Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"

[Andy Lutomirski]

> On Oct 11, 2018, at 5:52 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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@xxxxxxxxxx> 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.