Re: [PATCH] fs: Fix data race in inode_set_ctime_to_ts

[Jan Kara <jack@suse.cz>]

On Wed 20-11-24 10:43:06, Hao-ran Zheng wrote:
> A data race may occur when the function `inode_set_ctime_to_ts()` and
> the function `inode_get_ctime_sec()` are executed concurrently. When
> two threads call `aio_read` and `aio_write` respectively, they will
> be distributed to the read and write functions of the corresponding
> file system respectively. Taking the btrfs file system as an example,
> the `btrfs_file_read_iter` and `btrfs_file_write_iter` functions are
> finally called. These two functions created a data race when they
> finally called `inode_get_ctime_sec()` and `inode_set_ctime_to_ns()`.
> The specific call stack that appears during testing is as follows:
> 
> ```
> ============DATA_RACE============
> btrfs_delayed_update_inode+0x1f61/0x7ce0 [btrfs]
> btrfs_update_inode+0x45e/0xbb0 [btrfs]
> btrfs_dirty_inode+0x2b8/0x530 [btrfs]
> btrfs_update_time+0x1ad/0x230 [btrfs]
> touch_atime+0x211/0x440
> filemap_read+0x90f/0xa20
> btrfs_file_read_iter+0xeb/0x580 [btrfs]
> aio_read+0x275/0x3a0
> io_submit_one+0xd22/0x1ce0
> __se_sys_io_submit+0xb3/0x250
> do_syscall_64+0xc1/0x190
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> ============OTHER_INFO============
> btrfs_write_check+0xa15/0x1390 [btrfs]
> btrfs_buffered_write+0x52f/0x29d0 [btrfs]
> btrfs_do_write_iter+0x53d/0x1590 [btrfs]
> btrfs_file_write_iter+0x41/0x60 [btrfs]
> aio_write+0x41e/0x5f0
> io_submit_one+0xd42/0x1ce0
> __se_sys_io_submit+0xb3/0x250
> do_syscall_64+0xc1/0x190
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> ```
> 
> The call chain after traceability is as follows:
> 
> ```
> Thread1:
> btrfs_delayed_update_inode() ->
> fill_stack_inode_item() ->
> inode_get_ctime_sec()
> 
> Thread2:
> btrfs_write_check() ->
> update_time_for_write() ->
> inode_set_ctime_to_ts()
> ```
> 
> To address this issue, it is recommended to
> add WRITE_ONCE when writing the `inode->i_ctime_sec` variable.

Thanks for the patch! This is really, really theoretic but with LTO I
suppose the compiler could get inventive and compile this in some other way
than plain stores.  But WRITE_ONCE() alone is not enough. You should have
READ_ONCE() in the reading counterparts as well.

								Honza

> 
> Signed-off-by: Hao-ran Zheng <zhenghaoran@buaa.edu.cn>
> ---
>  include/linux/fs.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 3559446279c1..d11b257a35e1 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1674,8 +1674,8 @@ static inline struct timespec64 inode_get_ctime(const struct inode *inode)
>  static inline struct timespec64 inode_set_ctime_to_ts(struct inode *inode,
>  						      struct timespec64 ts)
>  {
> -	inode->i_ctime_sec = ts.tv_sec;
> -	inode->i_ctime_nsec = ts.tv_nsec;
> +	WRITE_ONCE(inode->i_ctime_sec, ts.tv_sec);
> +	WRITE_ONCE(inode->i_ctime_nsec, ts.tv_nsec);
>  	return ts;
>  }
>  
> -- 
> 2.34.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR