Re: Linux 5.19-rc8

Guenter Roeck
2022-07-29T00:11:22Z

On Thu, Jul 28, 2022 at 07:28:22PM +0100, Russell King (Oracle) wrote:
> On Tue, Jul 26, 2022 at 10:12:21AM +0100, Russell King (Oracle) wrote:
> > First I'm aware of it. Was it reported to linux-arm-kernel? I'm guessing
> > the report wasn't Cc'd to me - I can't find anything in my mailbox about
> > it.
> >
> > > I think the fix might be something like this:
> > >
> > > diff --git a/arch/arm/lib/findbit.S b/arch/arm/lib/findbit.S
> > > index b5e8b9ae4c7d..b36ca301892e 100644
> > > --- a/arch/arm/lib/findbit.S
> > > +++ b/arch/arm/lib/findbit.S
> > > @@ -83,6 +83,8 @@ ENDPROC(_find_first_bit_le)
> > > ENTRY(_find_next_bit_le)
> > > teq r1, #0
> > > beq 3b
> > > + cmp r2, r1
> > > + bhs 3b
> > > ands ip, r2, #7
> > > beq 1b @ If new byte, goto old routine
> > > ARM( ldrb r3, [r0, r2, lsr #3] )
> > >
> > > but my ARM asm is so broken that the above is just really random noise
> > > that may or may not build - much less work.
> > >
> > > I'll leave it to Russell &co to have a tested and working patch.
> >
> > I think it needs a bit more than that, but as you point out in later
> > emails, the compiler may do a better job for this.
>
> Okay, I've moved my patch that fixes this (without adding a single line
> of code!) to my fixes branch, which I'll ask you to pull in the next
> couple of days.
>
I downloaded your patch and ran it through my testbed.
With it applied, the problem is no longer seen.
Feel free to add

Tested-by: Guenter Roeck <linux@roeck-us.net>

Thanks,
Guenter

> Each of the _find_next_* functions had:
>
> teq r1, #0
> beq 3b
>
> at the beginning to catch the case where size == 0. This is now:
>
> cmp r2, r1
> bhs 3b
>
> which is the C equivalent of:
>
> if (offset >= size)
> goto 3b;
>
> where both are unsigned, and nicely covers the case where size == 0 as
> before (since if size is 0, the condition is always true irrespective
> of the value of offset.)
>
> We can sort out the question of keeping this code or not later, but I
> think as this has been spotted as an issue, it's important to get it
> fixed.
>
> --
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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