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).

1 files changed, 1 insertions, 1 deletions

diff --git a/src/malloc/malloc.c b/src/malloc/malloc.c
index ee6f170b..46cc21fb 100644
--- 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;