Re: RFC: Add SHT_GNU_PHDRS

[Carlos O'Donell <carlos at redhat dot com>]

On 9/27/18 9:20 AM, H.J. Lu wrote:
> 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.

Which other tools? Specific examples please.

The main problem we have to solve is:

* Segfault when trying to access program headers which are expected to be
  mapped in by the leading pages of the PT_LOAD segment.

We can't solve *all* the problems.

The correct solution to the above is to improve the semantics that the
toolchain relies upon to map the phdrs.

Some questions which we might get asked is:

* How does a running program know it's *safe* to look at it's own phdrs?

* How many downstream tools are impacted? Do they really need to understand
  SHT_GNU_PHDRS?

>>> 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 :-).
 
My apologies HJ, I did not intend this to sound like an attack on your
original design, just that a new design like SHT_GNU_PHDRS could be
created with reliable semantics.

-- 
Cheers,
Carlos.