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

[Anton Altaparmakov]

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.

Best regards,

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