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

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 6 May 2019 20:05:24 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:


> It would emulate the call that has had its first byte overwritten by
> 'int3'. Without doing any lookups of what it was supposed to change
> the call to, because it simply depends on what the rewriting code is
> doing on another CPU (or on the same CPU - it wouldn't care).

OK, so this is just about what to have it call.

> 
> So no need to look up anything, not at int3 time, and not at return
> time. It would just emulate the instruction atomically, with no state,
> and no need to look up what the 'ip' instruction is at the time.
> 
> It could literally just use a single flag: "is ftrace updating call
> instructions". Add another flag for the "I'm nop'ing out call
> instructions" so that it knows to emulate a jump-over instead. That's
> it.

Well we have that, and we have to look up the record regardless to know
if this was a ftrace int3 or not (the ftrace_location(ip) does that).
And the record has a counter to # of attached callers. Zero being to
turn it into a nop.

Note, if we are going from nop to call or call to nop, it would need to
read the offset to see if it is a nop (don't want to call with the nop
offset)

> 
> Because all the actual *values* would be entirely be determined by the
> actual rewriting that is going on independently of the 'int3'
> exception.

But still, we need to emulate the call, which requires pushing the
return code back onto the stack. I believe that part is the part we are
struggling with.

-- Steve