Messages in this thread | | | Subject | Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function" | From | Andy Lutomirski <> | Date | Thu, 11 Oct 2018 09:20:40 -0700 |
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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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