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SubjectRe: [RFC][PATCH 1/5] static_call: Make NULL static calls consistent
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@infradead.org> 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

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