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Date:   Fri, 3 May 2019 15:24:05 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Nicolai Stange <nstange@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
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        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
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        Petr Mladek <pmladek@suse.com>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Mimi Zohar <zohar@linux.ibm.com>,
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        <linux-kselftest@vger.kernel.org>, stable <stable@vger.kernel.org>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call
 functions
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On Thu, 2 May 2019 13:49:29 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Thu, May 2, 2019 at 1:22 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Something like so; it boots; but I could've made some horrible mistake
> > (again).  
> 
> This actually looks much better to me.
> 
> Maybe it's more lines (I didn't check), but it's a lot simpler in that
> now the magic of the int3 stack doesn't get exposed to anything else.
> 
> We *could* also make this kernel-mode-only do_int3() be a special
> function, and do something like
> 
>         # args: pt_regs pointer (no error code for int3)
>         movl %esp,%eax
>         # allocate a bit of extra room on the stack, so that
> 'kernel_int3' can move the pt_regs
>         subl $8,%esp
>         call kernel_int3
>         movl %eax,%esp
> 
> and not do any stack switching magic in the asm code AT ALL. We'd do
> 
>     struct pt_regs *kernel_int3(struct pt_regs *regs)
>     {
>         ..
>         return regs;
>     }
> 
> and now you the rule for call emulation ends up being that you need to
> "memmove()" the ptregs up and down properly, and return the new
> pt_regs pointer.
> 
> Hmm? That would simplify the asm code further, but some people might
> find it objectionable?
> 

The problem with this approach is that it would require doing the same
for x86_64, as the int3 C code is the same for both. And that may be a
bit more difficult on the x86_64 side because it's all done with a
simple flag in the idtentry macro to add the gap.

-- Steve