docs/internals/why-no-libc.txt

ate: Mon, 15 Dec 2008 15:23:31 -0500
From: Stephen McCamant <smcc@CSAIL.MIT.EDU>
To: Igor Shaul <mindmaze@gmail.com>
Cc: valgrind-developers@lists.sourceforge.net
Subject: Re: [Valgrind-developers] Using standard C library in valgrind tool

RW> On Dec 12, 2008, at 6:33 PM, "Igor Shaul" <mindmaze@gmail.com> wrote:

IS> Hi,
IS> I would like to write a valgrind tool that uses the standard c
IS> library (actually, I must link my tool to another library, which
IS> happens to use stdlib). I noticed that all the tools link with -
IS> nodefaultlibs flag, and if said flag is removed, then naturally no
IS> main() is found (stdlib requires a main). So, is there a natural
IS> way to use stdlib in my valgrind tool?

>>>>> "RW" == Robert Walsh <rjwalsh@durables.org> writes:

RW> Sadly, no. Valgrind shares the address space of the guest process,
RW> which would mean libc would get linked into the address space
RW> twice.  There's no telling how libc would react to that.

Though I agree that the short answer is "sorry, that's not supported",
I thought you might find a few more technical details helpful in
considering what to do.

Valgrind tools and the guest processes do run in the same address
space in terms of memory management, but in current versions they
don't share any dynamic linker context, and in fact Valgrind tools
don't link with libc at all, so there wouldn't be a double-linking
problem per se.

However, there are some incompatibilities between Valgrind and glibc
that are the reason Valgrind tools don't link with the standard
libraries. The most fundamental one is that Valgrind and glibc are
both designed with the assumption that they alone will be talking to
the kernel on behalf of their process, but obviously this can't be
true for both at once.

As of a few years ago, it was still possible (though unsupported) to
just link your tool directly with /usr/lib/libc.a, and it worked for
at least a reasonable subset of programs and glibc functionality. I
research tool I was working on did that for a while. However, we gave
that up because of a further issue that affects glibcs configured with
thread-local storage (which I think is standard these days). Glibc now
uses a segment pointed to by %gs to keep TLS, other thread data,
-fstack-protector canary values, and who knows what else. It relies on
its startup code to initialize this correctly, so if you call many
glibc functions before initializing this, it crashes. That's the point
where we gave up.

In theory, I don't think any of these Valgrind/glibc incompatibilities
are fundamental, and there would be ways of hacking around them. But
the glibc and Valgrind developers don't consider them bugs, and so
probably aren't interesting it expending much effort to fix them.

So if you need C standard library functionality that isn't covered by
the Valgrind core's somewhat non-standard subset, you'll have to get
it from somewhere else. What we found to be the easiest approach in
our Fjalar tool was to cut and paste the particular functions we need
from dietlibc (a nice lightweight implementation) or glibc itself. The
code is GPLed if you want to reuse it.

http://groups.csail.mit.edu/pag/fjalar/

Hope this helps,

 -- Stephen