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Re: [libvirt] [PATCH 11/11] hypervisor: Revisit Coverity issues regarding cpumap



On 02/11/2013 10:24 AM, Eric Blake wrote:
> On 01/30/2013 11:51 AM, John Ferlan wrote:
>> Turns out the issue regarding ptr_arith and sign_exension weren't false
>> positives. When shifting an 'unsigned char' as a target, it gets promoted
>> to an 'int'; however, that 'int' cannot be shifted 32 bits which was how
>> the algorithm was written. For the ptr_arith rather than index into the
>> cpumap, change the to address as necessary and assign directly.
>> ---
>>  src/xen/xen_hypervisor.c | 10 +++++-----
> 
>>          for (j = 0; j < maplen; j++) {
>> -            /* coverity[ptr_arith] */
>> -            /* coverity[sign_extension] */
>> -            *(pm + (j / 8)) |= cpumap[j] << (8 * (j & 7));
>> +            if ((j & 7) == 0)
>> +                pm = (uint64_t *)((uint64_t)&xen_cpumap + (j & ~0x7UL));
> 
> I'm not happy with how this turned out.  We are turning an address into
> a pointer but not via intptr_t (probably warnings on mingw); then doing
> math on that pointer, then turning the result back into a uint64_t
> pointer.  It looks like we are risking alignment errors.
> 
>> +            *pm |= (uint64_t)cpumap[j] << (8 * (j & 7));

More research - I think this is highly over-engineered, which explains
why we got it so wrong.  I checked a full range of xen-devel, from RHEL
5 (3.0.3, about as old as we support out-of-the-box with libvirt.git) to
Fedora 18 (4.2.1); ALL of those versions had:

/usr/include/xen/dom0_ops.h: typedef uint64_t cpumap_t;

which means this type has (more or less) _always_ been exactly 64 bits;
the code that tries to allow for iterating through 16 bytes instead of 8
is over-engineered.  Really, _ALL_ we are doing is reading a
little-endian 64-bit number in from the user (possibly in unaligned
memory), and turning it into a host-endian cpumap_t to hand to xen.

I should have a patch soon.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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