Re: [PATCH v3 4/5] archive-zip: support archives bigger than 4GB

From: René Scharfe <l.s.r@web.de>
Date: Apr 25, 2017, 16:24 UTC
Message-ID: <fdc17512-94dc-4f7f-4fd3-f933e1b18e8f@web.de>
In-Reply-To: <alpine.DEB.2.11.1704250851420.23677@perkele.intern.softwolves.pp.se>

Am 25.04.2017 um 09:55 schrieb Peter Krefting:

Show 18 quoted lines

> René Scharfe:
> 
>>> This needs to be >=. The spec says that if the value is 0xffffffff, 
>>> there should be a zip64 record with the actual size (even if it is 
>>> 0xffffffff).
>> Could you please cite the relevant part?
> 
> 4.4.8 compressed size: (4 bytes)
> 4.4.9 uncompressed size: (4 bytes)
> 
> "If an archive is in ZIP64 format and the value in this field is 
> 0xFFFFFFFF, the size will be in the corresponding 8 byte ZIP64 extended 
> information extra field."
> 
> 
> Of course, there is no definition of how they define that "an archive is 
> in ZIP64 format", but I would say that is whenever it has any ZIP64 
> structures.

I struggled with that sentence as well. There is no explicit "format" field AFAICS. The closest at the archive level are zip64 end of central directory record and locator. But what really matters is the presence of a zip64 extended information extra field to hold the 64-bit size value.

There's also this general note a bit higher up:

       "4.4.1.4  If one of the fields in the end of central directory
       record is too small to hold required data, the field should be
       set to -1 (0xFFFF or 0xFFFFFFFF) and the ZIP64 format record
       should be created."

My interpretation: An archiver that can only emit 32-bit ZIP files (either because it doesn't support ZIP64 or due to a compatibility option set by the user) writes 32-bit size fields and has no defined way to deal with overflows. An archiver that is allowed to use ZIP64 can emit zip64 extras as needed.

Or in other words: A legacy ZIP archive and a ZIP64 archive can be bit-wise the same if all values for all entries fit into the legacy fields, but the difference in terms of the spec is what the archiver was allowed to do when it created them.

	# 4-byte sizes, not ZIP64
	arch --format=zip ...

	# ZIP64, can use 8-byte sizes as needed
	arch --format=zip64 ...

Makes sense?

René