Re: [PATCH v2 08/11] arm64/static_call: Fix static call CFI violations

[Josh Poimboeuf]

On Wed, Mar 22, 2023 at 12:22:07PM +0000, Mark Rutland wrote:
> > The problem is that the __perf_guest_state() static call does an
> > indirect branch to __static_call_return0(), which isn't CFI-compliant.
> 
> IIUC that'd be broken even with the old CFI mechanism, since commit:
> 
>   87b940a0675e2526 ("perf/core: Use static_call to optimize perf_guest_info_callbacks")
> 
> If so, we probably want a Fixes tag?

Yeah, it should definitely get a Fixes tag.  I wasn't quite sure if this
bug started with the above commit or with the CFI_CLANG switch to kcfi.
And then I forgot to investigate.

> > +/* Generate a CFI-compliant static call NOP function */
> > +#define __ARCH_DEFINE_STATIC_CALL_CFI(name, insns)			\
> > +	asm(".align 4						\n"	\
> > +	    ".word __kcfi_typeid_" name "			\n"	\
> > +	    ".globl " name "					\n"	\
> > +	    name ":						\n"	\
> > +	    "bti c						\n"	\
> > +	    insns "						\n"	\
> > +	    "ret						\n"	\
> > +	    ".type " name ", @function				\n"	\
> > +	    ".size " name ", . - " name "			\n")
> > +
> > +#define __ARCH_DEFINE_STATIC_CALL_RET0_CFI(name)			\
> > +	GEN_CFI_SYM(STATIC_CALL_RET0_CFI(name));			\
> > +	__ARCH_DEFINE_STATIC_CALL_CFI(STATIC_CALL_RET0_CFI_STR(name), "mov x0, xzr")
> 
> This looks correct, but given we're generating a regular functions it's
> unfortunate we can't have the compiler generate the actual code with something
> like:
> 
> #define __ARCH_DEFINE_STATIC_CALL_RET0_CFI(rettype, name, args...)	\
> rettype name(args)							\
> {									\
> 	return (rettype)0;						\
> }
> 
> ... but I guess passing the rettype and args around is painful.

Hm, I hadn't considered that.  I'll play around with it.

> Regardless, I gave this a spin atop v6.3-rc3 using LLVM 16.0.0 and CFI_CLANG,
> and it does seem to work, so:
> 
> Tested-by: Mark Rutland <mark.rutland@xxxxxxx>

Thanks!

-- 
Josh