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

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Date:   Fri, 2 Jul 2021 20:56:21 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Zhen Lei <thunder.leizhen@...wei.com>
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 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?