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Re: [PATCH] Unzip fails without optional extended local header signature

From: Jim Meyering
Subject: Re: [PATCH] Unzip fails without optional extended local header signature
Date: Wed, 30 Nov 2011 10:57:16 +0100

Michael Gray wrote:
> I believe I've found a bug regarding the decompression of single-entry
> .zip files.
> As per the Section V.C of theĀ .ZIP File Format Specification
> (http://www.pkware.com/documents/casestudies/APPNOTE.TXT), data
> descriptors (called the extended local header in the gzip source) *may
> or may not* be preceded by a signature. Gzip always assumes this
> signature is present; if it is not, it reads the CRC and length values
> 4 bytes further into the file than it should, and the CRC and length
> checks fail even though the file is not corrupt.
> I've included a patch that works around the problem. First, it assumes
> that the signature *is not present*, as it's possible that the
> signature value is also a valid CRC, and no non-corrupt file should be
> rejected if it's CRC just happens to match the signature value. If the
> CRC or length check fails assuming the signature is not present, the
> signature is then checked for. If present, 4 more bytes of input are
> read, and the previously read values are shifted appropriately. The
> CRC and length checks then proceed as normal.
> Below is the text of the relevant text from the .ZIP spec:
> "
>       Although not originally assigned a signature, the value
>       0x08074b50 has commonly been adopted as a signature value
>       for the data descriptor record.  Implementers should be
>       aware that ZIP files may be encountered with or without this
>       signature marking data descriptors and should account for
>       either case when reading ZIP files to ensure compatibility.
>       When writing ZIP files, it is recommended to include the
>       signature value marking the data descriptor record.  When
>       the signature is used, the fields currently defined for
>       the data descriptor record will immediately follow the
>       signature.
> "
> -- Michael Gray
> address@hidden

Thank you for the analysis and patch.
However, I haven't looked at it yet, in case I have to
rewrite it based solely on your description.

Can you point to tools that produce ZIP files without that signature?

Assuming that we find a few that are still in non-trivial use,
we'll need a copyright assignment, since your patch is large enough to
require that.  Can you sign one?  If so, here are some details:
[that link is for the coreutils package, but it's the same policy for gzip]


Note: if you're in the US, you should be able to fax the signed form
rather than using actual stamp and envelope.


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