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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 3 May 2019 16:16:58 -0700
Message-ID: <CAHk-=wjZDhwStWvioV7totCnZfp74bqH0y1UJxkmFfdLg48wDA@mail.gmail.com>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        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>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
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On Fri, May 3, 2019 at 3:55 PM Andy Lutomirski <luto@amacapital.net> wrote:
>
> But I think this will end up worse than the version where the entry code fixes it up.  This is because, if the C code moves pt_regs, then we need some way to pass the new pointer back to the asm.

What? I already posted that code. Let me quote it again:

Message-ID: <CAHk-=wh8bi5c_GkyjPtDAiaXaZRqtmhWs30usUvs4qK_F+c9tg@mail.gmail.com>

        # 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

It's that easy (this is with the assumption that we've already applied
the "standalone simple int3" case, but I think the above might work
even with the current code model, just the "call do_int3" needs to
have the kernel/not-kernel distinction and do the above for the kernel
case)

That's *MUCH* easier than your code to move entries around on the
stack just as you return, and has the advantage of not changing any
C-visible layout.

The C interface looks like this

    /* Note: on x86-32, we can move 'regs' around for push/pop emulation */
    struct pt_regs *kernel_int3(struct pt_regs *regs)
    {
        ..
        .. need to pass regs to emulation functions
        .. and call emulation needs to return it
        ..
        return regs;
    }

and I just posted as a response to Stephen the *trivial* do_int3()
wrapper (so that x86-64 doesn't need to care), and the *trivial* code
to actually emulate a call instruction.

And when I say "trivial", I obviously mean "totally untested and
probably buggy", but it sure seems *simple*.,

Notice? Simple and minimal changes to entry code that only affect
int3, and nothing else.

                 Linus