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

[Linus Torvalds]

On Thu, May 2, 2019 at 1:22 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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