On Wed, Oct 10, 2018 at 11:17 AM Josh Poimboeuf <jpoim...@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? Anyway, your way is probably fine with a few caveats: - It won't really work if the call comes from a .S file. - There should probably be a comment to help de-confuse future people like me :)
