Re: [RFC] CryptoAPI & Compression

Herbert Xu
2005-03-31T09:57:13Z

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On Mon, Mar 28, 2005 at 05:22:36PM +0000, Artem B. Bityuckiy wrote:
>
> I made the changes to deflate_decompr() because the old version doesn't
> work properly for me. There are 2 changes.
>
> 1. I've added the following code:
>
> ------------------------------------------------------------------------
> if (slen > 2 && !(src[1] & PRESET_DICT) /* No preset dictionary */
> && ((src[0] & 0x0f) == Z_DEFLATED) /* Comp. method byte is OK */
> && !(((src[0] << 8) + src[1]) % 31)) { /* CMF*256 + FLG */
> stream->next_in += 2;
> stream->avail_in -= 2;
> }
> ------------------------------------------------------------------------

The reason you need to add this is because the window bits that
was used to produce the compressed data is positive while the window
bits crypto/deflate is using to perform the decompression isn't.

So what we should do here is turn window bits into a configurable
parameter.

Once you supply the correct window bits information, the above is
then simply an optimisation.

Rather than keeping the above optimisation, JFFS should simply do
the compression with a negative window bits value.

Of course to maintain backwards compatibility you'll need to do this
as a new compression type.

> 2. I've removed the "strange" (for me) uncompress sequence:
>
> ------------------------------------------------------------------------
> ret = zlib_inflate(stream, Z_SYNC_FLUSH);
> /*
> * Work around a bug in zlib, which sometimes wants to taste an extra
> * byte when being used in the (undocumented) raw deflate mode.
> * (From USAGI).
> */

I believe this bit of code originally came from FreeS/WAN and was
written by Svenning Sørensen. Maybe he or Yoshifuji-san can tell
us why?

Unless we're sure that zlib has been fixed we should leave it in.
It should be a no-op if zlib has been fixed. So this probably
isn't causing the breakage that you saw.

Cheers,
--
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Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
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