Re: [PATCH v3 5/6] x86/alternative: Use a single access in text_poke() where possible

[Alexandre Chartre]

On 01/11/2019 05:57 PM, Josh Poimboeuf wrote:
> On Fri, Jan 11, 2019 at 05:46:36PM +0100, Alexandre Chartre wrote:
> > On 01/11/2019 04:28 PM, Josh Poimboeuf wrote:
> > > On Fri, Jan 11, 2019 at 01:10:52PM +0100, Alexandre Chartre wrote:
> > > > To avoid any issue with live patching the call instruction, what about
> > > > toggling between two call instructions: one would be the currently active
> > > > call, while the other would currently be inactive but to be used after a
> > > > change is made. You can safely patch the inactive call and then toggle
> > > > the call flow (using a jump label) between the active and inactive calls.
> > > >
> > > > So instead of having a single call instruction:
> > > >
> > > > 	call function
> > > >
> > > > You would have:
> > > >
> > > > 	STATIC_JUMP_IF_TRUE label, key
> > > > 	call function1
> > > > 	jmp done
> > > > label:
> > > > 	call function2
> > > > done:
> > > >
> > > > If the key is set so that function1 is currently called then you can
> > > > safely update the call instruction for function2. Once this is done,
> > > > just flip the key to make the function2 call active. On a next update,
> > > > you would, of course, have to switch and update the call for function1.
> > >
> > > What about the following race?
> > >
> > > CPU1						CPU2
> > > static key is false, doesn't jump
> > > task gets preempted before calling function1
> > > 						change static key to true
> > > 						start patching "call function1"
> > > task resumes, sees inconsistent call instruction
> >
> > If the function1 call is active then it won't be changed, you will change
> > function2. However, I presume you can still have a race but if the function
> > is changed twice before calling function1:
> >
> > CPU1						CPU2
> > static key is false, doesn't jump
> > task gets preempted before calling function1
> >                                                 -- first function change --
> >                                                 patch "call function2"
> >                                                 change static key to true
> >                                                 -- second function change --
> >                                                 start patching "call function1"
> > task resumes, sees inconsistent call instruction
> >
> > So right, that's a problem.
>
> Right, that's what I meant to say :-)

Thinking more about it (and I've probably missed something or I am just being
totally stupid because this seems way too simple), can't we just replace the
"call" with "push+jmp" and patch the jmp instruction?

Instead of having:

    call target

Have:

    push $done
static_call:
    jmp target
done:

Then we can safely patch the "jmp" instruction to jump to a new target
with text_poke_bp(), using the new target as the text_poke_bp() handler:

  new_jmp_code = opcode of "jmp new_target"

  text_poke_bp(static_call, new_jmp_code, new_jmp_code_size, new_target);

Problems come with patching a call instruction, but there's no issue with patching
a jmp, no? (that's what jump labels do).

No change to the int3 handler, no thunk, this seems really too simple... :-)


alex.