linux-kernel - Re: [PATCH] fix comparison of unsigned expression

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Date:   Wed, 29 Nov 2023 04:01:47 +0100
From:   Andrey Konovalov <andreyknvl@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        kernel test robot <lkp@...el.com>,
        Haibo Li <haibo.li@...iatek.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        kasan-dev@...glegroups.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, xiaoming.yu@...iatek.com
Subject: Re: [PATCH] fix comparison of unsigned expression < 0

On Wed, Nov 29, 2023 at 2:22 AM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Tue, 28 Nov 2023 15:55:32 +0800 Haibo Li <haibo.li@...iatek.com> wrote:
>
> > Kernel test robot reported:
> >
> > '''
> > mm/kasan/report.c:637 kasan_non_canonical_hook() warn:
> > unsigned 'addr' is never less than zero.
> > '''
> > The KASAN_SHADOW_OFFSET is 0 on loongarch64.
> >
> > To fix it,check the KASAN_SHADOW_OFFSET before do comparison.
> >
> > --- a/mm/kasan/report.c
> > +++ b/mm/kasan/report.c
> > @@ -634,10 +634,10 @@ void kasan_non_canonical_hook(unsigned long addr)
> >  {
> >       unsigned long orig_addr;
> >       const char *bug_type;
> > -
> > +#if KASAN_SHADOW_OFFSET > 0
> >       if (addr < KASAN_SHADOW_OFFSET)
> >               return;
> > -
> > +#endif
>
> We'd rather not add ugly ifdefs for a simple test like this.  If we
> replace "<" with "<=", does it fix?  I suspect that's wrong.

Changing the comparison into "<=" would be wrong.

But I actually don't think we need to fix anything here.

This issue looks quite close to a similar comparison with 0 issue
Linus shared his opinion on here:

https://lore.kernel.org/all/Pine.LNX.4.58.0411230958260.20993@ppc970.osdl.org/

I don't know if the common consensus with the regard to issues like
that changed since then. But if not, perhaps we can treat this kernel
test robot report as a false positive.

Thanks!