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

[Alexandre Chartre]

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.

alex.