Re: [PATCH] Btrfs: deal with unexpected return value in flush_space

["Holger Hoffstätte" <holger@applied-asynchrony.com>]

On Wed, 27 Jul 2016 18:42:03 -0700, Liu Bo wrote:

> Function start_transaction() can return ERR_PTR(1) when flush is
> BTRFS_RESERVE_FLUSH_LIMIT, so the call graph is
> 
> start_transaction (return ERR_PTR(1))
>   -> btrfs_block_rsv_add (return 1)
>      -> reserve_metadata_bytes (return 1)
>         -> flush_space (return 1)
>            -> do_chunk_alloc  (return 1)
> 
> With BTRFS_RESERVE_FLUSH_LIMIT, if flush_space is already on the
> flush_state of ALLOC_CHUNK and it successfully allocates a new
> chunk, then instead of trying to reserve space again,
> reserve_metadata_bytes returns 1 immediately.
> 
> Eventually the callers who call start_transaction() usually just
> do the IS_ERR() check which ERR_PTR(1) can pass, then it'll get
> a panic when dereferencing a pointer which is ERR_PTR(1).
> 
> This makes flush_space() translate 'ret = 1' to 'ret = 0'.
> 
> Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> ---
> We found this 'NULL pointer dereference' on an old 3.8 kernel but
> it's not going to happen on the upstream since there is no caller
> of btrfs_start_transaction_lflush().
> 
>  fs/btrfs/extent-tree.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 7a35c9d..a00fb67 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -4457,6 +4457,15 @@ void check_system_chunk(struct btrfs_trans_handle *trans,
>  	}
>  }
>  
> +/*
> + * If force is CHUNK_ALLOC_FORCE:
> + *    - return 1 if it successfully allocates a chunk,
> + *    - return errors including -ENOSPC otherwise.
> + * If force is NOT CHUNK_ALLOC_FORCE:
> + *    - return 0 if it doesn't need to allocate a new chunk,
> + *    - return 1 if it successfully allocates a chunk,
> + *    - return errors including -ENOSPC otherwise.
> + */
>  static int do_chunk_alloc(struct btrfs_trans_handle *trans,
>  			  struct btrfs_root *extent_root, u64 flags, int force)
>  {
> @@ -4857,7 +4866,7 @@ static int flush_space(struct btrfs_root *root,
>  				     btrfs_get_alloc_profile(root, 0),
>  				     CHUNK_ALLOC_NO_FORCE);
>  		btrfs_end_transaction(trans, root);
> -		if (ret == -ENOSPC)
> +		if (ret == -ENOSPC || ret == 1)
>  			ret = 0;
>  		break;
>  	case COMMIT_TRANS:
> -- 
> 2.5.5

For reviewers - this came up before here:
https://patchwork.kernel.org/patch/7778651/

Same fix basically.

-h