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[PING][PATCH][BZ #15073] Fix race in free.
- From: OndÅej BÃlka <neleai at seznam dot cz>
- To: libc-alpha at sourceware dot org
- Date: Wed, 18 Dec 2013 11:27:57 +0100
- Subject: [PING][PATCH][BZ #15073] Fix race in free.
- Authentication-results: sourceware.org; auth=none
- References: <20131210194349 dot GA19644 at domone dot podge>
On Tue, Dec 10, 2013 at 08:43:49PM +0100, OndÅej BÃlka wrote:
> While actual implementation of atomic returning to fastbin is correct
> this could not be sayed about 'sanity' check surrounding it.
> When we read fastbin entry there is no quarantee that memory will be
> deallocated in meantime.
> Yet we try to dereference that pointer for a check that does not make
> any sense. We check if old chunk belongs to fastbin we are adding
> element. If old gets consolidated to bigger chunk we get a spurious
> memory corruption error, while if we continue a subsequent compare and
> swap will fail.
> Solution is simply drop that check.
> [BZ #15073]
> * malloc/malloc.c (_int_free): Remove sanity check that causes
> race condition.
> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index b1668b5..a05d4bb 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -3785,7 +3785,6 @@ _int_free(mstate av, mchunkptr p, int have_lock)
> mchunkptr fd;
> mchunkptr old = *fb;
> - unsigned int old_idx = ~0u;
> /* Another simple check: make sure the top of the bin is not the
> @@ -3795,13 +3794,10 @@ _int_free(mstate av, mchunkptr p, int have_lock)
> errstr = "double free or corruption (fasttop)";
> goto errout;
> - if (old != NULL)
> - old_idx = fastbin_index(chunksize(old));
> p->fd = fd = old;
> while ((old = catomic_compare_and_exchange_val_rel (fb, p, fd)) != fd);
> - if (fd != NULL && __builtin_expect (old_idx != idx, 0))
> + if (fd != NULL)
> errstr = "invalid fastbin entry (free)";
> goto errout;