linux-kernel - Re: [RFC][PATCH 2/3] x86_64: Allow breakpoints to emulate call functions

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Date:   Tue, 7 May 2019 14:49:25 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        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>,
        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 2/3] x86_64: Allow breakpoints to emulate call
 functions

On Tue, May 07, 2019 at 03:20:16PM -0400, Steven Rostedt wrote:
> On Tue, 7 May 2019 14:14:12 -0500
> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > On Tue, May 07, 2019 at 03:01:53PM -0400, Steven Rostedt wrote:
> > > How's this?
> > > 
> > > -- Steve
> > > 
> > > From d29dc2e9e0275c9857932b80cebc01551b669efb Mon Sep 17 00:00:00 2001
> > > From: Peter Zijlstra <peterz@...radead.org>
> > > Date: Wed, 1 May 2019 15:11:17 +0200
> > > Subject: [PATCH] x86_64: Allow breakpoints to emulate call functions
> > > 
> > > In order to allow breakpoints to emulate call functions, they need to push
> > > the return address onto the stack. But because the breakpoint exception
> > > frame is added to the stack when the breakpoint is hit, there's no room to
> > > add the address onto the stack and return to the address of the emulated
> > > called funtion.  
> > 
> > The 2nd sentence can probably be removed since it's technically no
> > longer true, thanks to the previous patch.
> > 
> > > This helper functions are added:  
> > 
> > "These"
> 
> New version:
> 
>     x86_64: Allow breakpoints to emulate call functions
>     
>     In order to allow breakpoints to emulate call functions, they need to push

Sorry to keep nitpicking, but "call functions" -> "function calls" would
sound more accurate to me (in both subject and description).

Otherwise it looks good.

>     the return address onto the stack. The x86_64 int3 handler adds a small gap
>     to allow the stack to grow some. Use this gap to add the return address to
>     be able to emulate a call instruction at the breakpoint location.
>     
>     These helper functions are added:
>     
>       int3_emulate_jmp(): changes the location of the regs->ip to return there.
>     
>      (The next two are only for x86_64)
>       int3_emulate_push(): to push the address onto the gap in the stack
>       int3_emulate_call(): push the return address and change regs->ip

-- 
Josh