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Date:   Fri, 25 Feb 2022 09:14:09 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>, x86@kernel.org,
        joao@overdrivepizza.com, hjl.tools@gmail.com, jpoimboe@redhat.com,
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        samitolvanen@google.com, mark.rutland@arm.com,
        alyssa.milburn@intel.com, mbenes@suse.cz,
        alexei.starovoitov@gmail.com
Subject: Re: [PATCH v2 15/39] x86/ibt,kprobes: Fix more +0 assumptions
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On Fri, 25 Feb 2022 11:46:23 +0100
Peter Zijlstra <peterz@infradead.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