RE: Debugging MIPS PIE executables

[Matthew Fortune <Matthew dot Fortune at imgtec dot com>]

Felix Radensky <felix.radensky@broadcom.com> writes:
> > -----Original Message-----
> > From: Matthew Fortune [mailto:Matthew.Fortune@imgtec.com]
> > Sent: Tuesday, June 16, 2015 3:07 PM
> > To: Felix Radensky; binutils@sourceware.org
> > Subject: RE: Debugging MIPS PIE executables
> >
> > Felix Radensky <felix.radensky@broadcom.com> writes:
> > > > -----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.
> >
> > Core files would be new territory for me so it will take a bit of exploring to
> > understand what piece of the puzzle is missing. Presumably the core file carries
> > some indication of where the r_debug map is and that is not getting set
> > correctly when generating the core file.
> >
> > I'll switch this over to the GDB list if I get stuck.
> >
> >
> 
> Actually core file inspection works fine for me with native GDB on the target, as well as
> debugging with native GDB. It's only the core file inspection in cross GDB that doesn't
> work.

OK, the penny drops. For some reason I thought the cross build of GDB would use
the local headers in binutils/gdb but having thought harder that couldn't possibly
be true. The problem is that the host's elf.h won't have DT_MIPS_RLD_MAP2 defined
so for a cross build this will have to be defined within the solib-svr4.c perhaps
as an #ifndef DT_MIPS_RLD_MAP2 #define...

thanks,
Matthew