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 struct static_call_mod {
166 struct static_call_mod *next;
167 struct module *mod; /* for vmlinux, mod == NULL */
168 struct static_call_site *sites;
169 };
170
171 /* For finding the key associated with a trampoline */
172 struct static_call_tramp_key {
173 s32 tramp;
174 s32 key;
175 };
176
177 extern void __static_call_update(struct static_call_key *key, void *tramp, void *func);
178 extern int static_call_mod_init(struct module *mod);
179 extern int static_call_text_reserved(void *start, void *end);
180
181 extern long __static_call_return0(void);
182
183 #define DEFINE_STATIC_CALL(name, _func) \
184 DECLARE_STATIC_CALL(name, _func); \
185 struct static_call_key STATIC_CALL_KEY(name) = { \
186 .func = _func, \
187 .type = 1, \
188 }; \
189 ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
190
191 #define DEFINE_STATIC_CALL_NULL(name, _func) \
192 DECLARE_STATIC_CALL(name, _func); \
193 struct static_call_key STATIC_CALL_KEY(name) = { \
194 .func = NULL, \
195 .type = 1, \
196 }; \
197 ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
198
199 #define DEFINE_STATIC_CALL_RET0(name, _func) \
200 DECLARE_STATIC_CALL(name, _func); \
201 struct static_call_key STATIC_CALL_KEY(name) = { \
202 .func = __static_call_return0, \
203 .type = 1, \
204 }; \
205 ARCH_DEFINE_STATIC_CALL_RET0_TRAMP(name)
206
207 #define static_call_cond(name) (void)__static_call(name)
208
209 #define EXPORT_STATIC_CALL(name) \
210 EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \
211 EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
212 #define EXPORT_STATIC_CALL_GPL(name) \
213 EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)); \
214 EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
215
216 /* Leave the key unexported, so modules can't change static call targets: */
217 #define EXPORT_STATIC_CALL_TRAMP(name) \
218 EXPORT_SYMBOL(STATIC_CALL_TRAMP(name)); \
219 ARCH_ADD_TRAMP_KEY(name)
220 #define EXPORT_STATIC_CALL_TRAMP_GPL(name) \
221 EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name)); \
222 ARCH_ADD_TRAMP_KEY(name)
223
224 #elif defined(CONFIG_HAVE_STATIC_CALL)
225
static_call_init(void)226 static inline int static_call_init(void) { return 0; }
227
228 #define DEFINE_STATIC_CALL(name, _func) \
229 DECLARE_STATIC_CALL(name, _func); \
230 struct static_call_key STATIC_CALL_KEY(name) = { \
231 .func = _func, \
232 }; \
233 ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
234
235 #define DEFINE_STATIC_CALL_NULL(name, _func) \
236 DECLARE_STATIC_CALL(name, _func); \
237 struct static_call_key STATIC_CALL_KEY(name) = { \
238 .func = NULL, \
239 }; \
240 ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
241
242 #define DEFINE_STATIC_CALL_RET0(name, _func) \
243 DECLARE_STATIC_CALL(name, _func); \
244 struct static_call_key STATIC_CALL_KEY(name) = { \
245 .func = __static_call_return0, \
246 }; \
247 ARCH_DEFINE_STATIC_CALL_RET0_TRAMP(name)
248
249 #define static_call_cond(name) (void)__static_call(name)
250
251 static inline
__static_call_update(struct static_call_key * key,void * tramp,void * func)252 void __static_call_update(struct static_call_key *key, void *tramp, void *func)
253 {
254 cpus_read_lock();
255 WRITE_ONCE(key->func, func);
256 arch_static_call_transform(NULL, tramp, func, false);
257 cpus_read_unlock();
258 }
259
static_call_text_reserved(void * start,void * end)260 static inline int static_call_text_reserved(void *start, void *end)
261 {
262 return 0;
263 }
264
265 extern long __static_call_return0(void);
266
267 #define EXPORT_STATIC_CALL(name) \
268 EXPORT_SYMBOL(STATIC_CALL_KEY(name)); \
269 EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
270 #define EXPORT_STATIC_CALL_GPL(name) \
271 EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name)); \
272 EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
273
274 /* Leave the key unexported, so modules can't change static call targets: */
275 #define EXPORT_STATIC_CALL_TRAMP(name) \
276 EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
277 #define EXPORT_STATIC_CALL_TRAMP_GPL(name) \
278 EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
279
280 #else /* Generic implementation */
281
static_call_init(void)282 static inline int static_call_init(void) { return 0; }
283
__static_call_return0(void)284 static inline long __static_call_return0(void)
285 {
286 return 0;
287 }
288
289 #define __DEFINE_STATIC_CALL(name, _func, _func_init) \
290 DECLARE_STATIC_CALL(name, _func); \
291 struct static_call_key STATIC_CALL_KEY(name) = { \
292 .func = _func_init, \
293 }
294
295 #define DEFINE_STATIC_CALL(name, _func) \
296 __DEFINE_STATIC_CALL(name, _func, _func)
297
298 #define DEFINE_STATIC_CALL_NULL(name, _func) \
299 __DEFINE_STATIC_CALL(name, _func, NULL)
300
301 #define DEFINE_STATIC_CALL_RET0(name, _func) \
302 __DEFINE_STATIC_CALL(name, _func, __static_call_return0)
303
__static_call_nop(void)304 static inline void __static_call_nop(void) { }
305
306 /*
307 * This horrific hack takes care of two things:
308 *
309 * - it ensures the compiler will only load the function pointer ONCE,
310 * which avoids a reload race.
311 *
312 * - it ensures the argument evaluation is unconditional, similar
313 * to the HAVE_STATIC_CALL variant.
314 *
315 * Sadly current GCC/Clang (10 for both) do not optimize this properly
316 * and will emit an indirect call for the NULL case :-(
317 */
318 #define __static_call_cond(name) \
319 ({ \
320 void *func = READ_ONCE(STATIC_CALL_KEY(name).func); \
321 if (!func) \
322 func = &__static_call_nop; \
323 (typeof(STATIC_CALL_TRAMP(name))*)func; \
324 })
325
326 #define static_call_cond(name) (void)__static_call_cond(name)
327
328 static inline
__static_call_update(struct static_call_key * key,void * tramp,void * func)329 void __static_call_update(struct static_call_key *key, void *tramp, void *func)
330 {
331 WRITE_ONCE(key->func, func);
332 }
333
static_call_text_reserved(void * start,void * end)334 static inline int static_call_text_reserved(void *start, void *end)
335 {
336 return 0;
337 }
338
339 #define EXPORT_STATIC_CALL(name) EXPORT_SYMBOL(STATIC_CALL_KEY(name))
340 #define EXPORT_STATIC_CALL_GPL(name) EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name))
341
342 #endif /* CONFIG_HAVE_STATIC_CALL */
343
344 #endif /* _LINUX_STATIC_CALL_H */
345