RFC: Add SHT_GNU_PHDRS

[H.J. Lu]

On Thu, Sep 27, 2018 at 3:34 PM, Cary Coutant <ccoutant@gmail.com> wrote:
>> > I'm now under the impression that the bits that are PT_LOAD'ed all need
>> > to be covered by (allocated) sections.  A zero-sized section doesn't
>> > cover anything, so it doesn't address this requirement of the ELF
>> > specification.
>>
>> I agree. What we did in the past by relying on phdrs to be accidentally
>> in the first PT_LOAD segment always irked me as bad design.
>>
>> If we need access to program header we need clear semantics for doing so,
>> not hackish kludges to force the linker to get it onto a page that also
>> happened to be mapped. This is just poor engineering on our part.
>
> It seems to me that the kernel loader should make the program headers
> available to the dynamic loader through the aux vector, whether
> they're part of a PT_LOAD segment or not. That should be part of the
> psABI. The gABI clearly requires that the dynamic loader has access to
> the program headers (e.g., it needs to find PT_DYNAMIC), but it
> doesn't care how the implementation accomplishes that.
>

Dynamic loader has no problem.  The problem is kernel passes
AT_PHDR to main, which points to the unmmaped address.   We can
ask for kernel change or make kernel happy.

My current .note.gnu.property patch only works for x86.   We can
add

#define GNU_PROPERTY_PHDRS 3

so that it can be used for all targets.


-- 
H.J.