Date: Sun, 22 Aug 2004 14:03:18 +0000 From: Bosko Milekic <bmilekic@FreeBSD.org> To: Paolo Pisati <flag@oltrelinux.com>, FreeBSD_Hackers <freebsd-hackers@freebsd.org> Subject: Re: Playing with mbuf in userland Message-ID: <20040822140318.GA45977@freefall.freebsd.org>
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Paolo Pisati wrote:
>Hi,
>
>i'm developing a little app that manipulates mbuf.
>Right now i'm still working on it as userland app but i
>would like to test it with some real mbufs straight
>from the stack.
>Do you know how i can get some of these structs in
>an easy way?
>I mean, is it possible to copy some of these struct from
>stack to userland?
>Or should i fake it in userland?
One way to do this would be to instrument a for-superuser-only
socket option that would copy out all of the data, including the
metadata and mbuf headers, out to userland, while taking care to
modify references within the mbufs to userland locations {*}. To do
this, in turn, you would need to obtain the userland target addresses
of all mbufs and clusters you're copying out beforehand, and overwrite
all mbufs' m_next, m_nextpkt, m_data, and in some cases, m_ext.ext_buf
references before doing the copyout in-kernel. This can be a pretty
involved copy and would require careful implementation.
{*} The data is the socket buffer is kept as an mbuf chain so this is
possible.
Another option to look into would be to implement a sysctl(8)-exported
handler that iterates over the mbuf chain and prints out the mbuf chains
in something like XML, which your userland application can then more or less
easily parse, and reproduce the chain ("fake it up") in userland. This
solution is rather attractive because you can do all sorts of things
with the intermediate-parsing-language right from the kernel, as well
as from userland (at the parsing stages). To see an example of a sysctl(8)
handler, refer to src/sys/vm/uma_core.c (the bottom) in FreeBSD 5.x.
In any case, I would be very interested in seeing what you come up with,
as this could be a very useful diagnostic tool.
-Bosko
--
Bosko Milekic <bmilekic@technokratis.com> <bmilekic@FreeBSD.org>
"For the wicked / Carry us away / Captivity require from us a song /
How can we sing king alpha's song in a strange land?" --Bob Marley
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