Re: RFC: Add SHT_GNU_PHDRS

[Szabolcs Nagy <nsz at port70 dot net>]

* Florian Weimer <fweimer@redhat.com> [2018-09-27 10:21:40 +0200]:
> * Jan Beulich:
> >>>> On 27.09.18 at 07:01, <fweimer@redhat.com> wrote:
> >> * H. J. Lu:
> >> 
> >>> I am proposing
> >>>
> >>> #define SHT_GNU_PHDRS  0x6ffffff4  /* Dummy section for program header */
> >>>
> >>> This is a special read-only SHF_ALLOC zero-size data section.  It is the
> >>> first output section, which will force a data PT_LOAD segment with program
> >>> header before the code-only PT_LOAD segment,
> >> 
> >> Is it actually a requirement in the ELF specification that all bits
> >> loaded via segments are covered by sections as well?
> >
> > Hardly, because the presence of a section table isn't required
> > in the first place in an executable (iirc).
> 
> I think so too, and that is why I don't understand this section hack is
> needed.

if there is no read-only alloc section then the program
headers are currently not part of a load segment.
https://sourceware.org/bugzilla/show_bug.cgi?id=23428

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?