Subject: Re: [RFC PATCH 05/21] ubifs: Pass worst-case buffer size to compression routines

From: Eric Biggers <[email-masked]>
Date: Tue, 18 Jul 2023 15:38:13 -0700

On Tue, Jul 18, 2023 at 02:58:31PM +0200, Ard Biesheuvel wrote:
> Currently, the ubifs code allocates a worst case buffer size to
> recompress a data node, but does not pass the size of that buffer to the
> compression code. This means that the compression code will never use
> the additional space, and might fail spuriously due to lack of space.
> 
> So let's multiply out_len by WORST_COMPR_FACTOR after allocating the
> buffer. Doing so is guaranteed not to overflow, given that the preceding
> kmalloc_array() call would have failed otherwise.
> 
> Signed-off-by: Ard Biesheuvel <[email-masked]>
> ---
>  fs/ubifs/journal.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/ubifs/journal.c b/fs/ubifs/journal.c
> index dc52ac0f4a345f30..4e5961878f336033 100644
> --- a/fs/ubifs/journal.c
> +++ b/fs/ubifs/journal.c
> @@ -1493,6 +1493,8 @@ static int truncate_data_node(const struct ubifs_info *c, const struct inode *in
>  	if (!buf)
>  		return -ENOMEM;
>  
> +	out_len *= WORST_COMPR_FACTOR;
> +
>  	dlen = le32_to_cpu(dn->ch.len) - UBIFS_DATA_NODE_SZ;
>  	data_size = dn_size - UBIFS_DATA_NODE_SZ;
>  	compr_type = le16_to_cpu(dn->compr_type);

This looks like another case where data that would be expanded by compression
should just be stored uncompressed instead.

In fact, it seems that UBIFS does that already.  ubifs_compress() has this:

        /*
         * If the data compressed only slightly, it is better to leave it
         * uncompressed to improve read speed.
         */
        if (in_len - *out_len < UBIFS_MIN_COMPRESS_DIFF)
                goto no_compr;

So it's unclear why the WORST_COMPR_FACTOR thing is needed at all.

- Eric