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Message-ID: <20250621192751.GL1827@brightrain.aerifal.cx>
Date: Sat, 21 Jun 2025 15:27:51 -0400
From: Rich Felker <dalias@...c.org>
To: Alejandro Colomar <alx@...nel.org>
Cc: libbsd@...ts.freedesktop.org, Guillem Jover <guillem@...rons.org>,
musl@...ts.openwall.com, freebsd-bugs@...ebsd.org
Subject: Re: Bug in reallocf(3) when calling realloc(3)
On Sat, Jun 21, 2025 at 09:10:58PM +0200, Alejandro Colomar wrote:
> Bon dia Guillem,
>
> I've just checked the implementation of reallocf(3bsd), and it looks
> like it has a memory leak. Admittedly, a small one, but I just wanted
> to report it. I'll suggest two proposals, plus keeping the status quo,
> and explain the advantages of each of them.
>
> Here's the current code in libbsd:
>
> $ grepc -tfd reallocf .
> ./src/reallocf.c:void *
> reallocf(void *ptr, size_t size)
> {
> void *nptr;
>
> nptr = realloc(ptr, size);
>
> /*
> * When the System V compatibility option (malloc "V" flag) is
> * in effect, realloc(ptr, 0) frees the memory and returns NULL.
> * So, to avoid double free, call free() only when size != 0.
> * realloc(ptr, 0) can't fail when ptr != NULL.
> */
> if (!nptr && ptr && size != 0)
> free(ptr);
> return (nptr);
> }
>
> The last sentence in that comment is false. realloc(nonnull,0) can fail
> in systems in which realloc(nonnull,0) normally returns non-null, such
> as the BSDs or musl. Let's say the system is unable to find enough
> space for the metadata necessary for the new 0-sized block of memory.
> It would return NULL and keep the old pointer intact.
>
> I've CCed musl@ so that they can verify that this is true on their
> implementation. I didn't see anything in their code suggesting that it
> can't fail.
Yes this is true. realloc returning null *always* indicates failure
(and accordingly, memory has not been freed) in our past and current
implementations, and indeed it can fail when resizing down (to zero or
otherwise).
Rich
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