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

[Ingo Molnar]

* Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:

> On Thu, Mar 14, 2024 at 4:05 PM 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.
> >
> > There's nothing to keep folks from reintroducing these kinds of issues
> > and evidently no way to detect when they happen without lengthy reproducers.
> 
> This change is fine with me FWIW,

thx, I've added your:

    Acked-by: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>

> but I agree that making it for kmemleak reasons feels kind of misguided.

Yeah, so it's a workaround, but kmemleak is also a useful debugging 
facility that is finding memory leaks that static checkers are missing.

The fact that we don't have an easy way to prevent these problems from 
being introduced is I think properly counterbalanced by the facts that:

  1) Only kmemleak users are inconvenienced by the false positives.

  2) kmemleak users & maintainers have created the patch. There was no 
     pressure on us x86 maintainers other than to apply a root-cause 
     analyzed patch.

  2) Over a timespan of ~10 years only 2 such alignment problems were 
     introduced, and they were fixed by the kmemleak folks. I think that's 
     a fair price to pay for a useful facility.

Ie. I don't think there's any long-term maintenance burder concern.

So I've applied this workaround to x86/urgent, with a change to the title 
to make sure this isn't understood as a real bug in the PM code, but a 
workaround:

   37fb408c99af x86/pm: Work around false positive kmemleak report in msr_build_context()

.. lemme know if you feel strongly about this. :-)

Thanks,

	Ingo