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Re: [RFC][MIPS] What to do about DT_MIPS_RLD_MAP and PIE
- From: Richard Sandiford <rdsandiford at googlemail dot com>
- To: Matthew Fortune <Matthew dot Fortune at imgtec dot com>
- Cc: "Maciej W. Rozycki" <macro at codesourcery dot com>, "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>, "Moore\, Catherine \(Catherine_Moore\ at mentor dot com\)" <Catherine_Moore at mentor dot com>, Nikola Veljkovic <Nikola dot Veljkovic at imgtec dot com>
- Date: Thu, 23 Oct 2014 20:34:04 +0100
- 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> <6D39441BF12EF246A7ABCE6654B0235320F30462 at LEMAIL01 dot le dot imgtec dot org>
Matthew Fortune <Matthew.Fortune@imgtec.com> writes:
> Maciej W. Rozycki <macro@codesourcery.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
>> 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.
Thanks,
Richard