Re: [PATCH] Use libbacktrace as libsanitizer's symbolizer

[Jakub Jelinek <jakub at redhat dot com>]

On Fri, Nov 22, 2013 at 10:19:02PM +0400, Alexey Samsonov wrote:
> On Tue, Nov 19, 2013 at 8:42 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> > Ok, here it is (untested though, because libsanitizer in gcc is older and I
> > don't have spare cycles to play with llvm).
> > Guess you need to write it up into compiler-rt buildsystem somehow,
> > and for testing add
> > -DSANITIZER_LIBBACKTRACE -I..../srcdir_of_libbacktrace -I..../builddir_of_libbacktrace -L..../builddir_of_libbacktrace/.libs -lbacktrace
> > This is against current libbacktrace (r205028 or later, before that
> > the syminfo callbacks had one less argument).
> 
> 
> Do you have an idea of how the libbacktrace symbolizer should look to
> the end user (if the static runtime was built with
> -DSANITIZER_LIBBACKTRACE)? I see the following options, none of which
> looks nice:
> 1) Force the user to provide -L/path/to/libbacktrace -lbacktrace at
> link time (otherwise one will see link errors like "undefined symbol:
> backtrace_create_state"). Certainly this is a bad user experience.
> 2) Make -fsanitize=address link flag imply "-lbacktrace" as we do for
> libpthread, librt etc. But libbacktrace may not be installed in the
> system.
> 3) Pull libbacktrace.a inside libasan.a. This means we should teach
> the build system to merge two archives (or have access to backtrace
> sources at build time and recompile them ourselves). This is doable,
> but kind of ugly and leads to funny consequences - we'll fail to build
> libbacktrace itself with ASan (or, arguably, any program which uses
> it).
> 
> I ask this because I'm slightly afraid libbacktrace symbolizer will be
> pretty undertested in the llvm tree and we may break it without
> noticing. Building it is not a problem, but making all the tests "just
> work" with it is harder.

In GCC, libbacktrace is built as a libtool convenience library only and
then linked into whatever libraries want to use it.  So indeed, the plan
is to link libbacktrace.la into libasan.so.1.0.0 and libasan.a
(and the earlier GCC patch I've posted included corresponding toplevel
configure/Makefile bits and libsanitizer Makefile/configure changes to make
it happen).  E.g. libgo.so.* links libbacktrace.la into itself too.
So, for GCC user experience, things will work automatically.

As Ian said, libbacktrace right now is not maintained as it's own separate
entity that would be individually installable etc.  So, for testing
in LLVM, if you don't want to copy over libbacktrace sources in there,
it means finding some way in the build system/configure to detect
or request libbacktrace availability (for libraries not included in
GCC tree GCC's configure typically uses --with-<package>=<path>
or --with-<package>-lib=<path>/--with-<package>-include=<path>) and just
link against it and you'd just unpack/configure/build libbacktrace
on one of your test bots.

	Jakub