Re: [RFC][PATCH 1/5] static_call: Make NULL static calls consistent

[Josh Poimboeuf]

On Mon, Mar 13, 2023 at 10:48:58AM -0700, Sami Tolvanen wrote:
> On Sun, Mar 12, 2023 at 8:17 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Fri, Mar 10, 2023 at 05:20:04PM -0800, Josh Poimboeuf wrote:
> > >   2) Create yet another "tier" of static call implementations, for
> > >      arches which can have the unfortunate combo of CFI_CLANG +
> > >      !HAVE_STATIC_CALL.  CONFIG_ALMOST_DONT_HAVE_STATIC_CALL?
> > >
> > >      The arch can define ARCH_DEFINE_STATIC_CALL_NOP() which uses inline
> > >      asm to create a CFI-compliant NOP/BUG/whatever version of the
> > >      function (insert lots of hand-waving).  Is the kcfi hash available
> > >      to inline asm at build time?
> >
> > Yes, clang creates magic symbol for everything it sees a declaration
> > for. This symbols can be referenced from asm, linking will make it all
> > work.
> >
> > And yes, C sucks, you can't actually create a function definition from a
> > type :/ Otherwise this could be trivially fixable.
> 
> Wouldn't creating a separate inline assembly nop function that
> references the CFI hash of another function with the correct type
> potentially solve this issue like Josh suggested?

Right, I was thinking something like this, where the nop function gets
generated by DEFINE_STATIC_CALL().

Completely untested of course...

#define STATIC_CALL_NOP_PREFIX		__SCN__
#define STATIC_CALL_NOP(name)		__PASTE(STATIC_CALL_NOP_PREFIX, name)
#define STATIC_CALL_NOP_STR(name)	__stringify(STATIC_CALL_NOP(name))

#define ARCH_DEFINE_STATIC_CALL_NOP(name, func)				\
	asm(".align 4						\n"	\
	    ".word __kcfi_typeid_" STATIC_CALL_NOP_STR(name) "  \n"	\
	    ".globl " STATIC_CALL_NOP_STR(name) "		\n"	\
	    STATIC_CALL_NOP_STR(name) ":			\n"	\
	    "bti c						\n"	\
	    "mov x0, xzr					\n"	\
	    "ret						\n"	\
	    ".type " STATIC_CALL_NOP_STR(name) ", @function	\n"	\
	    ".size " STATIC_CALL_NOP_STR(name) ", . - " STATIC_CALL_NOP_STR(name) " \n")

#define DECLARE_STATIC_CALL(name, func)					\
	extern struct static_call_key STATIC_CALL_KEY(name);		\
	extern typeof(func) STATIC_CALL_TRAMP(name)			\
	extern typeof(func) STATIC_CALL_NOP(name)

#define DEFINE_STATIC_CALL(name, _func, _func_init)			\
	DECLARE_STATIC_CALL(name, _func);				\
	ARCH_DEFINE_STATIC_CALL_NOP(name);				\
	struct static_call_key STATIC_CALL_KEY(name) = {		\
		.func = _func_init,					\
	}
-- 
Josh