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Date:	Fri, 25 Sep 2015 11:36:17 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrey Konovalov <andreyknvl@...gle.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.or>,
	"Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Alexander Potapenko <glider@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>
Subject: Re: Data race in __inode_add_bytes

Looking at __inode_add_bytes/ext4_inode_blocks_set I see much more
ways to screw things up.
For example, __inode_add_bytes:

void __inode_add_bytes(struct inode *inode, loff_t bytes)
{
        inode->i_blocks += bytes >> 9;
        bytes &= 511;
        inode->i_bytes += bytes;
        if (inode->i_bytes >= 512) {
                inode->i_blocks++;
                inode->i_bytes -= 512;
        }
}

can be compiled effectively as:

void __inode_add_bytes(struct inode *inode, loff_t bytes)
{
        inode->i_blocks += bytes >> 9 + 1;
        bytes = inode->i_bytes + (bytes & 511);
        if (bytes < 512)
                 inode->i_blocks--;
        inode->i_bytes = bytes & 511;
}

Which will produce invalid results on any bitness with any file size.

Or, for example, ext4_inode_blocks_set can read inode->i_blocks
several times and then also produce invalid results (lose high part or
something).

Since this is invalid C code, there is basically infinite number of
ways how this can screw up by compiler and/or hardware.





On Fri, Sep 25, 2015 at 11:00 AM, Jan Kara <jack@...e.cz> wrote:
> On Mon 31-08-15 21:33:46, Andrey Konovalov wrote:
>> Hi!
>>
>> We are working on a dynamic data race detector for the Linux kernel,
>> KernelThreadSanitizer (ktsan):
>> https://github.com/google/ktsan/wiki
>>
>> We got a report while running ktsan on 4.2:
>>
>> ==================================================================
>> ThreadSanitizer: data-race in __inode_add_bytes
>>
>> Write of size 8 by thread T210 (K740):
>>  [<ffffffff81266435>] __inode_add_bytes+0x55/0xd0 fs/stat.c:451
>>  [<ffffffff812f90c0>] inode_claim_rsv_space+0x60/0xa0 fs/quota/dquot.c:1557
>>  [<ffffffff812f9f7b>] dquot_claim_space_nodirty+0x3b/0x280 fs/quota/dquot.c:1721
>>  [<     inlined    >] ext4_da_update_reserve_space+0x13b/0x2c0
>> dquot_claim_block include/linux/quotaops.h:345
>>  [<ffffffff81335dab>] ext4_da_update_reserve_space+0x13b/0x2c0
>> fs/ext4/inode.c:350
>>  [<ffffffff81384cf0>] ext4_ext_map_blocks+0x1570/0x1a30 fs/ext4/extents.c:4597
>>  [<ffffffff8133610a>] ext4_map_blocks+0x1da/0x7b0 fs/ext4/inode.c:592
>>  [<     inlined    >] ext4_writepages+0x976/0x1480
>> mpage_map_one_extent fs/ext4/inode.c:2109
>>  [<     inlined    >] ext4_writepages+0x976/0x1480
>> mpage_map_and_submit_extent fs/ext4/inode.c:2165
>>  [<ffffffff8133b7b6>] ext4_writepages+0x976/0x1480 fs/ext4/inode.c:2508
>>  [<ffffffff811dbd23>] do_writepages+0x53/0x80 mm/page-writeback.c:2332
>>  [<ffffffff812a76bf>] __writeback_single_inode+0x7f/0x530
>> fs/fs-writeback.c:1259 (discriminator 3)
>>  [<ffffffff812a7fd4>] writeback_sb_inodes+0x464/0x690 fs/fs-writeback.c:1516
>>  [<ffffffff812a82c1>] __writeback_inodes_wb+0xc1/0x100 fs/fs-writeback.c:1562
>>  [<ffffffff812a86ae>] wb_writeback+0x3ae/0x450 fs/fs-writeback.c:1666
>>  [<     inlined    >] wb_workfn+0x203/0x780 wb_do_writeback
>> fs/fs-writeback.c:1801
>>  [<ffffffff812a91c3>] wb_workfn+0x203/0x780 fs/fs-writeback.c:1852
>>  [<ffffffff810b06ce>] process_one_work+0x28e/0x710 kernel/workqueue.c:2036
>>  [<ffffffff810b1299>] worker_thread+0xb9/0x750 kernel/workqueue.c:2170
>>  [<ffffffff810b9c61>] kthread+0x161/0x180 kernel/kthread.c:209
>>  [<ffffffff81eb0a1f>] ret_from_fork+0x3f/0x70 arch/x86/entry/entry_64.S:526
>>
>> Previous read of size 8 by thread T512 (K7200):
>>  [<     inlined    >] ext4_mark_iloc_dirty+0x454/0xe20
>> ext4_inode_blocks_set fs/ext4/inode.c:4272
>>  [<     inlined    >] ext4_mark_iloc_dirty+0x454/0xe20
>> ext4_do_update_inode fs/ext4/inode.c:4430
>>  [<ffffffff8133a014>] ext4_mark_iloc_dirty+0x454/0xe20 fs/ext4/inode.c:4937
>>  [<ffffffff8133ab8b>] ext4_mark_inode_dirty+0xdb/0x390 fs/ext4/inode.c:5053
>>  [<ffffffff8133f929>] ext4_dirty_inode+0x59/0x80 fs/ext4/inode.c:5085
>>  [<ffffffff812a7319>] __mark_inode_dirty+0x2c9/0x5f0 fs/fs-writeback.c:2015
>>  [<ffffffff8128a58e>] generic_update_time+0xbe/0x150 fs/inode.c:1566
>>  [<     inlined    >] file_update_time+0x112/0x1b0 update_time fs/inode.c:1582
>>  [<ffffffff812890f2>] file_update_time+0x112/0x1b0 fs/inode.c:1785
>>  [<ffffffff811cb175>] __generic_file_write_iter+0x105/0x2e0 mm/filemap.c:2570
>>  [<ffffffff8132c3a4>] ext4_file_write_iter+0x254/0x740 fs/ext4/file.c:170
>>  [<     inlined    >] __vfs_write+0x19c/0x1e0 new_sync_write fs/read_write.c:478
>>  [<ffffffff8125d48c>] __vfs_write+0x19c/0x1e0 fs/read_write.c:491
>>  [<ffffffff8125dde6>] vfs_write+0xf6/0x2a0 fs/read_write.c:538
>>  [<     inlined    >] SyS_write+0x6b/0xd0 SYSC_write fs/read_write.c:585
>>  [<ffffffff8125f37b>] SyS_write+0x6b/0xd0 fs/read_write.c:577
>>  [<ffffffff81eb062e>] entry_SYSCALL_64_fastpath+0x12/0x71
>> arch/x86/entry/entry_64.S:186
>> ==================================================================
>>
>> The 'inode->i_blocks' field is updated in one thread, while being read
>> and used in another.
>>
>> This can probably be fixed with a few READ_ONCE/WRITE_ONCE or by
>> taking inode->i_lock in ext4_inode_blocks_set.
>
> Yeah, the right fix would be to use inode->i_lock as quota code does,
> possibly with a wrapper function so that it can be avoided for 64-bit archs
> (see how i_size_read() / i_size_write() gets handled).  However if you're
> going to fix this (and I'd note that this race is mostly theoretical since
> it would require 32-bit architecture and a file using more than 2TB of
> space) it's not just about ext4_inode_blocks_set() but about auditing all
> the other places working with i_blocks which is kind of a pain given the
> theoretical nature of the race...
>
>                                                                 Honza
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR
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