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

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Message-ID: <20220225091409.0963e7d2@gandalf.local.home>
Date:   Fri, 25 Feb 2022 09:14:09 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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 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).

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.

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 ?

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