Messages in this thread | | | Date | Thu, 11 Oct 2018 07:52:58 -0500 | From | Josh Poimboeuf <> | Subject | Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function" |
| |
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.
#define STATIC_CALL_TRAMP(key) ____static_call_tramp_##key #define STATIC_CALL_PTR(key) ____static_call_ptr_##key
#define STATIC_CALL_TRAMP_STR(key) __stringify(STATIC_CALL_TRAMP(key)) #define STATIC_CALL_PTR_STR(key) __stringify(STATIC_CALL_PTR(key))
#define DECLARE_STATIC_CALL(key, func) \ extern typeof(func) STATIC_CALL_TRAMP(key); \ extern typeof(func) *STATIC_CALL_PTR(key)
#define DEFINE_STATIC_CALL(key, func) \ typeof(func) *STATIC_CALL_PTR(key) = func; \ asm(".pushsection .text, \"ax\" \n" \ ".align 4 \n" \ ".globl " STATIC_CALL_TRAMP_STR(key) " \n" \ ".type " STATIC_CALL_TRAMP_STR(key) ", @function \n" \ STATIC_CALL_TRAMP_STR(key) ": \n" \ "jmpq *" STATIC_CALL_PTR_STR(key) "(%rip) \n" \ ".popsection \n")
#define static_call(key, args...) STATIC_CALL_TRAMP(key)(args)
#define static_call_update(key, func) \ ({ \ STATIC_CALL_PTR(key) = func; \ __static_call_update((void **)&STATIC_CALL_PTR(key)); \ })
-- Josh
| |