         /* growing:  mark new extra memory clean */
-        memset(data + original_nbytes, CLEANBYTE,
-               nbytes - original_nbytes);
+        memset(data + original_nbytes, CLEANBYTE, nbytes - original_nbytes);
     }
 
     return data;