Re: [PATCH v2] crypto: deflate - fix decompression window size

[Giovanni Cabiddu <giovanni.cabiddu@intel.com>]

Hi Herbert,

On Fri, Apr 03, 2026 at 08:25:06AM +0800, Herbert Xu wrote:
> On Thu, Mar 26, 2026 at 09:59:22AM +0000, Giovanni Cabiddu wrote:
> > deflate_decompress() initializes the inflate stream with windowBits set
> > to -DEFLATE_DEF_WINBITS (11 bits, 2KB window). Valid raw DEFLATE streams
> > allow window sizes up to MAX_WBITS (15 bits, 32KB).
> > 
> > Data compressed with a history window larger than 2 KB, for example
> > produced by hardware compressors such as QAT or IAA, might not be
> > decompressed by deflate-generic since the inflate stream is initialized
> > with a 2 KB window. This might be seen, for example, when
> > deflate-generic is used as fallback.
> > 
> > Use -MAX_WBITS when calling zlib_inflateInit2() to accept all valid raw
> > DEFLATE streams. The inflate workspace allocated in deflate_alloc_stream()
> > is already sized using zlib_inflate_workspacesize(), which accounts for
> > the maximum window size, so no allocation change is needed.
> > 
> > Fixes: f6ded09de8bd ("crypto: acomp - add support for deflate via scomp")
> > Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
> > Reviewed-by: Laurent M Coquerel <laurent.m.coquerel@intel.com>
> > ---
> > Changes since v1:
> > - Updated commit message to clearly state why this is needed for the
> >   deflate algorithm (i.e. allow data produced by HW compressors with
> >   larger history windows to be decompressed by deflate-generic, which
> >   is used as fallback).
> > - Updated fixes tag to point to the commit that introduced deflate
> >   support in scomp.
> > 
> >  crypto/deflate.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> What happened to the parameters patch-set?
I'm reworking the acomp/BTRFS set, and that will be included there.

> Wouldn't this be something that should be treated as a parameter?
I don't think this should be treated as a parameter. A decompressor must
be able to handle any valid DEFLATE stream. RFC1951 (section 3.3) [1]
states that while a compressor may restrict parameters such as window
size, a compliant decompressor must accept the full range defined by the
specification.

In this case, deflate-generic does not accept valid streams compressed
with the full 32KB history window, which is why I proposed this change.

[1] https://datatracker.ietf.org/doc/html/rfc1951

Regards,

-- 
Giovanni