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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 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.

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