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/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_STATIC_CALL_H
#define _LINUX_STATIC_CALL_H
/*
* Static call support
*
* Static calls use code patching to hard-code function pointers into direct
* branch instructions. They give the flexibility of function pointers, but
* with improved performance. This is especially important for cases where
* retpolines would otherwise be used, as retpolines can significantly impact
* performance.
*
*
* API overview:
*
* DECLARE_STATIC_CALL(name, func);
* DEFINE_STATIC_CALL(name, func);
* DEFINE_STATIC_CALL_NULL(name, typename);
* DEFINE_STATIC_CALL_RET0(name, typename);
*
* __static_call_return0;
*
* static_call(name)(args...);
* static_call_cond(name)(args...);
* static_call_update(name, func);
* static_call_query(name);
*
* EXPORT_STATIC_CALL{,_TRAMP}{,_GPL}()
*
* Usage example:
*
* # Start with the following functions (with identical prototypes):
* int func_a(int arg1, int arg2);
* int func_b(int arg1, int arg2);
*
* # Define a 'my_name' reference, associated with func_a() by default
* DEFINE_STATIC_CALL(my_name, func_a);
*
* # Call func_a()
* static_call(my_name)(arg1, arg2);
*
* # Update 'my_name' to point to func_b()
* static_call_update(my_name, &func_b);
*
* # Call func_b()
* static_call(my_name)(arg1, arg2);
*
*
* Implementation details:
*
* This requires some arch-specific code (CONFIG_HAVE_STATIC_CALL).
* Otherwise basic indirect calls are used (with function pointers).
*
* Each static_call() site calls into a trampoline associated with the name.
* The trampoline has a direct branch to the default function. Updates to a
* name will modify the trampoline's branch destination.
*
* If the arch has CONFIG_HAVE_STATIC_CALL_INLINE, then the call sites
* themselves will be patched at runtime to call the functions directly,
* rather than calling through the trampoline. This requires objtool or a
* compiler plugin to detect all the static_call() sites and annotate them
* in the .static_call_sites section.
*
*
* Notes on NULL function pointers:
*
* Static_call()s support NULL functions, with many of the caveats that
* regular function pointers have.
*
* Clearly calling a NULL function pointer is 'BAD', so too for
* static_call()s (although when HAVE_STATIC_CALL it might not be immediately
* fatal). A NULL static_call can be the result of:
*
* DECLARE_STATIC_CALL_NULL(my_static_call, void (*)(int));
*
* which is equivalent to declaring a NULL function pointer with just a
* typename:
*
* void (*my_func_ptr)(int arg1) = NULL;
*
* or using static_call_update() with a NULL function. In both cases the
* HAVE_STATIC_CALL implementation will patch the trampoline with a RET
* instruction, instead of an immediate tail-call JMP. HAVE_STATIC_CALL_INLINE
* architectures can patch the trampoline call to a NOP.
*
* In all cases, any argument evaluation is unconditional. Unlike a regular
* conditional function pointer call:
*
* if (my_func_ptr)
* my_func_ptr(arg1)
*
* where the argument evaludation also depends on the pointer value.
*
* When calling a static_call that can be NULL, use:
*
* static_call_cond(name)(arg1);
*
* which will include the required value tests to avoid NULL-pointer
* dereferences.
*
* To query which function is currently set to be called, use:
*
* func = static_call_query(name);
*
*
* DEFINE_STATIC_CALL_RET0 / __static_call_return0:
*
* Just like how DEFINE_STATIC_CALL_NULL() / static_call_cond() optimize the
* conditional void function call, DEFINE_STATIC_CALL_RET0 /
* __static_call_return0 optimize the do nothing return 0 function.
*
* This feature is strictly UB per the C standard (since it casts a function
* pointer to a different signature) and relies on the architecture ABI to
* make things work. In particular it relies on Caller Stack-cleanup and the
* whole return register being clobbered for short return values. All normal
* CDECL style ABIs conform.
*
* In particular the x86_64 implementation replaces the 5 byte CALL
* instruction at the callsite with a 5 byte clear of the RAX register,
* completely eliding any function call overhead.
*
* Notably argument setup is unconditional.
*
*
* EXPORT_STATIC_CALL() vs EXPORT_STATIC_CALL_TRAMP():
*
* The difference is that the _TRAMP variant tries to only export the
* trampoline with the result that a module can use static_call{,_cond}() but
* not static_call_update().
*
*/
#include <linux/types.h>
#include <linux/cpu.h>
#include <linux/static_call_types.h>
#ifdef CONFIG_HAVE_STATIC_CALL
#include <asm/static_call.h>
/*
* Either @site or @tramp can be NULL.
*/
extern void arch_static_call_transform(void *site, void *tramp, void *func, bool tail);
#define STATIC_CALL_TRAMP_ADDR(name) &STATIC_CALL_TRAMP(name)
#else
#define STATIC_CALL_TRAMP_ADDR(name) NULL
#endif
#define static_call_update(name, func) \
({ \
typeof(&STATIC_CALL_TRAMP(name)) __F = (func); \
__static_call_update(&STATIC_CALL_KEY(name), \
STATIC_CALL_TRAMP_ADDR(name), __F); \
})
#define static_call_query(name) (READ_ONCE(STATIC_CALL_KEY(name).func))
#ifdef CONFIG_HAVE_STATIC_CALL_INLINE
extern int __init static_call_init(void);
struct static_call_mod {
struct static_call_mod *next;
struct module *mod; /* for vmlinux, mod == NULL */
struct static_call_site *sites;
};
/* For finding the key associated with a trampoline */
struct static_call_tramp_key {
s32 tramp;
s32 key;
};
extern void __static_call_update(struct static_call_key *key, void *tramp, void *func);
extern int static_call_mod_init(struct module *mod);
extern int static_call_text_reserved(void *start, void *end);
extern long __static_call_return0(void);
#define __DEFINE_STATIC_CALL(name, _func, _func_init) \
DECLARE_STATIC_CALL(name, _func); \
struct static_call_key STATIC_CALL_KEY(name) = { \
.func = _func_init, \
.type = 1, \
}; \
ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func_init)
#define DEFINE_STATIC_CALL_NULL(name, _func) \
DECLARE_STATIC_CALL(name, _func); \
struct static_call_key STATIC_CALL_KEY(name) = { \
.func = NULL, \
.type = 1, \
}; \
ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
#define static_call_cond(name) (void)__static_call(name)
#define EXPORT_STATIC_CALL(name) \
EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \
EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
#define EXPORT_STATIC_CALL_GPL(name) \
EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)); \
EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
/* Leave the key unexported, so modules can't change static call targets: */
#define EXPORT_STATIC_CALL_TRAMP(name) \
EXPORT_SYMBOL(STATIC_CALL_TRAMP(name)); \
ARCH_ADD_TRAMP_KEY(name)
#define EXPORT_STATIC_CALL_TRAMP_GPL(name) \
EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name)); \
ARCH_ADD_TRAMP_KEY(name)
#elif defined(CONFIG_HAVE_STATIC_CALL)
static inline int static_call_init(void) { return 0; }
#define __DEFINE_STATIC_CALL(name, _func, _func_init) \
DECLARE_STATIC_CALL(name, _func); \
struct static_call_key STATIC_CALL_KEY(name) = { \
.func = _func_init, \
}; \
ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func_init)
#define DEFINE_STATIC_CALL_NULL(name, _func) \
DECLARE_STATIC_CALL(name, _func); \
struct static_call_key STATIC_CALL_KEY(name) = { \
.func = NULL, \
}; \
ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
#define static_call_cond(name) (void)__static_call(name)
static inline
void __static_call_update(struct static_call_key *key, void *tramp, void *func)
{
cpus_read_lock();
WRITE_ONCE(key->func, func);
arch_static_call_transform(NULL, tramp, func, false);
cpus_read_unlock();
}
static inline int static_call_text_reserved(void *start, void *end)
{
return 0;
}
extern long __static_call_return0(void);
#define EXPORT_STATIC_CALL(name) \
EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \
EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
#define EXPORT_STATIC_CALL_GPL(name) \
EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)); \
EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
/* Leave the key unexported, so modules can't change static call targets: */
#define EXPORT_STATIC_CALL_TRAMP(name) \
EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
#define EXPORT_STATIC_CALL_TRAMP_GPL(name) \
EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
#else /* Generic implementation */
static inline int static_call_init(void) { return 0; }
static inline long __static_call_return0(void)
{
return 0;
}
#define __DEFINE_STATIC_CALL(name, _func, _func_init) \
DECLARE_STATIC_CALL(name, _func); \
struct static_call_key STATIC_CALL_KEY(name) = { \
.func = _func_init, \
}
#define DEFINE_STATIC_CALL_NULL(name, _func) \
DECLARE_STATIC_CALL(name, _func); \
struct static_call_key STATIC_CALL_KEY(name) = { \
.func = NULL, \
}
static inline void __static_call_nop(void) { }
/*
* This horrific hack takes care of two things:
*
* - it ensures the compiler will only load the function pointer ONCE,
* which avoids a reload race.
*
* - it ensures the argument evaluation is unconditional, similar
* to the HAVE_STATIC_CALL variant.
*
* Sadly current GCC/Clang (10 for both) do not optimize this properly
* and will emit an indirect call for the NULL case :-(
*/
#define __static_call_cond(name) \
({ \
void *func = READ_ONCE(STATIC_CALL_KEY(name).func); \
if (!func) \
func = &__static_call_nop; \
(typeof(STATIC_CALL_TRAMP(name))*)func; \
})
#define static_call_cond(name) (void)__static_call_cond(name)
static inline
void __static_call_update(struct static_call_key *key, void *tramp, void *func)
{
WRITE_ONCE(key->func, func);
}
static inline int static_call_text_reserved(void *start, void *end)
{
return 0;
}
#define EXPORT_STATIC_CALL(name) EXPORT_SYMBOL(STATIC_CALL_KEY(name))
#define EXPORT_STATIC_CALL_GPL(name) EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name))
#endif /* CONFIG_HAVE_STATIC_CALL */
#define DEFINE_STATIC_CALL(name, _func) \
__DEFINE_STATIC_CALL(name, _func, _func)
#define DEFINE_STATIC_CALL_RET0(name, _func) \
__DEFINE_STATIC_CALL(name, _func, __static_call_return0)
#endif /* _LINUX_STATIC_CALL_H */
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