linux-kernel - Re: [syzbot] [mm?] KCSAN: data-race in generic_fillattr / shmem

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Date:   Tue, 2 May 2023 19:13:43 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Cc:     akpm@...ux-foundation.org, hughd@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com,
        syzbot <syzbot+702361cf7e3d95758761@...kaller.appspotmail.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [syzbot] [mm?] KCSAN: data-race in generic_fillattr / shmem_mknod
 (2)

On 2023/05/01 23:05, Tetsuo Handa wrote:
>> Also, there was a similar report on updating i_{ctime,mtime} to current_time()
>> which means that i_size is not the only field that is causing data race.
>> https://syzkaller.appspot.com/bug?id=067d40ab9ab23a6fa0a8156857ed54e295062a29
> 
> Do we want to as well wrap i_{ctime,mtime} using data_race() ?
> 

I think we need to use inode_lock_shared()/inode_unlock_shared() when calling
generic_fillattr(), for i_{ctime,mtime} (128bits) are too large to copy atomically.

Is it safe to call inode_lock_shared()/inode_unlock_shared() from generic_fillattr()?
Is some filesystem already holding inode lock before calling generic_fillattr()?