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Re: [Coverity complaint] Re: [PATCH v5] pdx: introduce a new compression algorithm based on region offsets
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 14 Aug 2025 09:12:56 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Community Manager <community.manager@xxxxxxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Delivery-date: Thu, 14 Aug 2025 07:13:02 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 13.08.2025 14:41, Roger Pau Monné wrote:
> On Wed, Aug 13, 2025 at 12:47:37PM +0100, Andrew Cooper wrote:
>> On 12/08/2025 4:06 pm, Roger Pau Monne wrote:
>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>
>> Not this patch, but this is probably the best place to report it.
>>
>> https://scan5.scan.coverity.com/#/project-view/30554/10426?selectedIssue=1662707
>>
>> Something you did in the series made enough changes in
>> pfn_pdx_compression_setup() for Coverity to start reporting an issue in
>> some decade-old code.
>>
>> The complaint is on line:
>>
>> 277 ma_va_bottom_mask = (PAGE_SIZE << bottom_shift) - 1;
>> CID 1662707: (#1 of 1): Bad bit shift operation (BAD_SHIFT)
>>
>> 34. large_shift: In expression 0x1000 << bottom_shift, left shifting by more
>> than 31 bits
>> has undefined behavior. The shift amount, bottom_shift, is as much as 63.
>>
>>
>> The relevant part of earlier analysis seems to be the "i >=
>> BITS_PER_LONG" check in the order loop, causing Coverity to think that i
>> can be up to 63.
>
> Yes, I've also got the email. This is existing code, so the issue
> wasn't introduced by the previous patches, it's probably result of
> the code movement that Coverity picked it up.
>
> I will send a patch to fix this.
Isn't it simply because this file is now also built for a toolstack side
test?
Jan
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