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

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Message-ID: <YOQ5nuuoBVHABK1C@casper.infradead.org>
Date:   Tue, 6 Jul 2021 12:08:14 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Leizhen (ThunderTown)" <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 Mon, Jul 05, 2021 at 11:35:08AM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> 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.

We use -fwrapv on the gcc command line:

'-fwrapv'
     This option instructs the compiler to assume that signed arithmetic
     overflow of addition, subtraction and multiplication wraps around
     using twos-complement representation.  This flag enables some
     optimizations and disables others.

> lseek(r1, 0x8000000000000000, 0x3)

I'll see about adding this to xfstests ...