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

[luto@amacapital.net (Andy Lutomirski)]

> On May 3, 2019,@4:16 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
>> On Fri, May 3, 2019@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 at 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.
> 
>  

I can get on board with this.