[PATCH v2] elf: Don't set .note.GNU-stack type to SHT_NOTE

[H.J. Lu]

On Wed, Jan 14, 2026 at 1:33 AM Michael Matz <matz@suse.de> wrote:
>
> Hello,
>
> On Tue, 13 Jan 2026, Jan Beulich wrote:
>
> > >> Sam question to you then: Where exactly is this said in the spec? What am
> > >> I overlooking?
> > >
> > > https://gabi.xinuos.com/elf/07-pheader.html#note-sections
> >
> > Well, that's a different incarnation from what I usually use, but the
> > wording is very similar if not identical. Hence my question remains:
> > Where is it said there that zero entries isn't permitted? It says "holds
> > a variable amount of entries" (none at all fitting this well) and "each
> > entry is an array of {4,8} -byte words". Hence my request: Please give
> > an exact quotation that makes clear that "empty" isn't legal.
>
> It was clear to me from that language in the spec.  With your
> clarification on how you read it, yes I can see that that is a valid
> reading as well, maybe more valid even; i.e. zero sized section would
> contain no entries.  Hmm.
>
> > Your reference to it having been around since 2003 also is only partly
> > relevant. Iirc you were involved in the internal bug where suddenly a
> > helper component of Xen didn't work anymore.
>
> Ah, maybe that was causing the deja-vu, indeed.
>
> > > So you did find the section that mandates the header of SHT_NOTE
> > > sections, yet you are still convinved that a zero-length SHT_NOTE
> > > section is okay?  I'm confused.
> >
> > See above - I simply can't spot any statement to the effect of what
> > you're claiming. (And strictly speaking there's no "header of SHT_NOTE
> > sections" anyway. Every "entry" has to have such a header. Then: No
> > "entry", no header.
>
> I'm inclined to reverse my opinion from upthread.  I.e. from a pure
> specification perspective zero-sized SHT_NOTE sections are okay
> (corresponding to a no-entry note section/segment).  It will then
> automatically contribute into the PT_NOTE segment, but harmlessly so, as
> it contains no entries.  It will also continue to cause generation of the
> PT_GNU_STACK program header based on its name, exactly what we want.
>
> There are more warts of .note.GNU-stack that need at least a little
> pondering:
> (a) the usage of section flags to convey information, i.e. being
>     SHF_EXECINSTR to mean needing an executable stack.
>     It's slightly surprising to find such flag on a SHT_NOTE section.
>     (And what does it mean for the resulting PT_NOTE segment?)
> (b) in .s files the .note.GNU-stack section is often generated by
>     .section .note.GNU-stack,"",@progbits
>     it may be surprising if the resulting .o file has a SHT_NOTE type,
>     instead of the explicitely requested SHT_PROGBITS
>
> I think none of those would hinder the usage of SHT_NOTE for
> .note.GNU-stack.  (a) shouldn't pose a problem anywhere, and the surprise
> is very limited.  For (b) the asm directive should probably continue to
> then make the gnu-stack section be SHT_PROGBITS, but either way the link
> editor needs to continue accepting PROGBITS .note.GNU-stack anyway, as a
> compatility measure.
>
> Were there other reasons to reject the idea to .note.GNU-stack being
> SHT_NOTE (i.e. besides a presumed invalidity of zero-sized note sections
> and contribution into PT_NOTE)?  If there weren't I think I now agree with
> you that automatically generated .note.GNU-stack sections (or those not
> explicitely requesting a section type) can have type SHT_NOTE.
>
> But the question remains if it _should_ have such?  Due to the warts above
> .note.GNU-stack will always need special handling somewhere, so changing
> the section type doesn't get rid of all of them (and perhaps adds some
> more special casing for the PT_NOTEs flags).  I don't know the answer to
> that, I can see arguments both ways :-/
>

GCC generates:

.section .note.GNU-stack,"",@progbits

.note.GNU-stack type has been SHT_PROGBITS.   According to gABI,
an empty SHT_NOTE section doesn't contribute anything.   A linker can
discard an empty SHT_NOTE section.   Can someone show benefits of
changing .note.GNU-stack to SHT_NOTE?

-- 
H.J.