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

[Josh Poimboeuf <jpoimboe@redhat.com>]

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