Re: RFC: Add GNU_PROPERTY_NEED_PHDRS

[Alan Modra <amodra at gmail dot com>]

On Wed, Oct 03, 2018 at 04:46:10PM -0700, Cary Coutant wrote:
> > > > 3. Ld won't create a PT_LOAD segment just to hold phdrs.
> > >
> > > You seem to be breezing right past the idea of doing exactly this.
> > > Why? The scripting language already allows you to declare which
> > > segment should include FILEHDR and PHDRS. For -z separate-code, why
> > > not use a default linker script with something like the following?
> > >
> > >      PHDRS
> > >      {
> > >        headers PT_PHDR PHDRS ;
> > >        interp PT_INTERP ;
> > >        header_seg PT_LOAD FILEHDR PHDRS ;
> > >        text PT_LOAD ;
> > >        data PT_LOAD ;
> > >        dynamic PT_DYNAMIC ;
> > >      }
> >
> > The script idea is probably not practical in view of all the
> > variations of headers we'd need.  PT_NOTE, PT_TLS, PT_GNU_EH_FRAME,
> > PT_GNU_STACK, PT_GNU_RELRO come to mind, some of which depend on
> > executable contents.
> 
> I don't understand why you say it's not practical. How does the number
> of variations of individual program header table entries matter?

If you specify PHDRS in a script, ld uses exactly those program
headers, and I believe it should continue to operate that way.  So to
add PT_GNU_EH_FRAME, say, when an executable has .eh_frame_hdr, you'd
need a script with PT_GNU_EH_FRAME specified in PHDRS.  And another
without PT_GNU_EH_FRAME when the executable doesn't have
.eh_frame_hdr.  That soon becomes impractial considering the number of
optional headers, requiring auto-generation of the PHDRS script
snippet.

-- 
Alan Modra
Australia Development Lab, IBM