Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 11, 2019 at 08:54:23AM -0700, Andy Lutomirski wrote:
> > On Jun 11, 2019, at 1:03 AM, Peter Zijlstra <peterz@infradead.org> wrote:

> > arch_optimize_kprobe() then does the text_poke_bp() that replaces the
> > instruction @addr with int3, copies the rel jump address and overwrites
> > the int3 with jmp.
> > 
> > And I'm thinking the problem is with something like:
> > 
> > @addr: nop nop nop nop nop
> > 
> > We copy out the nops into the trampoline, overwrite the first nop with
> > an INT3, overwrite the remaining nops with the rel addr, but oops,
> > another CPU can still be executing one of those NOPs, right?
> > 
> > I'm thinking we could fix this by first writing INT3 into all relevant
> > instructions, which is going to be messy, given the current code base.
> 
> How does that help?  If RIP == x+2 and you want to put a 5-byte jump
> at address x, no amount of 0xcc is going to change the fact that RIP
> is in the middle of the jump.

What I proposed was doing 0xCC on every instruction that's inside of
[x,x+5). After that we do machine wide IPI+SYNC.

So if RIP is at x+2, then it will observe 0xCC and trigger the exception
there.

Now, the problem is that my exception cannot recover from anything
except NOPs, so while it could work for some corner cases, it doesn't
work for the optkprobe case in general.

Only then do we write the JMP offset and again to IPI+SYNC; then we
write the 0xE8 and again IPI-SYNC.

But at that point we already have the guarantee nobody is inside
[x,x+5). That is, except if we can get to x+2 without first going
through x, IOW if x+2 is a branch target we're screwed any which way
around and the poke is never valid.

Is that something optkprobes checks? If so, where and how?