On Wed, Dec 21, 2022 at 12:42 PM Tom Saeger <tom.saeger@xxxxxxxxxx> wrote:
>
> On Wed, Dec 21, 2022 at 11:56:33AM -0800, Nick Desaulniers wrote:
> > On Thu, Dec 15, 2022 at 3:18 PM Tom Saeger <tom.saeger@xxxxxxxxxx> wrote:
> > >
> v1 cover has a simple example if someone has capability/time to adapt to
> another architecture.
>
> - enable CONFIG_MODVERSIONS
> - build
> - readelf -n vmlinux
Keep this info in the commit message.
>
> >
> > >
> > > Linus's tree doesn't have this issue since 0d362be5b142 was merged
> > > after df202b452fe6 which included:
> > > commit 7b4537199a4a ("kbuild: link symbol CRCs at final link, removing CONFIG_MODULE_REL_CRCS")
> > >
> > > This kernel's KBUILD CONFIG_MODVERSIONS tooling compiles and links .S targets
> > > with relocatable (-r) and now (-z noexecstack)
> > > which results in ld adding a .note.GNU-stack section to .o files.
> > > Final linking of vmlinux should add a .NOTES segment containing the
> > > Build ID, but does NOT (on some architectures like arm64) if a
> > > .note.GNU-stack section is found in .o's supplied during link
> > > of vmlinux.
> >
> > Is that a bug in BFD? That the behavior differs per target
> > architecture is subtle. If it's not documented behavior that you can
> > link to, can you file a bug about your findings and cc me?
> > https://sourceware.org/bugzilla/enter_bug.cgi?product=binutils
>
> I've found:
> https://sourceware.org/bugzilla/show_bug.cgi?id=16744
> Comment 1: https://sourceware.org/bugzilla/show_bug.cgi?id=16744#c1
>
> "the semantics of a .note.GNU-stack presence is target-dependent."
I wonder if that's an observation, or a statement of intended design.
A comment in a bug tracker is perhaps less normative than explicit
documentation.
Probably doesn't hurt to include that link in the commit message as well.
>
> corresponding to this commit:
> https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=76f0cad6f4e0fdfc4cfeee135b44b6a090919c60
Seems x86 specific...
>
> So - I'm not entirely sure if this is a bug or expected behavior.
Nick Clifton is cc'ed and might be able to provide more details
(holiday timing permitting; no rush).
>
> >
> > If it is a bug in BFD, then I'm not opposed to working around it, but
> > it would be good to have as precise a report as possible in the commit
> > message if we're going to do hijinks in a stable-only patch for
> > existing tooling.
> >
> > If it's a feature, having some explanation _why_ we get per-arch
> > behavior like this may be helpful for us to link to in the future
> > should this come up again.
>
> While I agree - *I* don't have an explanation (despite digging), only
> work-arounds.
That's fine. That's why I'd rather have a bug on file that we link to
stating we're working around this until we have a more definitive
review of this surprising behavior. Please file a bug wrt. this
behavior.
https://sourceware.org/bugzilla/enter_bug.cgi?product=binutils
>
> >
> > >
> > > DISCARD .note.GNU-stack sections of .S targets. Final link of
> >
> > That's going to give them an executable stack again.
> > https://www.redhat.com/en/blog/linkers-warnings-about-executable-stacks-and-segments
> > >> missing .note.GNU-stack section implies executable stack
> > The intent of 0d362be5b142 is that we don't want translation units to
> > have executable stacks, though I do note that assembler sources need
> > to opt in.
> >
> > Is it possible to force a build-id via linker flag `--build-id=sha1`?
> That's an idea - I'll see if this works.
Yes, please try this first.
>
> >
> > If not, can we just use `-z execstack` rather than concatenating a
> > DISCARD section into a linker script?
>
> so... something like v1 patch, but replace `-z noexecstack` with `-z
> execstack`? And for arm64 only? I'll try this.
If --build-id doesn't work, then I'd try this. Doesn't have to be
arm64 only if it's difficult to express that.
>
>
> > Either command line flags feel
> > cleaner than modifying a linker script at build time, if they work
> > that is.
>
> well... that entire linker script is generated at build-time.
Fair, but yuck!
--
Thanks,
~Nick Desaulniers
