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

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