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Re: [RFC][MIPS] What to do about DT_MIPS_RLD_MAP and PIE

Matthew Fortune <> writes:
> Maciej W. Rozycki <> 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 (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 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
>> > 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
>> me.
> 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.

Sounds good to me too FWIW.


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