From: David Sterba <dsterba@suse.cz>
To: Qian Cai <cai@lca.pw>
Cc: viro@zeniv.linux.org.uk, hch@infradead.org,
darrick.wong@oracle.com, elver@google.com,
linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fs: fix a data race in i_size_write/i_size_read
Date: Wed, 19 Feb 2020 13:08:43 +0100 [thread overview]
Message-ID: <20200219120843.GU2902@twin.jikos.cz> (raw)
In-Reply-To: <20200219040426.1140-1-cai@lca.pw>
On Tue, Feb 18, 2020 at 11:04:26PM -0500, Qian Cai wrote:
> inode::i_size could be accessed concurently as noticed by KCSAN,
>
> BUG: KCSAN: data-race in iomap_do_writepage / iomap_write_end
>
> The write is protected by exclusive inode::i_rwsem (in
> xfs_file_buffered_aio_write()) but the read is not. A shattered value
> could introduce a logic bug. Fixed it using a pair of WRITE/READ_ONCE().
We had a different problem with lack of READ_ONCE/WRITE_ONCE for i_size,
the fix was the same though. There was i_size_read(inode) used in max()
macro and compiled code two reads (unlocked), and this led to a race
when where different value was checked and then used.
The thread:
https://lore.kernel.org/linux-fsdevel/20191011202050.8656-1-josef@toxicpanda.com/
We had to apply a workaround to btrfs code because the generic fix was
not merged, even with several reviews and fixing a real bug. The report
from KCSAN seems to require some sort of splitting the values. What we
saw happened on 64bit platform without that effect so I'd call that a
more likely to happen because the pattern max(i_size_read(inode), ...) is
not something we'd instinctively consider unsafe.
prev parent reply other threads:[~2020-02-19 12:09 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-19 4:04 [PATCH] fs: fix a data race in i_size_write/i_size_read Qian Cai
2020-02-19 4:52 ` Al Viro
2020-02-19 5:08 ` Qian Cai
2020-02-19 5:23 ` Al Viro
2020-02-19 9:21 ` Marco Elver
2020-02-19 12:18 ` David Sterba
2020-02-21 4:19 ` Qian Cai
2020-02-19 12:08 ` David Sterba [this message]
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