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Date:   Fri, 25 Feb 2022 16:41:15 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     x86@kernel.org, joao@overdrivepizza.com, hjl.tools@gmail.com,
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        mark.rutland@arm.com, alyssa.milburn@intel.com, mbenes@suse.cz,
        rostedt@goodmis.org, alexei.starovoitov@gmail.com
Subject: Re: [PATCH v2 15/39] x86/ibt,kprobes: Fix more +0 assumptions
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On Fri, Feb 25, 2022 at 10:42:49PM +0900, Masami Hiramatsu wrote:

> OK, this sounds like kp->addr should be "call fentry" if there is ENDBR.
> 
> > 
> > This patch takes the approach that sym+0 means __fentry__, irrespective
> > of where it might actually live. I *think* that's more or less
> > consistent with what other architectures do; specifically see
> > arch/powerpc/kernel/kprobes.c:kprobe_lookup_name(). I'm not quite sure
> > what ARM64 does when it has BTI on (which is then very similar to what
> > we have here).
> 
> Yeah, I know the powerpc does such thing, but I think that is not what
> user expected. I actually would like to fix that, because in powerpc
> and other non-x86 case (without BTI/IBT), the instructions on sym+0 is
> actually executed.
> 
> > 
> > What do you think makes most sense here?
> 
> Are there any way to distinguish the "preparing instructions" (part of
> calling mcount) and this kind of trap instruction online[1]? If possible,
> I would like to skip such traps, but put the probe on preparing
> instructions.

None that exist, but we could easily create one. See also my email here:

  https://lkml.kernel.org/r/Yhj1oFcTl2RnghBz@hirez.programming.kicks-ass.net

That skip_endbr() function is basically what you're looking for; it just
needs a better name and a Power/ARM64 implementation to get what you
want, right?

The alternative 'hack' I've been contemplating is (ab)using
INT_MIN/INT_MAX offset for __fentry__ and __fexit__ points (that latter
is something we'll probably have to grow when CET-SHSTK or backward-edge
CFI gets to be done, because then ROP tricks as used by function-graph
and kretprobes are out the window).

That way sym+[0..size) is still a valid reference to the actual
instruction in the symbol, but sym+INT_MIN will hard map to __fentry__
while sym+INT_MAX will get us __fexit__.