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Date:   Tue, 6 Apr 2021 16:05:04 +0200
From:   Jan Kara <jack@suse.cz>
To:     Hao Sun <sunhao.th@gmail.com>
Cc:     Jan Kara <jack@suse.cz>, tytso@mit.edu, jack@suse.com,
        linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: KCSAN: data-race in __jbd2_journal_file_buffer /
 jbd2_journal_dirty_metadata
Message-ID: <20210406140504.GE19407@quack2.suse.cz>
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On Tue 06-04-21 21:27:48, Hao Sun wrote:
> > Thanks for report but I'm not sure what KCSAN is complaining about - isn't the report truncated?
> 
> Yes, the full KCSAN report is available in the attached log file.

Sorry, I missed that in your original email.

> Here is the report :
>  ==================================================================
> BUG: KCSAN: data-race in __jbd2_journal_file_buffer /
> jbd2_journal_dirty_metadata
> 
> write to 0xffff88800af6da38 of 8 bytes by task 4822 on cpu 1:
>  __jbd2_journal_file_buffer+0x18d/0x370
>  __jbd2_journal_refile_buffer+0x155/0x230
>  jbd2_journal_commit_transaction+0x24c6/0x3200
>  kjournald2+0x253/0x470
>  kthread+0x1f0/0x220
>  ret_from_fork+0x1f/0x30
> 
> read to 0xffff88800af6da38 of 8 bytes by task 1955 on cpu 0:
>  jbd2_journal_dirty_metadata+0x17f/0x670
>  __ext4_handle_dirty_metadata+0xc6/0x590
>  ext4_mark_iloc_dirty+0x12dd/0x16e0
>  __ext4_mark_inode_dirty+0x4d2/0x5d0
>  ext4_writepages+0x1262/0x1e50
>  do_writepages+0x7b/0x150
>  __writeback_single_inode+0x84/0x4e0
>  writeback_sb_inodes+0x69f/0x1020
>  __writeback_inodes_wb+0xb0/0x2a0
>  wb_writeback+0x290/0x650
>  wb_do_writeback+0x582/0x5d0
>  wb_workfn+0xb8/0x410
>  process_one_work+0x3e1/0x940
>  worker_thread+0x64a/0xaa0
>  kthread+0x1f0/0x220
>  ret_from_fork+0x1f/0x30
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 PID: 1955 Comm: kworker/u5:2 Not tainted 5.11.0+ #5
> 
> 
> Sorry, I couldn't symbolize it because the original Linux binary was lost.
> Do you think this is an actual bug?

So it is difficult to be 100% sure without knowing which particular access
caused the KCSAN warning but I'm quite confident it was caused by one of
unlocked accesses to jh->b_transaction in jbd2_journal_dirty_metadata().
And as the comments explain, these are only assertions which we redo under
proper lock if they look like they would fail. So the code is in fact
correct.

								Honza

> Jan Kara <jack@suse.cz> 于2021年4月6日周二 下午8:32写道：
> >
> > Hello!
> >
> > On Sun 04-04-21 17:40:44, Hao Sun wrote:
> > > When using Healer(https://github.com/SunHao-0/healer/tree/dev) to fuzz
> > > the Linux kernel, I found a data-race vulnerability in
> > > __jbd2_journal_file_buffer / jbd2_journal_dirty_metadata.
> > > Sorry, data-race is usually difficult to reproduce. I cannot provide
> > > you with a reproducing program.
> > > I hope that the call stack information in the crash log can help you
> > > locate the problem.
> > > Kernel config and full log can be found in the attachment.
> > >
> > > Here is the detailed information:
> > > commit:   3b9cdafb5358eb9f3790de2f728f765fef100731
> > > version:   linux 5.11
> > > git tree:    upstream
> > > report:
> > > ==================================================================
> > > BUG: KCSAN: data-race in __jbd2_journal_file_buffer /
> > > jbd2_journal_dirty_metadata
> > > write to 0xffff88800af6da38 of 8 bytes by task 4822 on cpu 1:
> > >  __jbd2_journal_file_buffer+0x18d/0x370 linux/fs/jbd2/transaction.c:2518
> > >  __jbd2_journal_refile_buffer+0x155/0x230 linux/fs/jbd2/transaction.c:2612
> > >  jbd2_journal_commit_transaction+0x24c6/0x3200 linux/fs/jbd2/commit.c:1084
> > >  kjournald2+0x253/0x470 linux/fs/jbd2/journal.c:213
> > >  kthread+0x1f0/0x220 linux/kernel/kthread.c:292
> > >  ret_from_fork+0x1f/0x30 linux/arch/x86/entry/entry_64.S:294
> >
> > Thanks for report but I'm not sure what KCSAN is complaining about - isn't
> > the report truncated? I'm missing 'read' part of the report... The complaint
> > is on line:
> >
> > jh->b_transaction = transaction;
> >
> > I would guess the complaint is because of the check:
> >
> >         /*
> >          * This and the following assertions are unreliable since we may see jh
> >          * in inconsistent state unless we grab bh_state lock. But this is
> >          * crucial to catch bugs so let's do a reliable check until the
> >          * lockless handling is fully proven.
> >          */
> >         if (jh->b_transaction != transaction &&
> >             jh->b_next_transaction != transaction) {
> >
> > And the comment explains, why we do this unreliable check. Again, if we
> > wanted to silence KCSAN, we could use data_race() macro but AFAIU Ted isn't
> > very fond of that annotation.
> >
> >                                                                 Honza
> >
> > --
> > Jan Kara <jack@suse.com>
> > SUSE Labs, CR
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR