1 /* SPDX-License-Identifier: GPL-2.0 */
2 #ifndef _LINUX_STATIC_CALL_H
3 #define _LINUX_STATIC_CALL_H
4
5 /*
6 * Static call support
7 *
8 * Static calls use code patching to hard-code function pointers into direct
9 * branch instructions. They give the flexibility of function pointers, but
10 * with improved performance. This is especially important for cases where
11 * retpolines would otherwise be used, as retpolines can significantly impact
12 * performance.
13 *
14 *
15 * API overview:
16 *
17 * DECLARE_STATIC_CALL(name, func);
18 * DEFINE_STATIC_CALL(name, func);
19 * DEFINE_STATIC_CALL_NULL(name, typename);
20 * DEFINE_STATIC_CALL_RET0(name, typename);
21 *
22 * __static_call_return0;
23 *
24 * static_call(name)(args...);
25 * static_call_cond(name)(args...);
26 * static_call_update(name, func);
27 * static_call_query(name);
28 *
29 * EXPORT_STATIC_CALL{,_TRAMP}{,_GPL}()
30 *
31 * Usage example:
32 *
33 * # Start with the following functions (with identical prototypes):
34 * int func_a(int arg1, int arg2);
35 * int func_b(int arg1, int arg2);
36 *
37 * # Define a 'my_name' reference, associated with func_a() by default
38 * DEFINE_STATIC_CALL(my_name, func_a);
39 *
40 * # Call func_a()
41 * static_call(my_name)(arg1, arg2);
42 *
43 * # Update 'my_name' to point to func_b()
44 * static_call_update(my_name, &func_b);
45 *
46 * # Call func_b()
47 * static_call(my_name)(arg1, arg2);
48 *
49 *
50 * Implementation details:
51 *
52 * This requires some arch-specific code (CONFIG_HAVE_STATIC_CALL).
53 * Otherwise basic indirect calls are used (with function pointers).
54 *
55 * Each static_call() site calls into a trampoline associated with the name.
56 * The trampoline has a direct branch to the default function. Updates to a
57 * name will modify the trampoline's branch destination.
58 *
59 * If the arch has CONFIG_HAVE_STATIC_CALL_INLINE, then the call sites
60 * themselves will be patched at runtime to call the functions directly,
61 * rather than calling through the trampoline. This requires objtool or a
62 * compiler plugin to detect all the static_call() sites and annotate them
63 * in the .static_call_sites section.
64 *
65 *
66 * Notes on NULL function pointers:
67 *
68 * Static_call()s support NULL functions, with many of the caveats that
69 * regular function pointers have.
70 *
71 * Clearly calling a NULL function pointer is 'BAD', so too for
72 * static_call()s (although when HAVE_STATIC_CALL it might not be immediately
73 * fatal). A NULL static_call can be the result of:
74 *
75 * DECLARE_STATIC_CALL_NULL(my_static_call, void (*)(int));
76 *
77 * which is equivalent to declaring a NULL function pointer with just a
78 * typename:
79 *
80 * void (*my_func_ptr)(int arg1) = NULL;
81 *
82 * or using static_call_update() with a NULL function. In both cases the
83 * HAVE_STATIC_CALL implementation will patch the trampoline with a RET
84 * instruction, instead of an immediate tail-call JMP. HAVE_STATIC_CALL_INLINE
85 * architectures can patch the trampoline call to a NOP.
86 *
87 * In all cases, any argument evaluation is unconditional. Unlike a regular
88 * conditional function pointer call:
89 *
90 * if (my_func_ptr)
91 * my_func_ptr(arg1)
92 *
93 * where the argument evaludation also depends on the pointer value.
94 *
95 * When calling a static_call that can be NULL, use:
96 *
97 * static_call_cond(name)(arg1);
98 *
99 * which will include the required value tests to avoid NULL-pointer
100 * dereferences.
101 *
102 * To query which function is currently set to be called, use:
103 *
104 * func = static_call_query(name);
105 *
106 *
107 * DEFINE_STATIC_CALL_RET0 / __static_call_return0:
108 *
109 * Just like how DEFINE_STATIC_CALL_NULL() / static_call_cond() optimize the
110 * conditional void function call, DEFINE_STATIC_CALL_RET0 /
111 * __static_call_return0 optimize the do nothing return 0 function.
112 *
113 * This feature is strictly UB per the C standard (since it casts a function
114 * pointer to a different signature) and relies on the architecture ABI to
115 * make things work. In particular it relies on Caller Stack-cleanup and the
116 * whole return register being clobbered for short return values. All normal
117 * CDECL style ABIs conform.
118 *
119 * In particular the x86_64 implementation replaces the 5 byte CALL
120 * instruction at the callsite with a 5 byte clear of the RAX register,
121 * completely eliding any function call overhead.
122 *
123 * Notably argument setup is unconditional.
124 *
125 *
126 * EXPORT_STATIC_CALL() vs EXPORT_STATIC_CALL_TRAMP():
127 *
128 * The difference is that the _TRAMP variant tries to only export the
129 * trampoline with the result that a module can use static_call{,_cond}() but
130 * not static_call_update().
131 *
132 */
133
134 #include <linux/types.h>
135 #include <linux/cpu.h>
136 #include <linux/static_call_types.h>
137
138 #ifdef CONFIG_HAVE_STATIC_CALL
139 #include <asm/static_call.h>
140
141 /*
142 * Either @site or @tramp can be NULL.
143 */
144 extern void arch_static_call_transform(void *site, void *tramp, void *func, bool tail);
145
146 #define STATIC_CALL_TRAMP_ADDR(name) &STATIC_CALL_TRAMP(name)
147
148 #else
149 #define STATIC_CALL_TRAMP_ADDR(name) NULL
150 #endif
151
152 #define static_call_update(name, func) \
153 ({ \
154 typeof(&STATIC_CALL_TRAMP(name)) __F = (func); \
155 __static_call_update(&STATIC_CALL_KEY(name), \
156 STATIC_CALL_TRAMP_ADDR(name), __F); \
157 })
158
159 #define static_call_query(name) (READ_ONCE(STATIC_CALL_KEY(name).func))
160
161 #ifdef CONFIG_HAVE_STATIC_CALL_INLINE
162
163 extern int __init static_call_init(void);
164
165 extern void static_call_force_reinit(void);
166
167 struct static_call_mod {
168 struct static_call_mod *next;
169 struct module *mod; /* for vmlinux, mod == NULL */
170 struct static_call_site *sites;
171 };
172
173 /* For finding the key associated with a trampoline */
174 struct static_call_tramp_key {
175 s32 tramp;
176 s32 key;
177 };
178
179 extern void __static_call_update(struct static_call_key *key, void *tramp, void *func);
180 extern int static_call_mod_init(struct module *mod);
181 extern int static_call_text_reserved(void *start, void *end);
182
183 extern long __static_call_return0(void);
184
185 #define DEFINE_STATIC_CALL(name, _func) \
186 DECLARE_STATIC_CALL(name, _func); \
187 struct static_call_key STATIC_CALL_KEY(name) = { \
188 .func = _func, \
189 .type = 1, \
190 }; \
191 ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
192
193 #define DEFINE_STATIC_CALL_NULL(name, _func) \
194 DECLARE_STATIC_CALL(name, _func); \
195 struct static_call_key STATIC_CALL_KEY(name) = { \
196 .func = NULL, \
197 .type = 1, \
198 }; \
199 ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
200
201 #define DEFINE_STATIC_CALL_RET0(name, _func) \
202 DECLARE_STATIC_CALL(name, _func); \
203 struct static_call_key STATIC_CALL_KEY(name) = { \
204 .func = __static_call_return0, \
205 .type = 1, \
206 }; \
207 ARCH_DEFINE_STATIC_CALL_RET0_TRAMP(name)
208
209 #define static_call_cond(name) (void)__static_call(name)
210
211 #define EXPORT_STATIC_CALL(name) \
212 EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \
213 EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
214 #define EXPORT_STATIC_CALL_GPL(name) \
215 EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)); \
216 EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
217
218 /* Leave the key unexported, so modules can't change static call targets: */
219 #define EXPORT_STATIC_CALL_TRAMP(name) \
220 EXPORT_SYMBOL(STATIC_CALL_TRAMP(name)); \
221 ARCH_ADD_TRAMP_KEY(name)
222 #define EXPORT_STATIC_CALL_TRAMP_GPL(name) \
223 EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name)); \
224 ARCH_ADD_TRAMP_KEY(name)
225
226 #elif defined(CONFIG_HAVE_STATIC_CALL)
227
static_call_init(void)228 static inline int static_call_init(void) { return 0; }
229
230 #define DEFINE_STATIC_CALL(name, _func) \
231 DECLARE_STATIC_CALL(name, _func); \
232 struct static_call_key STATIC_CALL_KEY(name) = { \
233 .func = _func, \
234 }; \
235 ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
236
237 #define DEFINE_STATIC_CALL_NULL(name, _func) \
238 DECLARE_STATIC_CALL(name, _func); \
239 struct static_call_key STATIC_CALL_KEY(name) = { \
240 .func = NULL, \
241 }; \
242 ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
243
244 #define DEFINE_STATIC_CALL_RET0(name, _func) \
245 DECLARE_STATIC_CALL(name, _func); \
246 struct static_call_key STATIC_CALL_KEY(name) = { \
247 .func = __static_call_return0, \
248 }; \
249 ARCH_DEFINE_STATIC_CALL_RET0_TRAMP(name)
250
251 #define static_call_cond(name) (void)__static_call(name)
252
253 static inline
__static_call_update(struct static_call_key * key,void * tramp,void * func)254 void __static_call_update(struct static_call_key *key, void *tramp, void *func)
255 {
256 cpus_read_lock();
257 WRITE_ONCE(key->func, func);
258 arch_static_call_transform(NULL, tramp, func, false);
259 cpus_read_unlock();
260 }
261
static_call_text_reserved(void * start,void * end)262 static inline int static_call_text_reserved(void *start, void *end)
263 {
264 return 0;
265 }
266
267 extern long __static_call_return0(void);
268
269 #define EXPORT_STATIC_CALL(name) \
270 EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \
271 EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
272 #define EXPORT_STATIC_CALL_GPL(name) \
273 EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)); \
274 EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
275
276 /* Leave the key unexported, so modules can't change static call targets: */
277 #define EXPORT_STATIC_CALL_TRAMP(name) \
278 EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
279 #define EXPORT_STATIC_CALL_TRAMP_GPL(name) \
280 EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
281
282 #else /* Generic implementation */
283
static_call_init(void)284 static inline int static_call_init(void) { return 0; }
285
__static_call_return0(void)286 static inline long __static_call_return0(void)
287 {
288 return 0;
289 }
290
291 #define __DEFINE_STATIC_CALL(name, _func, _func_init) \
292 DECLARE_STATIC_CALL(name, _func); \
293 struct static_call_key STATIC_CALL_KEY(name) = { \
294 .func = _func_init, \
295 }
296
297 #define DEFINE_STATIC_CALL(name, _func) \
298 __DEFINE_STATIC_CALL(name, _func, _func)
299
300 #define DEFINE_STATIC_CALL_NULL(name, _func) \
301 __DEFINE_STATIC_CALL(name, _func, NULL)
302
303 #define DEFINE_STATIC_CALL_RET0(name, _func) \
304 __DEFINE_STATIC_CALL(name, _func, __static_call_return0)
305
__static_call_nop(void)306 static inline void __static_call_nop(void) { }
307
308 /*
309 * This horrific hack takes care of two things:
310 *
311 * - it ensures the compiler will only load the function pointer ONCE,
312 * which avoids a reload race.
313 *
314 * - it ensures the argument evaluation is unconditional, similar
315 * to the HAVE_STATIC_CALL variant.
316 *
317 * Sadly current GCC/Clang (10 for both) do not optimize this properly
318 * and will emit an indirect call for the NULL case :-(
319 */
320 #define __static_call_cond(name) \
321 ({ \
322 void *func = READ_ONCE(STATIC_CALL_KEY(name).func); \
323 if (!func) \
324 func = &__static_call_nop; \
325 (typeof(STATIC_CALL_TRAMP(name))*)func; \
326 })
327
328 #define static_call_cond(name) (void)__static_call_cond(name)
329
330 static inline
__static_call_update(struct static_call_key * key,void * tramp,void * func)331 void __static_call_update(struct static_call_key *key, void *tramp, void *func)
332 {
333 WRITE_ONCE(key->func, func);
334 }
335
static_call_text_reserved(void * start,void * end)336 static inline int static_call_text_reserved(void *start, void *end)
337 {
338 return 0;
339 }
340
341 #define EXPORT_STATIC_CALL(name) EXPORT_SYMBOL(STATIC_CALL_KEY(name))
342 #define EXPORT_STATIC_CALL_GPL(name) EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name))
343
344 #endif /* CONFIG_HAVE_STATIC_CALL */
345
346 #endif /* _LINUX_STATIC_CALL_H */
347