RE: Debugging MIPS PIE executables

[Felix Radensky <felix dot radensky at broadcom dot com>]

Hi Mattew,

> -----Original Message-----
> From: Matthew Fortune [mailto:Matthew.Fortune@imgtec.com]
> Sent: Monday, June 15, 2015 9:03 PM
> To: Felix Radensky; binutils@sourceware.org
> Subject: RE: Debugging MIPS PIE executables
> 
> Felix Radensky <felix.radensky@broadcom.com> writes:
> > Hi Mattew
> >
> > > -----Original Message-----
> > > From: Matthew Fortune [mailto:Matthew.Fortune@imgtec.com]
> > > Sent: Saturday, June 13, 2015 11:10 AM
> > > To: Felix Radensky; binutils@sourceware.org
> > > Subject: RE: Debugging MIPS PIE executables
> > >
> > > Felix Radensky <felix.radensky@broadcom.com> writes:
> > > > Hi Matthew,
> > > >
> > > > > -----Original Message-----
> > > > > From: Matthew Fortune [mailto:Matthew.Fortune@imgtec.com]
> > > > > Sent: Sunday, June 07, 2015 11:10 AM
> > > > > To: Felix Radensky; binutils@sourceware.org
> > > > > Subject: RE: Debugging MIPS PIE executables
> > > > >
> > > > > Hi Felix,
> > > > >
> > > > > Felix Radensky <felix.radensky@broadcom.com> writes:
> > > > > > I was wondering if any progress was made to resolve the
> > > > > > problem discussed in the following threads:
> > > > > >
> > > > > > https://sourceware.org/ml/binutils/2013-06/msg00183.html
> > > > > > https://sourceware.org/ml/binutils/2014-10/msg00200.html
> > > > > >
> > > > > > Are there any experimental patches for binutils/gdb/glibc that
> > I
> > > > can
> > > > > > try ? PIE executables are must in many security-conscious
> > > > > > projects, and the lack of ability to debug them is a serious
> > problem.
> > > > >
> > > > > This request has come up a few times again recently and I am
> > > > > hoping
> > > > to
> > > > > submit patches for this in the coming week. I still have not
> > > > > managed to build a PIE version of glibc which is a bit of a
> > > > > blocker. I'm currently trying to use HJ's recent patches to
> > > > > support PIE by default from GCC but now have link assertions in
> > > > > glibc instead of reloc
> > > > failures.
> > > > >
> > > > > I have binutils and gdb patches that have been used with bionic
> > > > > but they still need some cleaning up.
> > > >
> > > > Thanks a lot for a prompt reply. I'm looking forward to test your
> > > > patches.
> > >
> > > I've attached the patches I intend to submit but I still have to
> > > work through the binutils testsuite to account for the new tag and
> > > try to
> > write a useful test case.
> > > I'll also need to get advice on doing a GDB test for this as I don't
> > > know if/how to do that.
> > >
> > > The patches are manually tested for o32/n32/n64 PIE and ordinary
> > > executables with the expected behaviour that new PIEs with new ld.so
> > > show the correct library list.
> > >
> >
> > I've build a MIPS o32 little-endian toolchain using crosstool-ng-
> > 1.21.0, with gcc-5.1.0, binutils-2.25 and gdb-7.9.1, and your patches
> > applied. However the problem still exists, I'm unable to debug PIE
> > executables with shared libraries or inspect core files produced by
> > such executables. The shared library is not listed by "info
> > sharedlibrary", the backtrace cannot be extracted from core file. I
> > can send you my crosstool-ng configuration file and the trivial
> > application I use for testing.
> 
> The support in the GDB side of this depends on the elf.h header having the
> DT_MIPS_RLD_MAP2 macro defined. Are you using native GDB or remote with
> the gdbserver? The gdbserver does the work for finding shared library debug if
> you are using a remote so has to be built from the patched sources.
> 
> Debugging tips would be to firstly check that your PIE has the
> DT_MIPS_RLD_MAP2 tag using readelf. To get a textual description of the tag
> you will have to use readelf built from the patched binutils source.
> 
> Then double check that GDB has been built against headers that define
> DT_MIPS_RLD_MAP2, the GLIBC patch includes changes to add this to the
> headers.
> 

Thanks for the tips. I've verified that my PIE executable has DT_MIPS_RLD_MAP2 tag. Also recompiled cross GDB, gdbserver and native GDB with DT_MIPS_RLD_MAP2 defined. I can now successfully debug my PIE executable using gdbserver and cross GDB. What I still cannot do is to debug using native GDB and to inspect core files using cross GDB. 

Felix.