Re: Adding libxml2 as optional Wireshark dependency

[Graham Bloice <graham.bloice () trihedral com>]

On 5 April 2017 at 14:11, Ahmad Fatoum <ahmad () a3f at> wrote:

> Hello everyone,
>
> I was advised on Gerrit to post this issue here as to garner wider input.
>
> This concerns proposed Change-Id I13c0a2f408fb5c21bad7ab3d7971e0fa8ed7d783
> [1] intending to add libxml2 as optional dependency to Wireshark.
>
> I am currently preparing to submit upstream, changes I did to the EPL v2
> dissector (packet-epl.c).
>
> A significant change is the ability to optionally read in user-supplied
> XML device descriptions and to extract type/description/mapping information
> for aiding the dissection. See this previous submission of mine to the
> mailing list: https://www.wireshark.org/lists/wireshark-dev/
> 201701/msg00154.html
>
>
> Seeing as there also has been interest for libxml2 support in dissectors
> in the past:
>
> https://www.wireshark.org/lists/wireshark-dev/201005/msg00108.html
>
> https://ask.wireshark.org/questions/36063/using-libxml2-
> in-my-own-dissector
>
>
> I think, it would be a good idea to have this as optional dependency as
> Glib's GMarkup may be inadequate or inconvenient for parsing actual XML.
>
>
> Looking forward to your feedback.
>
> Best regards,
> Ahmad Fatoum
>
> [1] https://code.wireshark.org/review/#/c/20912/
Thanks for the post,

1.  Where will the Windows binaries come from and are these supported long
term?  The  libXml2 downloads page indicates another site provides Windows
binaries [1].  The binaries at that site in the 64 bit directory seem to be
the most recent and are labelled as libXml2-2.9.3 [2].  The current release
of libXml2 is 2.9.4 which has a number of security fixes among other bug
fixes and enhancements [3] so it would appear that the Windows binaries are
not being maintained.

2.  According to the diagram at [1], libXml2 depends on iconv and zlib.  We
currently build our own zlib, will that be suitable for the libXml2
dependency?  What will be the source of the iconv binary (iconv-1.14 is
available in the same download area as libXml2 [2])?

3. The readme.txt in the download area ([2]) has some "interesting" text:

These are experimental 64bit binaries. For completeness, 32bit binaries
built using the same method are also included.

The libraries in these packages are made using GCC (MinGW) toolchain. It is
presently not possible to use these libraries with any recent version of the
Microsoft Visual C compiler because of conflicting C-runtimes. To help you
resist the temptation, the import libraries (.LIB) are not provided at all.
If you need these libraries in an environment which mandates the use of the
Microsoft toolchain, you will have to build them from source yourself.

and inspection of the download shows this is true, so it appears that we'll
need to rebuild to obtain the import .lib file.

4. Microsoft have a Visual Studio porting effort underway called vcpkg [4],
that does include libXml2, but unfortunately is only for VS2015 or later.
If we move to VS2015 for main releases (post 2.4 release) then this may be
a viable source for libXml2 and other packages we use.  It might be
possible to use this to build VS2013 libXml2.

5.  Are there any manufacturers or tools that produce XML device
description files for the EPL dissector such that choosing XML as the input
format is the most sensible choice, or would another format be just as
applicable?


[1]: https://www.zlatkovic.com/libxml.en.html
[2]: ftp://ftp.zlatkovic.com/libxml/64bit/
[3]: http://xmlsoft.org/news.html
[4]: https://github.com/Microsoft/vcpkg

-- 
Graham Bloice

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