[PATCH v3 2/2] x86: Fix /proc/cpuinfo cpumask warning

[Andrew Jones]

On Wed, Nov 02, 2022 at 07:44:02PM +0100, Borislav Petkov wrote:
> On Mon, Oct 31, 2022 at 11:03:27AM +0100, Andrew Jones wrote:
> > Currently (after the revert of 78e5a3399421)
> 
> After the revert?
> 
> That commit is still in the latest Linus tree.

The revert commit is 80493877d7d0 ("Revert "cpumask: fix checking valid
cpu range".")

> 
> > with DEBUG_PER_CPU_MAPS we'll get a warning splat when the cpu is
> > outside the range [-1, nr_cpu_ids)
> 
> Yah, that range makes sense.
> 
> > and cpumask_next() will call find_next_bit() with the input plus one anyway.
> > find_next_bit() doesn't explicity document what happens when an input is
> > outside the range, but it currently returns the bitmap size without any
> > side effects, which means cpumask_next() will return nr_cpu_ids.
> 
> That is good to have in the commit message.
> 
> > show_cpuinfo() doesn't try to show anything in that case and stops its
> > loop, or, IOW, things work fine now with an input of nr_cpu_ids - 1. But,
> > show_cpuinfo() is just getting away with a violated cpumask_next()
> > contract, which 78e5a3399421 exposed. How about a new commit message like
> > this
> 
> You're making it sound more complex than it is. All you wanna say is:
> 
> "Filter out invalid cpumask_next() inputs by checking its first argument
> against nr_cpu_ids because cpumask_next() will call find_next_bit() with
> the input plus one but the valid range for n is [-1, nr_cpu_ids)."

The patch I'm proposing ensures cpumask_next()'s range, which is actually
[-1, nr_cpus_ids - 1), isn't violated. Violating that range will generate
the warning for kernels which have commit 78e5a3399421 ("cpumask: fix
checking valid cpu range"), but not its revert.

Since 78e5a3399421 has been reverted, the value of this proposed fix is
less, and indeed the warning may even go away completely for these types
of cpumask calls[1]. However, it seems reasonable for callers to implement
their own checks until the cpumask API has documented what they should
expect.

[1] https://lore.kernel.org/all/CAHk-=wihz-GXx66MmEyaADgS1fQE_LDcB9wrHAmkvXkd8nx9tA@mail.gmail.com/

> 
> But that thing with the revert above needs to be clarified first.

I'll send a v4 with another stab at the commit message.

Thanks,
drew

> 
> Thx.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette