Re: RFC: Add PT_GNU_PROPERTY to cover .note.gnu.property section

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, Dec 12, 2018 at 8:12 AM Cary Coutant <ccoutant@gmail.com> wrote:
>
> > > > As you might expect, I support this new program header. Ideally, I'd
> > > > have liked to replace the input SHT_NOTE sections with
> > > > SHT_GNU_PROPERTY sections and dispense with all the note section
> > > > overhead, but I'll take this as a compromise.
> > >
> > > Why can't we switch to SHT_GNU_PROPERTY? My fear with combining
> > > PT_GNU_PROPERTY with SHT_NOTE is that it will be even more confusing
> >
> > There is no requirement for PT_XXX to have SHT_XXX, like PT_GNU_RELRO.
>
> But it is not normal for the linker to perform such special processing
> on an SHT_NOTE section. When a section requires special processing, it
> is customary to use a new section type. Otherwise, the linker has to
> resort to string matching on the section name. Section names in ELF
> are not supposed to have special meaning to the linker.

Good point.  It is too late now.

> > > Also I thought there was still a question whether any or all
> > > newly proposed property features and flags are actually needed
> > > as loadable segments. There is a clear overlap with the GNU
> > > Attributes (which are non-loadable). I would like to see consensus
> > > first on the new property format/flags and which are and which
> > > aren't needed as loadable properties at runtime.
> >
> > Yes, they are needed in loadable segment.  That is the main motivation
> > for GNU program property,
>
> The only properties required in a loadable segment are those that will
> be used by the loader. From what I can tell so far, the USED bits
> can't be used by the loader, so why can't they go in the GNU
> attributes section?

There are couple reasons:

1. GNU attributes section has a very different format and isn't
designed for loader.
2. X86 needs loadable properties.
3. X86 should use a uniform format for all properties.
4. USED bits can be useful.  For example, if AVX512 isn't marked as USED, OS
loader may optimize OS for this process with this info.


-- 
H.J.