linux-kernel - Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions

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Date:   Mon, 6 May 2019 12:46:11 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Nicolai Stange <nstange@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Juergen Gross <jgross@...e.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Joerg Roedel <jroedel@...e.de>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, stable <stable@...r.kernel.org>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions

On Mon, May 6, 2019 at 11:57 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> You should have waited another week to open that merge window ;-)

Hmm. I'm looking at it while the test builds happen, and since I don't
see what's wrong in the low-level entry code, I'm looking at the
ftrace code instead.

What's going on here?

               *pregs = int3_emulate_call(regs, (unsigned
long)ftrace_regs_caller);

that line makes no sense. Why would we emulate a call to
ftrace_regs_caller()? That function sets up a pt_regs, and then calls
ftrace_stub().

But we *have* pt_regs here already with the right values. Why isn't
this just a direct call to ftrace_stub() from within the int3 handler?

And the thing is, calling ftrace_regs_caller() looks wrong, because
that's not what happens for *real* mcount handling, which uses that
"create_trampoline()" to create the thing we're supposed to really
use?

Anyway, I simply don't understand the code, so I'm confused. But why
is the int3 emulation creating a call that doesn't seem to match what
the instruction that we're actually rewriting is supposed to do?

IOW, it looks to me like ftrace_int3_handler() is actually emulating
something different than what ftrace_modify_code() is actually
modifying the code to do!

Since the only caller of ftrace_modify_code() is update_ftrace_func(),
why is that function not just saving the target and we'd emulate the
call to that? Using anything else looks crazy?

But as mentioned, I just don't understand the ftrace logic. It looks
insane to me, and much more likely to be buggy than the very simple
entry code.

             Linus