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

[Ard Biesheuvel]

On Wed, 19 Jul 2023 at 00:38, Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>
> 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 <ardb@xxxxxxxxxx>
> > ---
> >  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.
>

It is not. The buffer is used for decompression in the truncation
path, so none of this logic even matters. Even if the subsequent
recompression of the truncated data node could result in expansion
beyond the uncompressed size of the original data (which seems
impossible to me), increasing the size of this buffer would not help
as it is the input buffer for the compression not the output buffer.