From: Carlos O'Donell <carlos@redhat.com>
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: Florian Weimer <fweimer@redhat.com>,
Szabolcs Nagy <nsz@port70.net>, Jan Beulich <JBeulich@suse.com>,
Rich Felker <dalias@libc.org>, Binutils <binutils@sourceware.org>,
gnu-gabi@sourceware.org
Subject: Re: RFC: Add SHT_GNU_PHDRS
Date: Mon, 01 Jan 2018 00:00:00 -0000 [thread overview]
Message-ID: <0d529e75-beb2-3ba6-f4cc-e99d50880220@redhat.com> (raw)
In-Reply-To: <CAMe9rOrW38N+2H-bVGrFaZYiHoRSSzCvzaazuyHFDE24rxDRUQ@mail.gmail.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.
next prev parent reply other threads:[~2018-09-27 15:18 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-01 0:00 H.J. Lu
[not found] ` <CAORpzuOWtHeqBLEE+MMN4-TZyp6Z1r-MdmyNv7Zj-BhxMstr=g@mail.gmail.com>
2018-01-01 0:00 ` H.J. Lu
2018-01-01 0:00 ` Florian Weimer
2018-01-01 0:00 ` Jan Beulich
2018-01-01 0:00 ` Florian Weimer
2018-01-01 0:00 ` Szabolcs Nagy
2018-01-01 0:00 ` Florian Weimer
2018-01-01 0:00 ` Cary Coutant
2018-01-01 0:00 ` H.J. Lu
2018-01-01 0:00 ` Florian Weimer
2018-01-01 0:00 ` H.J. Lu
2018-01-01 0:00 ` Florian Weimer
2018-01-01 0:00 ` Carlos O'Donell
2018-01-01 0:00 ` Cary Coutant
2018-01-01 0:00 ` H.J. Lu
2018-01-01 0:00 ` Rich Felker
2018-01-01 0:00 ` H.J. Lu
2018-01-01 0:00 ` Carlos O'Donell [this message]
2018-01-01 0:00 ` H.J. Lu
2018-01-01 0:00 ` Carlos O'Donell
2018-01-01 0:00 ` Michael Matz
2018-01-01 0:00 ` Florian Weimer
2018-01-01 0:00 ` Michael Matz
2018-01-01 0:00 ` Carlos O'Donell
2018-01-01 0:00 ` H.J. Lu
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