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Date:   Tue, 28 Nov 2023 17:22:38 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Haibo Li <haibo.li@mediatek.com>
Cc:     <linux-kernel@vger.kernel.org>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        <kasan-dev@googlegroups.com>, <linux-mm@kvack.org>,
        <linux-arm-kernel@lists.infradead.org>,
        <linux-mediatek@lists.infradead.org>, <xiaoming.yu@mediatek.com>,
        kernel test robot <lkp@intel.com>
Subject: Re: [PATCH] fix comparison of unsigned expression < 0
Message-Id: <20231128172238.f80ed8dd74ab2a13eba33091@linux-foundation.org>
In-Reply-To: <20231128075532.110251-1-haibo.li@mediatek.com>
References: <20231128075532.110251-1-haibo.li@mediatek.com>
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On Tue, 28 Nov 2023 15:55:32 +0800 Haibo Li <haibo.li@mediatek.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.

But really, some hardwired comparison with an absolute address seems
lazy.  If KASAN_SHADOW_OFFSET is variable on a per-architecture basis
then the expression which checks the validity of an arbitrary address
should also be per-architecture.