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RFC: Add SHT_GNU_PHDRS

H.J. Lu hjl.tools@gmail.com
Thu Sep 27 13:21:00 GMT 2018 
On Thu, Sep 27, 2018 at 6:07 AM, Carlos O'Donell <carlos@redhat.com> wrote:
> 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

It depends on how we define it.  I did experiment SHT_GNU_PHDRS
to cover the whole program header.  But other tools don't expect a
section covering the program header.

>> 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.
>

My current dummy program property note section sounds much better
now :-).

-- 
H.J.
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