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From: Matthew Wilcox <willy@infradead.org>
To: "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	"Darrick J . Wong" <djwong@kernel.org>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH -next 1/1] iomap: Fix a false positive of UBSAN in iomap_seek_data()
Date: Tue, 6 Jul 2021 12:08:14 +0100	[thread overview]
Message-ID: <YOQ5nuuoBVHABK1C@casper.infradead.org> (raw)
In-Reply-To: <492c7a7b-6f2e-de45-c733-51c80422305e@huawei.com>

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 ...

  reply	other threads:[~2021-07-06 11:08 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-02  9:21 [PATCH -next 1/1] iomap: Fix a false positive of UBSAN in iomap_seek_data() Zhen Lei
2021-07-02  9:34 ` Christoph Hellwig
2021-07-02 11:50   ` Leizhen (ThunderTown)
2021-07-05  3:29     ` Leizhen (ThunderTown)
2021-07-05  3:43       ` Matthew Wilcox
2021-07-05  4:05         ` Leizhen (ThunderTown)
2021-07-04 13:51   ` Matthew Wilcox
2021-07-02 19:56 ` Matthew Wilcox
2021-07-05  3:35   ` Leizhen (ThunderTown)
2021-07-06 11:08     ` Matthew Wilcox [this message]
2021-07-06 11:59       ` Matthew Wilcox

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