Re: [Xen-devel] New Defects reported by Coverity Scan for XenProject

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 25/02/16 10:00, Ian Campbell wrote:
> 
> On Wed, 2016-02-24 at 21:02 -0800, scan-admin@xxxxxxxxxxxx wrote:
>> Hi,
>>
>> Please find the latest report on new defect(s) introduced to XenProject 
>> found with Coverity Scan.
>>
>> 2 new defect(s) introduced to XenProject found with Coverity Scan.
>> 12 defect(s), reported by Coverity Scan earlier, were marked fixed in the 
>> recent build analyzed by Coverity Scan.
>>
>> New defect(s) Reported-by: Coverity Scan
>> Showing 2 of 2 defect(s)
>>
>>
>> ** CID 1354244:  Null pointer dereferences  (FORWARD_NULL)
>> /tools/libxc/xc_tbuf.c: 72 in xc_tbuf_get_size()
>>
>>
>> ________________________________________________________________________________________________________
>> *** CID 1354244:  Null pointer dereferences  (FORWARD_NULL)
>> /tools/libxc/xc_tbuf.c: 72 in xc_tbuf_get_size()
>> 66             return rc;
>> 67     
>> 68         t_info = xc_map_foreign_range(xch, DOMID_XEN,
>> 69                         sysctl.u.tbuf_op.size, PROT_READ | PROT_WRITE,
>> 70                         sysctl.u.tbuf_op.buffer_mfn);
>> 71     
>>>>>     CID 1354244:  Null pointer dereferences  (FORWARD_NULL)
>>>>>     Comparing "t_info" to null implies that "t_info" might be null.
>> 72         if ( t_info == NULL || t_info->tbuf_size == 0 )
>> 73             rc = -1;
>> 74         else
>> 75           *size = t_info->tbuf_size;
>> 76     
>> 77         xenforeignmemory_unmap(xch->fmem, t_info, sysctl.u.tbuf_op.size);
> 
> This is complaining about the eventual munmap(t_info) => munmap(NULL) which
> is behind xenforeignmemory_unmap().
> 
> Looks like it was newly added by the fix to CID 1351228 in 7c479883b04a
> ("libxc: fix leak of t_info in xc_tbuf_get_size()").
> xenforeignmemory_unmap() should behave like munmap WRT tollerance of NULL,
> I'm not 100% sure what that behaviour is since 0 is a valid address.
> xenforeignmemory.h no doubt wants updating with the desired semantics and
> either this code of the implementation adjusting to match.
> 
> While here I notice that using xc_map_*() to create the mapping and
> xenforeignmemory_unmap() to destroy it is a bit odd since they are strictly
> two separate APIs, even if one happens to be implemented in terms of the
> other. Being libxc internal this code is at liberty to use xc_map_* but
> should then use plain munmap to undo it, or it would also be reasonable to
> port this code fully to the xenforeignmemory interface.
> 
>>
>> ** CID 1354243:  Control flow issues  (DEADCODE)
>> /tools/xentrace/xenalyze.c: 4148 in cr3_dump_list()
>>
>>
>> ________________________________________________________________________________________________________
>> *** CID 1354243:  Control flow issues  (DEADCODE)
>> /tools/xentrace/xenalyze.c: 4148 in cr3_dump_list()
>> 4142     
>> 4143         /* Count the number of elements */
>> 4144         for(p=head; p; p=p->next)
>> 4145             N++;
>> 4146     
>> 4147         if(!N)
>>>>>     CID 1354243:  Control flow issues  (DEADCODE)
>>>>>     Execution cannot reach this statement: "return;".
> 
> Here it has observed that due to the (above, just out of the context given
> here) "if (!head) return" that the for loop must run at least once, so N
> cannot be 0.
> 
> My guess is that this is a prexisting issue which was exposed to coverities
> beady eye somehow by 28ab9f3d0e7c ("tools/xenalyze: Fix build with clang").
> Or maybe this was previous marked deliberate but the change has caused
> coverity to think this is a different instance of the same thing, eitherway
> I don't think the issue itself is new.
> 
> FWIW having both if (!head) return and if (!N) return looks redundant to
> me, the other two similar looking instances (from grepping for N++) have
> only the latter check.

Yes, they're certainly redundant, and I definitely prefer the latter
check rather than the former.  I'll send a patch.

 -George


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