linux-kernel - Re: [PATCH -next 1/1] iomap: Fix a false positive of UBSAN in iomap_seek

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Date:   Mon, 5 Jul 2021 11:35:08 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Christoph Hellwig <hch@...radead.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next 1/1] iomap: Fix a false positive of UBSAN in
 iomap_seek_data()



On 2021/7/3 3:56, Matthew Wilcox wrote:
> On Fri, Jul 02, 2021 at 05:21:09PM +0800, Zhen Lei wrote:
>> Move the evaluation expression "size - offset" after the "if (offset < 0)"
>> judgment statement to eliminate a false positive produced by the UBSAN.
>>
>> No functional changes.
>>
>> ==========================================================================
>> UBSAN: Undefined behaviour in fs/iomap.c:1435:9
>> signed integer overflow:
>> 0 - -9223372036854775808 cannot be represented in type 'long long int'
> 
> I don't understand.  I thought we defined the behaviour of signed
> integer overflow in the kernel with whatever-the-gcc-flag-is?

-9223372036854775808 ==> 0x8000000000000000 ==> -0

I don't fully understand what you mean. This is triggered by explicit error
injection '-0' at runtime, which should not be detected by compilation options.

lseek(r1, 0x8000000000000000, 0x3)

> 
> 
> .
>