From: Jeff King <peff@peff.net>
To: Derrick Stolee <stolee@gmail.com>
Cc: git@vger.kernel.org
Subject: Re: [PATCH] xrealloc: do not reuse pointer freed by zero-length realloc()
Date: Tue, 1 Sep 2020 09:51:05 -0400 [thread overview]
Message-ID: <20200901135105.GA3284077@coredump.intra.peff.net> (raw)
In-Reply-To: <c81b7225-a663-1598-62b3-bd80457d5648@gmail.com>
On Tue, Sep 01, 2020 at 09:04:36AM -0400, Derrick Stolee wrote:
> > The simplest fix here is to just pass "ret" (which we know to be NULL)
> > to the follow-up realloc(). That does mean that a system which _doesn't_
> > free the original pointer would leak it. But that interpretation of the
> > standard seems unlikely (if a system didn't deallocate in this case, I'd
> > expect it to simply return the original pointer). If it turns out to be
> > an issue, we can handle the "!size" case up front instead, before we
> > call realloc() at all.
>
> Adding an `if (!size) {free(ptr); return NULL;}` block was what I
> expected. Was that chosen just so we can rely more on the system
> realloc(), or is there a performance implication that I'm not
> seeing?
I went back and forth on whether to do that or not. This case should
basically never happen, so I like both the performance and readability
of only triggering it when realloc() returns NULL. But it would get rid
of the hand-waving above, and I doubt the performance is measurable.
If we do handle it up-front, then I think we'd actually want:
if (!size) {
free(ptr);
return xmalloc(0);
}
(i.e., to never return NULL for consistency with xmalloc() and
xcalloc()).
> > @@ -120,7 +120,7 @@ void *xrealloc(void *ptr, size_t size)
> > memory_limit_check(size, 0);
> > ret = realloc(ptr, size);
> > if (!ret && !size)
> > - ret = realloc(ptr, 1);
> > + ret = realloc(ret, 1);
>
> I appreciate all the additional context for such a small change.
Somebody's got to complete with you for ratio of commit message to diff
lines. :)
-Peff
next prev parent reply other threads:[~2020-09-01 13:53 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-01 11:18 [PATCH] xrealloc: do not reuse pointer freed by zero-length realloc() Jeff King
2020-09-01 13:04 ` Derrick Stolee
2020-09-01 13:51 ` Jeff King [this message]
2020-09-01 14:24 ` Derrick Stolee
2020-09-01 15:58 ` Junio C Hamano
2020-09-02 7:54 ` Jeff King
2020-09-02 19:19 ` Junio C Hamano
2020-09-03 3:50 ` Jonathan Nieder
2020-09-01 15:20 ` Andreas Schwab
2020-09-01 15:56 ` Junio C Hamano
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