Re: [PATCH] x86/pm: Fix false positive kmemleak report in msr_build_context().

[Anton Altaparmakov]

Hi Ingo,

> On 22 Mar 2024, at 10:03, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> * Anton Altaparmakov <anton@xxxxxxxxxx> wrote:
>> Hi Dave,
>>> On 14 Mar 2024, at 15:05, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>>> On 3/14/24 07:26, Anton Altaparmakov wrote:
>>>> /* image of the saved processor state */
>>>> struct saved_context {
>>>> - /*
>>>> - * On x86_32, all segment registers except gs are saved at kernel
>>>> - * entry in pt_regs.
>>>> - */
>>>> - u16 gs;
>>>> unsigned long cr0, cr2, cr3, cr4;
>>>> u64 misc_enable;
>>>> struct saved_msrs saved_msrs;
>>>> @@ -27,6 +22,11 @@ struct saved_context {
>>>> unsigned long tr;
>>>> unsigned long safety;
>>>> unsigned long return_address;
>>>> + /*
>>>> + * On x86_32, all segment registers except gs are saved at kernel
>>>> + * entry in pt_regs.
>>>> + */
>>>> + u16 gs;
>>>> bool misc_enable_saved;
>>>> } __attribute__((packed));
>>> 
>>> Isn't this just kinda poking at the symptoms?  This seems to be
>>> basically the exact same bug as b0b592cf08, just with a different source
>>> of unaligned structure members.
>> 
>> Yes, that is exactly the same bug.  That's how we figured out the solution in fact - it is totally the same problem with another struct member...
>> 
>>> There's nothing to keep folks from reintroducing these kinds of issues
>>> and evidently no way to detect when they happen without lengthy reproducers.
>> 
>> Correct.  But short of adding asserts / documentation that pointers must be aligned or kmemleak won't work or fixing kmemleak (which I expect is not tractical as it would become a lot slower if nothing else) not sure what else can be done.
>> 
>> Given I cannot see any alternative to fixing the kmemleak failures I think it is worth applying this fix.
>> 
>> Unless you have better ideas how to fix this issue?
>> 
>> What I can say is that we run a lot of tests with our CI and applying 
>> this fix we do not see any kmemleak issues any more whilst without it we 
>> see hundreds of the above - from a single, simple test run consisting of 
>> 416 individual test cases on kernel 5.10 x86 with kmemleak enabled we got 
>> 20 failures due to this which is quite a lot.  With this fix applied we 
>> get zero kmemleak related failures.
> 
> I turned this tidbit into the following paragraph in the commit:
> 
>    Testing:
> 
>    We run a lot of tests with our CI, and after applying this fix we do not
>    see any kmemleak issues any more whilst without it we see hundreds of
>    the above report. From a single, simple test run consisting of 416 individual test
>    cases on kernel 5.10 x86 with kmemleak enabled we got 20 failures due to this,
>    which is quite a lot. With this fix applied we get zero kmemleak related failures.
> 
> Describing the impact of a fix in a changelog is always helpful.

That's a good idea, thank you!  Also, thank you for taking the patch.  Always nice not to have to maintain too many custom kernel patches!

Best regards,

Anton

> 
> Thanks,
> 
> Ingo


-- 
Anton Altaparmakov <anton at tuxera.com> (replace at with @)
Lead in File System Development, Tuxera Inc., http://www.tuxera.com/