linux-kernel - Re: [PATCH v2 15/39] x86/ibt,kprobes: Fix more +0 assumptions

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Message-Id: <20220226160946.cd0d45b859449ea4a2c40b75@kernel.org>
Date:   Sat, 26 Feb 2022 16:09:46 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>, x86@...nel.org,
        joao@...rdrivepizza.com, hjl.tools@...il.com, jpoimboe@...hat.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz,
        alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 15/39] x86/ibt,kprobes: Fix more +0 assumptions

On Fri, 25 Feb 2022 09:14:09 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 25 Feb 2022 11:46:23 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > Given all that, kprobe users are in a bit of a bind. Determining the
> > __fentry__ point basically means they *have* to first read the function
> > assembly to figure out where it is.
> 
> Technically I think that's what kprobes has been designed for. But
> realistically, I do not think anyone actually does that (outside of
> academic and niche uses).

Yeah, raw kprobe user must check the instruction boundary anyway.
And if possible, I would like to keep the kprobe (in kprobe level) as it is.

> Really, when people use func+0 they just want to trace the function, and
> ftrace is the fastest way to do so, and if it's not *exactly* at function
> entry, but includes the arguments, then it should be fine.

Yes, that is another (sub) reason why I introduced fprobe. ;-)

OK, I understand that we should not allow to probe on endbr unless
user really wants it. Let me add a KPROBE_FLAG_RAW_ENTRY for that special
purpose. If the flag is not set (by default), the kprobe::addr will be
shifted automatically.
ANyway, this address translation must be done in check_ftrace_location
instead of kprobe_lookup_name(). Let me make another patch.
Also, selftest and document must be updated with that.

> That said, perhaps we should add a config to know if the architecture
> uses function entry or the old mcount that is after the frame set up (that
> is, you can not get to the arguments).
> 
> CONFIG_HAVE_FTRACE_FUNCTION_START ?

Hmm, ENDBR is not always there, and except x86, most of the arch will
make it 'n'. (x86 is a special case.)

> 
> Because, if the arch still uses the old mcount method (where it's after the
> frame set up), then a kprobe at func+0 really wants the breakpoint.
> 
> -- Steve
> 

Thank you,

-- 
Masami Hiramatsu <mhiramat@...nel.org>