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RE: [RFC][MIPS] What to do about DT_MIPS_RLD_MAP and PIE
- From: Matthew Fortune <Matthew dot Fortune at imgtec dot com>
- To: "Maciej W. Rozycki" <macro at codesourcery dot com>
- Cc: "binutils at sourceware dot org" <binutils at sourceware dot org>, "libc-alpha at sourceware dot org" <libc-alpha at sourceware dot org>, "Joseph Myers (joseph at codesourcery dot com)" <joseph at codesourcery dot com>, Richard Sandiford <rdsandiford at googlemail dot com>, "Moore, Catherine (Catherine_Moore at mentor dot com)" <Catherine_Moore at mentor dot com>, Nikola Veljkovic <Nikola dot Veljkovic at imgtec dot com>
- Date: Wed, 22 Oct 2014 18:57:58 +0000
- Subject: RE: [RFC][MIPS] What to do about DT_MIPS_RLD_MAP and PIE
- Authentication-results: sourceware.org; auth=none
- References: <6D39441BF12EF246A7ABCE6654B0235320F3027E at LEMAIL01 dot le dot imgtec dot org> <alpine dot DEB dot 1 dot 10 dot 1410221755430 dot 7896 at tp dot orcam dot me dot uk>
Maciej W. Rozycki <email@example.com> writes:
> On Wed, 22 Oct 2014, Matthew Fortune wrote:
> > Currently binutils will not emit a DT_MIPS_RLD_MAP entry for PIE
> > as the condition for emitting this is '!shared' rather than
> > 'executable'. If binutils were to start emitting DT_MIPS_RLD_MAP
> > for PIE then ld.so (and GDB) would need to calculate the real
> > address of the debug pointer by adding the base of the executable
> > to the offset in the DT_MIPS_RLD_MAP entry. This is possible
> > but will result in new PIE binaries crashing ld.so as the value
> > of DT_MIPS_RLD_MAP is dereferenced directly.
> > Realistically I don't think there is any way to avoid this crash
> > if we start emiting DT_MIPS_RLD_MAP for PIE. So we either have
> > to take that as a known issue (and do the work in ld.so
> > and gdb/gdbserver to account for PIE) OR we choose to define
> > the ABI for MIPS PIE to use DT_DEBUG instead of DT_MIPS_RLD_MAP
> > and allow the dynamic section to be writable for PIE (and only
> > PIE). The dynamic linker logic would be to fill out
> > DT_MIPS_RLD_MAP if it is present and iff not then fill out
> > DT_DEBUG if it is present. GDB probably wouldn't need any
> > changes but I didn't check in detail.
> > There's work to do either way but which one is 'better'?
> Or we could define an entirely new dynamic tag that'll be ignored by old
> software. It's not like we're short of them. It can point to the same
> area DT_MIPS_RLD_MAP would; in fact we can even produce both at a time for
> maximum backwards compatibility, and maybe go as far as to rename the old
> macro and use the old name with the new tag. Deliberately letting newer
> binaries crash with older software sounds like an awful design decision to
That sounds like a reasonable plan too. I wasn't expecting to go with the
let it crash plan but all options need to be presented.
I'm not sure about using the old name with a new tag as it is listed in the
original psABI I believe. For safety a PIE could only get the new tag rather
than both of course but a standard executable could have both I guess.
I'm a bit stumped as to what the technical benefit was/is about having a read
only dynamic section. Does anyone know?
I think overall Maciej's suggestion of a new tag sounds like the most
pleasant option so far as it doesn't need .dynamic to be writable and is
therefore more similar to non-PIE.