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

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Date:   Mon, 6 May 2019 23:21:58 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.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>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call
 functions

On Mon, 6 May 2019 20:05:24 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:


> It would emulate the call that has had its first byte overwritten by
> 'int3'. Without doing any lookups of what it was supposed to change
> the call to, because it simply depends on what the rewriting code is
> doing on another CPU (or on the same CPU - it wouldn't care).

OK, so this is just about what to have it call.

> 
> So no need to look up anything, not at int3 time, and not at return
> time. It would just emulate the instruction atomically, with no state,
> and no need to look up what the 'ip' instruction is at the time.
> 
> It could literally just use a single flag: "is ftrace updating call
> instructions". Add another flag for the "I'm nop'ing out call
> instructions" so that it knows to emulate a jump-over instead. That's
> it.

Well we have that, and we have to look up the record regardless to know
if this was a ftrace int3 or not (the ftrace_location(ip) does that).
And the record has a counter to # of attached callers. Zero being to
turn it into a nop.

Note, if we are going from nop to call or call to nop, it would need to
read the offset to see if it is a nop (don't want to call with the nop
offset)

> 
> Because all the actual *values* would be entirely be determined by the
> actual rewriting that is going on independently of the 'int3'
> exception.

But still, we need to emulate the call, which requires pushing the
return code back onto the stack. I believe that part is the part we are
struggling with.

-- Steve