[libvirt] [PATCH] libxl: Check for regcomp failure
Jim Fehlig
jfehlig at suse.com
Wed Sep 4 22:57:51 UTC 2013
Eric Blake wrote:
> On 09/04/2013 02:03 PM, Jim Fehlig wrote:
>
>>> Yeah, good question. I found a few occurrences of regcomp() and friends
>>> throughout the sources and most seem to do regfree() even when regcomp()
>>> fails. The man page is not very clear, but the notes on regfree()
>>> suggest it is not necessary
>>>
>>> POSIX Pattern Buffer Freeing
>>> Supplying regfree() with a precompiled pattern buffer, preg will
>>> free the memory allocated to the pattern buffer by the compiling
>>> process, regcomp().
>>>
>>> But does the pattern buffer contain any allocated memory when regcomp()
>>> fails? The notes on regcomp() are not clear about this.
>>>
>
> Thankfully, we can read the source :)
>
Nod :). Was about to do that before seeing your message...
> In glibc, regcomp assigns into preg, but is careful to undo any
> allocation on failure; it is also careful to make regfree() a no-op on
> an already-freed buffer (whether by calling regfree() twice in a row, or
> using it on preg after a failed regcomp). Gnulib copies this behavior.
> But it is not universally standard:
>
>
>>>
>>>
>> The System Interfaces volume of POSIX.1-2008 [1] says this about
>> regcomp() return value
>>
>> Upon successful completion, the regcomp() function shall return 0.
>> Otherwise, it shall return an integer value indicating an error as
>> described in /<regex.h>/
>> <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/regex.h.html>,
>> and the content of preg is undefined. If a code is returned, the
>> interpretation shall be as given in /<regex.h>/
>> <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/regex.h.html>.
>>
>> I don't think we want to call regfree() on an undefined preg right?
>>
>
> Correct - regfree() is only needed on successful regcomp(). We can
> probably get away with calling regfree() even on failure because we use
> gnulib, but that's not a good reason, so it wouldn't hurt to audit the
> code and guarantee a free only on success.
>
I've pushed this patch, given Michal's ACK before raising the regfree()
question.
Also sent a small series to remove other unnecessary uses of regfree()
https://www.redhat.com/archives/libvir-list/2013-September/msg00272.html
Regards,
Jim
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