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Sender: Guenter Roeck <groeck7@gmail.com>
Date:   Thu, 28 Jul 2022 17:11:13 -0700
From:   Guenter Roeck <linux@roeck-us.net>
To:     "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Yury Norov <yury.norov@gmail.com>,
        Dennis Zhou <dennis@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Linux 5.19-rc8
Message-ID: <20220729001113.GA1979316@roeck-us.net>
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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/
> FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!