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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 2 May 2019 13:49:29 -0700
Message-ID: <CAHk-=wh8bi5c_GkyjPtDAiaXaZRqtmhWs30usUvs4qK_F+c9tg@mail.gmail.com>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Steven Rostedt <rostedt@goodmis.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>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        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>,
        Juergen Gross <jgross@suse.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nayna Jain <nayna@linux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Joerg Roedel <jroedel@suse.de>,
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        <linux-kselftest@vger.kernel.org>, stable <stable@vger.kernel.org>
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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?

                  Linus