Messages in this thread | | | Subject | Re: [PATCH v3 5/6] x86/alternative: Use a single access in text_poke() where possible | From | Alexandre Chartre <> | Date | Tue, 15 Jan 2019 12:10:19 +0100 |
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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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