Re: [libvirt] [PATCH] libxl: Check for regcomp failure
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 :)
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.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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