fix rare but nasty under-allocation bug in malloc with large requests

the bug appeared only with requests roughly 2*sizeof(size_t) to
4*sizeof(size_t) bytes smaller than a multiple of the page size, and
only for requests large enough to be serviced by mmap instead of the
normal heap. it was only ever observed on 64-bit machines but
presumably could also affect 32-bit (albeit with a smaller window of
opportunity).

diff --git a/src/malloc/malloc.c b/src/malloc/malloc.c
index ee6f170..46cc21f 100644 (file)
--- a/src/malloc/malloc.c
+++ b/src/malloc/malloc.c
@@ -333,7 +333,7 @@ void *malloc(size_t n)
        if (adjust_size(&n) < 0) return 0;
 
        if (n > MMAP_THRESHOLD) {
-               size_t len = n + PAGE_SIZE - 1 & -PAGE_SIZE;
+               size_t len = n + OVERHEAD + PAGE_SIZE - 1 & -PAGE_SIZE;
                char *base = __mmap(0, len, PROT_READ|PROT_WRITE,
                        MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
                if (base == (void *)-1) return 0;