linux-kernel - Re: [PATCH] x86/pm: Fix false positive kmemleak report in msr_build

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Date: Fri, 22 Mar 2024 10:58:06 +0100
From: Ingo Molnar <mingo@...nel.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Dave Hansen <dave.hansen@...el.com>,
	Anton Altaparmakov <anton@...era.com>, Pavel Machek <pavel@....cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H . Peter Anvin" <hpa@...or.com>, Chen Yu <yu.c.chen@...el.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Catalin Marinas <catalin.marinas@....com>, linux-mm@...ck.org,
	Matthieu Baerts <matthieu.baerts@...sares.net>,
	Mat Martineau <mathew.j.martineau@...ux.intel.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] x86/pm: Fix false positive kmemleak report in
 msr_build_context().


* Rafael J. Wysocki <rafael@...nel.org> wrote:

> On Thu, Mar 14, 2024 at 4:05 PM Dave Hansen <dave.hansen@...el.com> 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@...nel.org>

> 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