From: Junio C Hamano <gitster@pobox.com>
To: David Turner <dturner@twopensource.com>
Cc: git@vger.kernel.org, Keith McGuigan <kmcguigan@twitter.com>
Subject: Re: [PATCH v2] merge: fix cache_entry use-after-free
Date: Wed, 14 Oct 2015 14:17:34 -0700 [thread overview]
Message-ID: <xmqqk2qp6tg1.fsf@gitster.mtv.corp.google.com> (raw)
In-Reply-To: <1444687413-928-1-git-send-email-dturner@twitter.com> (David Turner's message of "Mon, 12 Oct 2015 18:03:33 -0400")
David Turner <dturner@twopensource.com> writes:
> + unsigned int ref_count; /* count the number of refs to this in dir_hash */
Me makes a mental note of the type used...
> @@ -213,6 +214,32 @@ struct cache_entry {
> struct pathspec;
>
> /*
> + * Increment the cache_entry reference count. Should be called
> + * whenever a pointer to a cache_entry is retained in a data structure,
> + * thus marking it as alive.
> + */
> +static inline void add_ce_ref(struct cache_entry *ce)
> +{
> + assert(ce != NULL && ce->ref_count >= 0);
... and notices that ce->ref_count will always be non-negative here
> + ce->ref_count++;
> +}
> +
> +/*
> + * Decrement the cache_entry reference count. Should be called whenever
> + * a pointer to a cache_entry is dropped. Once the counter drops to 0
> + * the cache_entry memory will be safely freed.
> + */
> +static inline void drop_ce_ref(struct cache_entry *ce)
> +{
> + if (ce != NULL) {
> + assert(ce->ref_count >= 0);
... and here.
By not checking integer overflow/wraparound, the code is assuming
that a ce entry will never referenced more than 4 billion times on
32-bit platform. And that is a sensible assumption as there aren't
that many pointers in the address space to make that many reference
anyway.
Perhaps the code can assume the number won't be more than 2 billion
and use a signed type instead for the reference counting?
next prev parent reply other threads:[~2015-10-14 21:17 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-12 22:03 [PATCH v2] merge: fix cache_entry use-after-free David Turner
2015-10-12 22:28 ` Junio C Hamano
2015-10-13 19:22 ` David Turner
2015-10-13 21:05 ` Junio C Hamano
2015-10-14 21:17 ` Junio C Hamano [this message]
2015-10-14 22:00 ` David Turner
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