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Date:   Sat, 28 May 2022 16:09:13 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@kernel.org>
Cc:     x86@kernel.org, jpoimboe@redhat.com, linux-kernel@vger.kernel.org,
        elver@google.com, jbaron@akamai.com, rostedt@goodmis.org,
        ardb@kernel.org, mark.rutland@arm.com
Subject: Re: [PATCH 2/7] objtool: Extend UNWIND_HINT based ENDBR rules
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On Thu, May 26, 2022 at 08:05:49AM -0700, Josh Poimboeuf wrote:
> On Thu, May 26, 2022 at 12:52:54PM +0200, Peter Zijlstra wrote:
> > Extend the UNWIND hint driven rules for ENDBR placement. Currently
> > objtool expects an ENDBR at any UNWINT_HINT_IRET_REGS that is at +0 of
> > an STB_GLOBAL symbol, with the expectation that this is an exception
> > entry point.
> > 
> > Extend this to also expect ENDBR at UNWIND_HINT_EMPTY at +0 for
> > STB_GLOBAL symbols, with the expectation that these are also machine
> > entry points (SYSCALL et. al.).
> > 
> > This guarantees all machine entry points are covered by objtool rules at
> > the expense of a few additional false positives:
> > 
> >   vmlinux.o: warning: objtool: startup_64+0x0: UNWIND_HINT_EMPTY without ENDBR
> >   vmlinux.o: warning: objtool: start_cpu0+0x0: UNWIND_HINT_EMPTY without ENDBR
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> 
> I can't remember if this was my bright idea, but it feels kind of
> arbitrary.  Hopefully there won't be a lot of false positives.

The existing UNWIND_HINT_IRET_REGS at +0 was your idea, I'm just trying
to cover more.

> Anyway, won't SYSCALL-type symbols typically be referenced elsewhere in
> the kernel and thus be found by the regular IBT validation?

They do indeed, and that's what we've been relying on. I just figured it
would be more consistent to have rules covering all machine entry
points.

(also all the Xen entry points are EMPTY like).

> Do you have any examples of where this warning would trigger if there
> were a missing ENDBR?

No.

Anyway, I can drop these first two patches for now.