Re: [PATCH] libfs: fix infinite directory reads for offset dir

[Jan Kara <jack@suse.cz>]

On Wed 31-07-24 12:38:35, yangerkun wrote:
> After we switch tmpfs dir operations from simple_dir_operations to
> simple_offset_dir_operations, every rename happened will fill new dentry
> to dest dir's maple tree(&SHMEM_I(inode)->dir_offsets->mt) with a free
> key starting with octx->newx_offset, and then set newx_offset equals to
> free key + 1. This will lead to infinite readdir combine with rename
> happened at the same time, which fail generic/736 in xfstests(detail show
> as below).
> 
> 1. create 5000 files(1 2 3...) under one dir
> 2. call readdir(man 3 readdir) once, and get one entry
> 3. rename(entry, "TEMPFILE"), then rename("TEMPFILE", entry)
> 4. loop 2~3, until readdir return nothing or we loop too many
>    times(tmpfs break test with the second condition)
> 
> We choose the same logic what commit 9b378f6ad48cf ("btrfs: fix infinite
> directory reads") to fix it, record the last_index when we open dir, and
> do not emit the entry which index >= last_index. The file->private_data
> now used in offset dir can use directly to do this, and we also update
> the last_index when we llseek the dir file.

The patch looks good! Just I'm not sure about the llseek part. As far as I
understand it was added due to this sentence in the standard:

"If a file is removed from or added to the directory after the most recent
call to opendir() or rewinddir(), whether a subsequent call to readdir()
returns an entry for that file is unspecified."

So if the offset used in offset_dir_llseek() is 0, then we should update
last_index. But otherwise I'd leave it alone because IMHO it would do more
harm than good.

								Honza

> 
> Fixes: a2e459555c5f ("shmem: stable directory offsets")
> Signed-off-by: yangerkun <yangerkun@huawei.com>
> ---
>  fs/libfs.c | 34 +++++++++++++++++++++++-----------
>  1 file changed, 23 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index 8aa34870449f..38b306738c00 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -450,6 +450,14 @@ void simple_offset_destroy(struct offset_ctx *octx)
>  	mtree_destroy(&octx->mt);
>  }
>  
> +static int offset_dir_open(struct inode *inode, struct file *file)
> +{
> +	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> +
> +	file->private_data = (void *)ctx->next_offset;
> +	return 0;
> +}
> +
>  /**
>   * offset_dir_llseek - Advance the read position of a directory descriptor
>   * @file: an open directory whose position is to be updated
> @@ -463,6 +471,9 @@ void simple_offset_destroy(struct offset_ctx *octx)
>   */
>  static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
>  {
> +	struct inode *inode = file->f_inode;
> +	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> +
>  	switch (whence) {
>  	case SEEK_CUR:
>  		offset += file->f_pos;
> @@ -476,7 +487,7 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
>  	}
>  
>  	/* In this case, ->private_data is protected by f_pos_lock */
> -	file->private_data = NULL;
> +	file->private_data = (void *)ctx->next_offset;
>  	return vfs_setpos(file, offset, LONG_MAX);
>  }
>  
> @@ -507,7 +518,7 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
>  			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
>  }
>  
> -static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
> +static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, long last_index)
>  {
>  	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
>  	struct dentry *dentry;
> @@ -515,17 +526,21 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
>  	while (true) {
>  		dentry = offset_find_next(octx, ctx->pos);
>  		if (!dentry)
> -			return ERR_PTR(-ENOENT);
> +			return;
> +
> +		if (dentry2offset(dentry) >= last_index) {
> +			dput(dentry);
> +			return;
> +		}
>  
>  		if (!offset_dir_emit(ctx, dentry)) {
>  			dput(dentry);
> -			break;
> +			return;
>  		}
>  
>  		ctx->pos = dentry2offset(dentry) + 1;
>  		dput(dentry);
>  	}
> -	return NULL;
>  }
>  
>  /**
> @@ -552,22 +567,19 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
>  static int offset_readdir(struct file *file, struct dir_context *ctx)
>  {
>  	struct dentry *dir = file->f_path.dentry;
> +	long last_index = (long)file->private_data;
>  
>  	lockdep_assert_held(&d_inode(dir)->i_rwsem);
>  
>  	if (!dir_emit_dots(file, ctx))
>  		return 0;
>  
> -	/* In this case, ->private_data is protected by f_pos_lock */
> -	if (ctx->pos == DIR_OFFSET_MIN)
> -		file->private_data = NULL;
> -	else if (file->private_data == ERR_PTR(-ENOENT))
> -		return 0;
> -	file->private_data = offset_iterate_dir(d_inode(dir), ctx);
> +	offset_iterate_dir(d_inode(dir), ctx, last_index);
>  	return 0;
>  }
>  
>  const struct file_operations simple_offset_dir_operations = {
> +	.open		= offset_dir_open,
>  	.llseek		= offset_dir_llseek,
>  	.iterate_shared	= offset_readdir,
>  	.read		= generic_read_dir,
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR