Re: [f2fs-dev] [PATCH] f2fs: synchronize atomic write aborts

[Chao Yu <chao@kernel.org>]

On 2023/2/10 2:18, Daeho Jeong wrote:
> From: Daeho Jeong <daehojeong@google.com>
> 
> To fix a race condition between atomic write aborts, I use the inode
> lock and make COW inode to be re-usable thoroughout the whole
> atomic file inode lifetime.
> 
> Reported-by: syzbot+823000d23b3400619f7c@syzkaller.appspotmail.com
> Fixes: 3db1de0e582c ("f2fs: change the current atomic write way")
> Signed-off-by: Daeho Jeong <daehojeong@google.com>
> ---
>   fs/f2fs/file.c    | 44 +++++++++++++++++++++++++++++---------------
>   fs/f2fs/inode.c   | 11 +++++++++--
>   fs/f2fs/segment.c |  3 ---
>   fs/f2fs/super.c   |  2 --
>   4 files changed, 38 insertions(+), 22 deletions(-)
> 
> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> index 300eae8b5415..6436c52e7913 100644
> --- a/fs/f2fs/file.c
> +++ b/fs/f2fs/file.c
> @@ -1863,7 +1863,10 @@ static int f2fs_release_file(struct inode *inode, struct file *filp)
>   			atomic_read(&inode->i_writecount) != 1)
>   		return 0;
>   
> +	inode_lock(inode);
>   	f2fs_abort_atomic_write(inode, true);
> +	inode_unlock(inode);
> +
>   	return 0;
>   }
>   
> @@ -1878,8 +1881,12 @@ static int f2fs_file_flush(struct file *file, fl_owner_t id)
>   	 * before dropping file lock, it needs to do in ->flush.
>   	 */
>   	if (F2FS_I(inode)->atomic_write_task == current &&
> -				(current->flags & PF_EXITING))
> +				(current->flags & PF_EXITING)) {
> +		inode_lock(inode);
>   		f2fs_abort_atomic_write(inode, true);
> +		inode_unlock(inode);
> +	}
> +
>   	return 0;
>   }
>   
> @@ -2085,19 +2092,28 @@ static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate)
>   		goto out;
>   	}
>   
> -	/* Create a COW inode for atomic write */
> -	pinode = f2fs_iget(inode->i_sb, fi->i_pino);
> -	if (IS_ERR(pinode)) {
> -		f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
> -		ret = PTR_ERR(pinode);
> -		goto out;
> -	}
> +	/* Check if the inode already has a COW inode */
> +	if (fi->cow_inode == NULL) {
> +		/* Create a COW inode for atomic write */
> +		pinode = f2fs_iget(inode->i_sb, fi->i_pino);
> +		if (IS_ERR(pinode)) {
> +			f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
> +			ret = PTR_ERR(pinode);
> +			goto out;
> +		}
>   
> -	ret = f2fs_get_tmpfile(mnt_userns, pinode, &fi->cow_inode);
> -	iput(pinode);
> -	if (ret) {
> -		f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
> -		goto out;
> +		ret = f2fs_get_tmpfile(mnt_userns, pinode, &fi->cow_inode);
> +		iput(pinode);
> +		if (ret) {
> +			f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
> +			goto out;
> +		}
> +
> +		set_inode_flag(fi->cow_inode, FI_COW_FILE);
> +		clear_inode_flag(fi->cow_inode, FI_INLINE_DATA);
> +	} else {
> +		/* Reuse the already created COW inode */
> +		f2fs_do_truncate_blocks(fi->cow_inode, 0, true);
>   	}
>   
>   	f2fs_write_inode(inode, NULL);
> @@ -2105,8 +2121,6 @@ static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate)
>   	stat_inc_atomic_inode(inode);
>   
>   	set_inode_flag(inode, FI_ATOMIC_FILE);
> -	set_inode_flag(fi->cow_inode, FI_COW_FILE);
> -	clear_inode_flag(fi->cow_inode, FI_INLINE_DATA);
>   
>   	isize = i_size_read(inode);
>   	fi->original_i_size = isize;
> diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
> index 28c9c72dda2a..7bf660d4cad9 100644
> --- a/fs/f2fs/inode.c
> +++ b/fs/f2fs/inode.c
> @@ -777,11 +777,18 @@ int f2fs_write_inode(struct inode *inode, struct writeback_control *wbc)
>   void f2fs_evict_inode(struct inode *inode)
>   {
>   	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
> -	nid_t xnid = F2FS_I(inode)->i_xattr_nid;
> +	struct f2fs_inode_info *fi = F2FS_I(inode);
> +	nid_t xnid = fi->i_xattr_nid;
>   	int err = 0;
>   
>   	f2fs_abort_atomic_write(inode, true);
>   
> +	if (fi->cow_inode) {
> +		clear_inode_flag(fi->cow_inode, FI_COW_FILE);
> +		iput(fi->cow_inode);
> +		fi->cow_inode = NULL;
> +	}

It looks "fi->cow_inode = NULL" here may race w/ cow_inode allocation in
f2fs_ioc_start_atomic_write due to f2fs_write_inode() has not been covered
by inode_lock()? IIUC.

Thanks,

> +
>   	trace_f2fs_evict_inode(inode);
>   	truncate_inode_pages_final(&inode->i_data);
>   
> @@ -866,7 +873,7 @@ void f2fs_evict_inode(struct inode *inode)
>   	stat_dec_inline_inode(inode);
>   	stat_dec_compr_inode(inode);
>   	stat_sub_compr_blocks(inode,
> -			atomic_read(&F2FS_I(inode)->i_compr_blocks));
> +			atomic_read(&fi->i_compr_blocks));
>   
>   	if (likely(!f2fs_cp_error(sbi) &&
>   				!is_sbi_flag_set(sbi, SBI_CP_DISABLED)))
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index ead3f35f501d..719329c1808c 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -192,9 +192,6 @@ void f2fs_abort_atomic_write(struct inode *inode, bool clean)
>   	if (!f2fs_is_atomic_file(inode))
>   		return;
>   
> -	clear_inode_flag(fi->cow_inode, FI_COW_FILE);
> -	iput(fi->cow_inode);
> -	fi->cow_inode = NULL;
>   	release_atomic_write_cnt(inode);
>   	clear_inode_flag(inode, FI_ATOMIC_COMMITTED);
>   	clear_inode_flag(inode, FI_ATOMIC_REPLACE);
> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> index c11a161ba5be..aa55dc12aff2 100644
> --- a/fs/f2fs/super.c
> +++ b/fs/f2fs/super.c
> @@ -1427,8 +1427,6 @@ static int f2fs_drop_inode(struct inode *inode)
>   			atomic_inc(&inode->i_count);
>   			spin_unlock(&inode->i_lock);
>   
> -			f2fs_abort_atomic_write(inode, true);
> -
>   			/* should remain fi->extent_tree for writepage */
>   			f2fs_destroy_extent_node(inode);
>   


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