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SubjectRe: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"
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> On Oct 11, 2018, at 5:52 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
>> On Wed, Oct 10, 2018 at 10:07:38PM -0500, Josh Poimboeuf wrote:
>>> On Wed, Oct 10, 2018 at 02:13:22PM -0700, Andy Lutomirski wrote:
>>>> On Wed, Oct 10, 2018 at 11:17 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>>>>
>>>>> On Wed, Oct 10, 2018 at 01:16:05PM -0500, Josh Poimboeuf wrote:
>>>>> On Wed, Oct 10, 2018 at 11:03:43AM -0700, Andy Lutomirski wrote:
>>>>>>> +#define DECLARE_STATIC_CALL(tramp, func) \
>>>>>>> + extern typeof(func) tramp; \
>>>>>>> + static void __used __section(.discard.static_call_tramps) \
>>>>>>> + *__static_call_tramp_##tramp = tramp
>>>>>>> +
>>>>>>
>>>>>> Confused. What's the __static_call_tramp_##tramp variable for? And
>>>>>> why is a DECLARE_ macro defining a variable?
>>>>>
>>>>> This is the magic needed for objtool to find all the call sites.
>>>>>
>>>>> The variable itself isn't needed, but the .discard.static_call_tramps
>>>>> entry is. Objtool reads that section to find out which function call
>>>>> sites are targeted to a static call trampoline.
>>>>
>>>> To clarify: objtool reads that section to find out which functions are
>>>> really static call trampolines. Then it annotates all the instructions
>>>> which call/jmp to those trampolines. Those annotations are then read by
>>>> the kernel.
>>>>
>>>
>>> Ah, right, and objtool runs on a per-object basis so it has no other
>>> way to know what symbols are actually static calls.
>>>
>>> There's another way to skin this cat, though:
>>>
>>> extern typeof(func) __static_call_trampoline_##tramp;
>>> #define tramp __static_call_trampoline_##tramp
>>>
>>> And objtool could recognize it by name. But, of course, you can't put
>>> a #define in a macro. But maybe there's a way to hack it up with a
>>> static inline?
>
> I went with something similar in the latest version. It's less
> surprising in a couple of ways:
>
> - DECLARE_STATIC_CALL doesn't have the magic objtool definition.
>
> - Call sites use the static_call() wrapper, which makes static calls
> clearly visible.

Seems reasonable. Also, for a real patch, it should be straightforward to have a fallback implementation in include/linux/static_call.h that just dereferences the pointer.

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