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

[Ard Biesheuvel]

On Wed, 19 Jul 2023 at 16:23, Zhihao Cheng <chengzhihao1 at huawei.com> wrote:
>
> 在 2023/7/19 16:33, Ard Biesheuvel 写道:
> > On Wed, 19 Jul 2023 at 00:38, Eric Biggers <ebiggers at kernel.org> 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
>
> I think you mean the 'out_len' which describes the lengh of 'buf' is
> passed into ubifs_decompress, which effects the result of
> decompressor(eg. lz4 uses length to calculate the buffer end pos).
> So, we should pass the real lenghth of 'buf'.
>

Yes, that is what I meant.

But Eric makes a good point, and looking a bit more closely, there is
really no need for the multiplication here: we know the size of the
decompressed data, so we don't need the additional space.

I intend to drop this patch, and replace it with the following:

----------------8<--------------

Currently, when truncating a data node, a decompression buffer is
allocated that is twice the size of the data node's uncompressed size.
However, the fact that this space is available is not communicated to
the compression routines, as out_len itself is not updated.

The additional space is not needed even in the theoretical worst case
where compression might lead to inadvertent expansion: first of all,
increasing the size of the input buffer does not help mitigate that
issue. And given the truncation of the data node and the fact that the
original data compressed well enough to pass the UBIFS_MIN_COMPRESS_DIFF
test, there is no way on this particular code path that compression
could result in expansion beyond the original decompressed size, and so
no mitigation is necessary to begin with.

So let's just drop WORST_COMPR_FACTOR here.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>

diff --git a/fs/ubifs/journal.c b/fs/ubifs/journal.c
index dc52ac0f4a345f30..0b55cbfe0c30505e 100644
--- a/fs/ubifs/journal.c
+++ b/fs/ubifs/journal.c
@@ -1489,7 +1489,7 @@ static int truncate_data_node(const struct
ubifs_info *c, const struct inode *in
        int err, dlen, compr_type, out_len, data_size;

        out_len = le32_to_cpu(dn->size);
-       buf = kmalloc_array(out_len, WORST_COMPR_FACTOR, GFP_NOFS);
+       buf = kmalloc(out_len, GFP_NOFS);
        if (!buf)
                return -ENOMEM;