Re: [musl] PAC/BTI Support on aarch64

[William Roberts <bill.c.roberts@gmail.com>]

On Mon, Feb 12, 2024 at 12:42 PM Rich Felker <dalias@libc.org> wrote:
>
> On Mon, Feb 12, 2024 at 10:38:50AM -0600, William Roberts wrote:
> > Hello,
> >
> > I was just wondering if there was any work being done to support PAC
> > and BTI in aarch64? I could add support but didn't want to duplicate
> > the work.
>
> I'm not aware of any active work on this, but before writing a full
> implementation, it would be really helpful to start with a basic
> proposal for the scope of changes needed to make it work to assess
> whether these are manageable and acceptable cost.

It's a matter of building with -mbranch-protection=standard

Just the ASM labels need the first instruction to be a BTI. They're in
the NOP space
so they are backwards compatible, older hardware will just NOP it.

It's been done for many projects, glibc and bionic have it. The
problem with BTI is that when one item in the link
list doesn't support BTI the loader/linker turns it off. So when it's
something like a libc that is fundamental in the link chain,
it turns it off for everything.

The initial scope of code changes would be what's reported when
LDFLAGS=-Wl,-zforce-bti,--fatal-warnings

/usr/bin/ld: obj/src/fenv/aarch64/fenv.lo: warning: BTI turned on by
-z force-bti when all inputs do not have BTI in NOTE section.
/usr/bin/ld: obj/src/ldso/aarch64/dlsym.lo: warning: BTI turned on by
-z force-bti when all inputs do not have BTI in NOTE section.
/usr/bin/ld: obj/src/ldso/aarch64/tlsdesc.lo: warning: BTI turned on
by -z force-bti when all inputs do not have BTI in NOTE section.
/usr/bin/ld: obj/src/process/aarch64/vfork.lo: warning: BTI turned on
by -z force-bti when all inputs do not have BTI in NOTE section.
/usr/bin/ld: obj/src/setjmp/aarch64/longjmp.lo: warning: BTI turned on
by -z force-bti when all inputs do not have BTI in NOTE section.
/usr/bin/ld: obj/src/setjmp/aarch64/setjmp.lo: warning: BTI turned on
by -z force-bti when all inputs do not have BTI in NOTE section.
/usr/bin/ld: obj/src/signal/aarch64/restore.lo: warning: BTI turned on
by -z force-bti when all inputs do not have BTI in NOTE section.
/usr/bin/ld: obj/src/signal/aarch64/sigsetjmp.lo: warning: BTI turned
on by -z force-bti when all inputs do not have BTI in NOTE section.
/usr/bin/ld: obj/src/string/aarch64/memcpy.lo: warning: BTI turned on
by -z force-bti when all inputs do not have BTI in NOTE section.
/usr/bin/ld: obj/src/string/aarch64/memset.lo: warning: BTI turned on
by -z force-bti when all inputs do not have BTI in NOTE section.
/usr/bin/ld: obj/src/thread/aarch64/__set_thread_area.lo: warning: BTI
turned on by -z force-bti when all inputs do not have BTI in NOTE
section.
/usr/bin/ld: obj/src/thread/aarch64/__unmapself.lo: warning: BTI
turned on by -z force-bti when all inputs do not have BTI in NOTE
section.
/usr/bin/ld: obj/src/thread/aarch64/clone.lo: warning: BTI turned on
by -z force-bti when all inputs do not have BTI in NOTE section.
/usr/bin/ld: obj/src/thread/aarch64/syscall_cp.lo: warning: BTI turned
on by -z force-bti when all inputs do not have BTI in NOTE section.

>
> Rich