345 lines
10 KiB
C
345 lines
10 KiB
C
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/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _LINUX_STATIC_CALL_H
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#define _LINUX_STATIC_CALL_H
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/*
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* Static call support
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*
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* Static calls use code patching to hard-code function pointers into direct
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* branch instructions. They give the flexibility of function pointers, but
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* with improved performance. This is especially important for cases where
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* retpolines would otherwise be used, as retpolines can significantly impact
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* performance.
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*
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*
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* API overview:
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*
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* DECLARE_STATIC_CALL(name, func);
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* DEFINE_STATIC_CALL(name, func);
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* DEFINE_STATIC_CALL_NULL(name, typename);
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* DEFINE_STATIC_CALL_RET0(name, typename);
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*
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* __static_call_return0;
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*
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* static_call(name)(args...);
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* static_call_cond(name)(args...);
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* static_call_update(name, func);
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* static_call_query(name);
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*
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* EXPORT_STATIC_CALL{,_TRAMP}{,_GPL}()
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*
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* Usage example:
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*
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* # Start with the following functions (with identical prototypes):
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* int func_a(int arg1, int arg2);
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* int func_b(int arg1, int arg2);
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*
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* # Define a 'my_name' reference, associated with func_a() by default
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* DEFINE_STATIC_CALL(my_name, func_a);
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*
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* # Call func_a()
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* static_call(my_name)(arg1, arg2);
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*
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* # Update 'my_name' to point to func_b()
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* static_call_update(my_name, &func_b);
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*
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* # Call func_b()
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* static_call(my_name)(arg1, arg2);
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*
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*
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* Implementation details:
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*
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* This requires some arch-specific code (CONFIG_HAVE_STATIC_CALL).
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* Otherwise basic indirect calls are used (with function pointers).
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*
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* Each static_call() site calls into a trampoline associated with the name.
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* The trampoline has a direct branch to the default function. Updates to a
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* name will modify the trampoline's branch destination.
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*
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* If the arch has CONFIG_HAVE_STATIC_CALL_INLINE, then the call sites
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* themselves will be patched at runtime to call the functions directly,
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* rather than calling through the trampoline. This requires objtool or a
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* compiler plugin to detect all the static_call() sites and annotate them
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* in the .static_call_sites section.
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*
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*
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* Notes on NULL function pointers:
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*
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* Static_call()s support NULL functions, with many of the caveats that
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* regular function pointers have.
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*
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* Clearly calling a NULL function pointer is 'BAD', so too for
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* static_call()s (although when HAVE_STATIC_CALL it might not be immediately
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* fatal). A NULL static_call can be the result of:
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*
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* DECLARE_STATIC_CALL_NULL(my_static_call, void (*)(int));
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*
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* which is equivalent to declaring a NULL function pointer with just a
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* typename:
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*
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* void (*my_func_ptr)(int arg1) = NULL;
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*
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* or using static_call_update() with a NULL function. In both cases the
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* HAVE_STATIC_CALL implementation will patch the trampoline with a RET
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* instruction, instead of an immediate tail-call JMP. HAVE_STATIC_CALL_INLINE
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* architectures can patch the trampoline call to a NOP.
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*
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* In all cases, any argument evaluation is unconditional. Unlike a regular
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* conditional function pointer call:
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*
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* if (my_func_ptr)
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* my_func_ptr(arg1)
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*
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* where the argument evaludation also depends on the pointer value.
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*
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* When calling a static_call that can be NULL, use:
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*
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* static_call_cond(name)(arg1);
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*
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* which will include the required value tests to avoid NULL-pointer
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* dereferences.
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*
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* To query which function is currently set to be called, use:
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*
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* func = static_call_query(name);
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*
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*
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* DEFINE_STATIC_CALL_RET0 / __static_call_return0:
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*
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* Just like how DEFINE_STATIC_CALL_NULL() / static_call_cond() optimize the
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* conditional void function call, DEFINE_STATIC_CALL_RET0 /
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* __static_call_return0 optimize the do nothing return 0 function.
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*
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* This feature is strictly UB per the C standard (since it casts a function
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* pointer to a different signature) and relies on the architecture ABI to
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* make things work. In particular it relies on Caller Stack-cleanup and the
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* whole return register being clobbered for short return values. All normal
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* CDECL style ABIs conform.
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*
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* In particular the x86_64 implementation replaces the 5 byte CALL
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* instruction at the callsite with a 5 byte clear of the RAX register,
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* completely eliding any function call overhead.
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*
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* Notably argument setup is unconditional.
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*
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*
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* EXPORT_STATIC_CALL() vs EXPORT_STATIC_CALL_TRAMP():
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*
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* The difference is that the _TRAMP variant tries to only export the
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* trampoline with the result that a module can use static_call{,_cond}() but
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* not static_call_update().
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*
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*/
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#include <linux/types.h>
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#include <linux/cpu.h>
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#include <linux/static_call_types.h>
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#ifdef CONFIG_HAVE_STATIC_CALL
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#include <asm/static_call.h>
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/*
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* Either @site or @tramp can be NULL.
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*/
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extern void arch_static_call_transform(void *site, void *tramp, void *func, bool tail);
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#define STATIC_CALL_TRAMP_ADDR(name) &STATIC_CALL_TRAMP(name)
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#else
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#define STATIC_CALL_TRAMP_ADDR(name) NULL
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#endif
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#define static_call_update(name, func) \
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({ \
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typeof(&STATIC_CALL_TRAMP(name)) __F = (func); \
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__static_call_update(&STATIC_CALL_KEY(name), \
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STATIC_CALL_TRAMP_ADDR(name), __F); \
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})
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#define static_call_query(name) (READ_ONCE(STATIC_CALL_KEY(name).func))
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#ifdef CONFIG_HAVE_STATIC_CALL_INLINE
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extern int __init static_call_init(void);
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struct static_call_mod {
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struct static_call_mod *next;
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struct module *mod; /* for vmlinux, mod == NULL */
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struct static_call_site *sites;
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};
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/* For finding the key associated with a trampoline */
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struct static_call_tramp_key {
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s32 tramp;
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s32 key;
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};
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extern void __static_call_update(struct static_call_key *key, void *tramp, void *func);
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extern int static_call_mod_init(struct module *mod);
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extern int static_call_text_reserved(void *start, void *end);
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extern long __static_call_return0(void);
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#define DEFINE_STATIC_CALL(name, _func) \
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DECLARE_STATIC_CALL(name, _func); \
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struct static_call_key STATIC_CALL_KEY(name) = { \
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.func = _func, \
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.type = 1, \
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}; \
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ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
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#define DEFINE_STATIC_CALL_NULL(name, _func) \
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DECLARE_STATIC_CALL(name, _func); \
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struct static_call_key STATIC_CALL_KEY(name) = { \
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.func = NULL, \
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.type = 1, \
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}; \
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ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
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#define DEFINE_STATIC_CALL_RET0(name, _func) \
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DECLARE_STATIC_CALL(name, _func); \
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struct static_call_key STATIC_CALL_KEY(name) = { \
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.func = __static_call_return0, \
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.type = 1, \
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}; \
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ARCH_DEFINE_STATIC_CALL_RET0_TRAMP(name)
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#define static_call_cond(name) (void)__static_call(name)
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#define EXPORT_STATIC_CALL(name) \
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EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \
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EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
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#define EXPORT_STATIC_CALL_GPL(name) \
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EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)); \
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EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
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/* Leave the key unexported, so modules can't change static call targets: */
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#define EXPORT_STATIC_CALL_TRAMP(name) \
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EXPORT_SYMBOL(STATIC_CALL_TRAMP(name)); \
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ARCH_ADD_TRAMP_KEY(name)
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#define EXPORT_STATIC_CALL_TRAMP_GPL(name) \
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EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name)); \
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ARCH_ADD_TRAMP_KEY(name)
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#elif defined(CONFIG_HAVE_STATIC_CALL)
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static inline int static_call_init(void) { return 0; }
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#define DEFINE_STATIC_CALL(name, _func) \
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DECLARE_STATIC_CALL(name, _func); \
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struct static_call_key STATIC_CALL_KEY(name) = { \
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.func = _func, \
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}; \
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ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
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#define DEFINE_STATIC_CALL_NULL(name, _func) \
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DECLARE_STATIC_CALL(name, _func); \
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struct static_call_key STATIC_CALL_KEY(name) = { \
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.func = NULL, \
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}; \
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ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
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#define DEFINE_STATIC_CALL_RET0(name, _func) \
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DECLARE_STATIC_CALL(name, _func); \
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struct static_call_key STATIC_CALL_KEY(name) = { \
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.func = __static_call_return0, \
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}; \
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ARCH_DEFINE_STATIC_CALL_RET0_TRAMP(name)
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#define static_call_cond(name) (void)__static_call(name)
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static inline
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void __static_call_update(struct static_call_key *key, void *tramp, void *func)
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{
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cpus_read_lock();
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WRITE_ONCE(key->func, func);
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arch_static_call_transform(NULL, tramp, func, false);
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cpus_read_unlock();
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}
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static inline int static_call_text_reserved(void *start, void *end)
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{
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return 0;
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}
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extern long __static_call_return0(void);
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#define EXPORT_STATIC_CALL(name) \
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EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \
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EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
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#define EXPORT_STATIC_CALL_GPL(name) \
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EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)); \
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EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
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/* Leave the key unexported, so modules can't change static call targets: */
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#define EXPORT_STATIC_CALL_TRAMP(name) \
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EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
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#define EXPORT_STATIC_CALL_TRAMP_GPL(name) \
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EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
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#else /* Generic implementation */
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static inline int static_call_init(void) { return 0; }
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static inline long __static_call_return0(void)
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{
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return 0;
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}
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#define __DEFINE_STATIC_CALL(name, _func, _func_init) \
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DECLARE_STATIC_CALL(name, _func); \
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struct static_call_key STATIC_CALL_KEY(name) = { \
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.func = _func_init, \
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}
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#define DEFINE_STATIC_CALL(name, _func) \
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__DEFINE_STATIC_CALL(name, _func, _func)
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#define DEFINE_STATIC_CALL_NULL(name, _func) \
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__DEFINE_STATIC_CALL(name, _func, NULL)
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#define DEFINE_STATIC_CALL_RET0(name, _func) \
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__DEFINE_STATIC_CALL(name, _func, __static_call_return0)
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static inline void __static_call_nop(void) { }
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/*
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* This horrific hack takes care of two things:
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*
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* - it ensures the compiler will only load the function pointer ONCE,
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* which avoids a reload race.
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*
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* - it ensures the argument evaluation is unconditional, similar
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* to the HAVE_STATIC_CALL variant.
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*
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* Sadly current GCC/Clang (10 for both) do not optimize this properly
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* and will emit an indirect call for the NULL case :-(
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*/
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#define __static_call_cond(name) \
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({ \
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void *func = READ_ONCE(STATIC_CALL_KEY(name).func); \
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if (!func) \
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func = &__static_call_nop; \
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(typeof(STATIC_CALL_TRAMP(name))*)func; \
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})
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#define static_call_cond(name) (void)__static_call_cond(name)
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static inline
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void __static_call_update(struct static_call_key *key, void *tramp, void *func)
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{
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WRITE_ONCE(key->func, func);
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}
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static inline int static_call_text_reserved(void *start, void *end)
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{
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return 0;
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}
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#define EXPORT_STATIC_CALL(name) EXPORT_SYMBOL(STATIC_CALL_KEY(name))
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#define EXPORT_STATIC_CALL_GPL(name) EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name))
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#endif /* CONFIG_HAVE_STATIC_CALL */
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#endif /* _LINUX_STATIC_CALL_H */
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