RFC: Add SHT_GNU_PHDRS

[Carlos O'Donell]

On 9/27/18 8:57 AM, Florian Weimer wrote:
> * H. J. Lu:
> 
>> On Thu, Sep 27, 2018 at 5:42 AM, Florian Weimer <fweimer@redhat.com> wrote:
>>> * H. J. Lu:
>>>
>>>> On Thu, Sep 27, 2018 at 3:35 AM, Szabolcs Nagy <nsz@port70.net> wrote:
>>>>> an alloc .phdr section covering the program headers solves
>>>>> this problem. if sections are not required for segments
>>>>> then simply the linker should ensure that there is always
>>>>> a load segment covering the program headers, possibly
>>>>> without containing any sections, however elf says
>>>>> "An object file segment contains one or more sections".
>>>>>
>>>>> i don't understand why a zero-size section is enough, what
>>>>> if phdr > pagesize? will that get covered by the load
>>>>> segment that is created for the zero-size section?
>>>>
>>>> Linker must keep this zero-size section in output and
>>>> create a PT_LOAD segment to cover it even if it is
>>>> the only SHF_ALLOC section in the PT_LOAD segment.
>>>
>>> Based on Szabolcs' comment, I don't think the section can be zero-sized.
>>>
>>
>> Why can't we put a zero-size section in a PT_LOAD segment?
>> Of course, we need to change linker to do it.
> 
> 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.

-- 
Cheers,
Carlos.