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SubjectRe: [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

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