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

[Steven Rostedt]

On Thu, 2 May 2019 13:49:29 -0700
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> 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?
> 

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