[libvirt] [PATCH] qemu: avoid dereference of null pointer

[Eric Blake]

On 08/04/2011 11:14 AM, Alex Jia wrote:
>> Also, how does checking for a non-zero union value prevent a null
>> dereference?
>>
> To be honest, I'm not sure this, however, it's okay for ccc-analyzer if
> I add these judgements,

Is ccc-analyzer different from clang?  If so, how can I set it up, to 
reproduce the problem you saw?

I previously saw a false positive in one of these functions 
(qemudDomainGetMemoryParameters) when using Coverity, but that was fixed 
by commit f768b4c3, but Coverity was silent for the other 4 functions 
you touched.  I'm now trying to do a clang run to see if that differs 
from Coverity.  The Coverity false positive was that our logic confused 
the static analyzers:

type var; // uninit
if (flags & _CONFIG)
   var = something
if (flags & _LIVE)
   do something else
if (flags & _CONFIG)
   use var

this pattern was enough to make the analyzers think that var could be 
used uninitialized, or initialized to NULL, in a setting where it must 
not be NULL; but once you see that it is merely a case of the analyzer 
getting it wrong (var is _only_ used under the same conditions where it 
was previously assigned earlier on), the solution is to add sa_assert() 
hints to the analyzers.

NACK to this patch; we need to get to the real root of why the analyzers 
are complaining, and fix the real bug if there is one (but I didn't see 
one in my manual inspection), or more likely add sa_assert() hints to 
silence the analyzer.

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org