include/linux/static_call.h

   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  *   static_call(name)(args...);
  21  *   static_call_cond(name)(args...);
  22  *   static_call_update(name, func);
  23  *
  24  * Usage example:
  25  *
  26  *   # Start with the following functions (with identical prototypes):
  27  *   int func_a(int arg1, int arg2);
  28  *   int func_b(int arg1, int arg2);
  29  *
  30  *   # Define a 'my_name' reference, associated with func_a() by default
  31  *   DEFINE_STATIC_CALL(my_name, func_a);
  32  *
  33  *   # Call func_a()
  34  *   static_call(my_name)(arg1, arg2);
  35  *
  36  *   # Update 'my_name' to point to func_b()
  37  *   static_call_update(my_name, &func_b);
  38  *
  39  *   # Call func_b()
  40  *   static_call(my_name)(arg1, arg2);
  41  *
  42  *
  43  * Implementation details:
  44  *
  45  *   This requires some arch-specific code (CONFIG_HAVE_STATIC_CALL).
  46  *   Otherwise basic indirect calls are used (with function pointers).
  47  *
  48  *   Each static_call() site calls into a trampoline associated with the name.
  49  *   The trampoline has a direct branch to the default function.  Updates to a
  50  *   name will modify the trampoline's branch destination.
  51  *
  52  *   If the arch has CONFIG_HAVE_STATIC_CALL_INLINE, then the call sites
  53  *   themselves will be patched at runtime to call the functions directly,
  54  *   rather than calling through the trampoline.  This requires objtool or a
  55  *   compiler plugin to detect all the static_call() sites and annotate them
  56  *   in the .static_call_sites section.
  57  *
  58  *
  59  * Notes on NULL function pointers:
  60  *
  61  *   Static_call()s support NULL functions, with many of the caveats that
  62  *   regular function pointers have.
  63  *
  64  *   Clearly calling a NULL function pointer is 'BAD', so too for
  65  *   static_call()s (although when HAVE_STATIC_CALL it might not be immediately
  66  *   fatal). A NULL static_call can be the result of:
  67  *
  68  *     DECLARE_STATIC_CALL_NULL(my_static_call, void (*)(int));
  69  *
  70  *   which is equivalent to declaring a NULL function pointer with just a
  71  *   typename:
  72  *
  73  *     void (*my_func_ptr)(int arg1) = NULL;
  74  *
  75  *   or using static_call_update() with a NULL function. In both cases the
  76  *   HAVE_STATIC_CALL implementation will patch the trampoline with a RET
  77  *   instruction, instead of an immediate tail-call JMP. HAVE_STATIC_CALL_INLINE
  78  *   architectures can patch the trampoline call to a NOP.
  79  *
  80  *   In all cases, any argument evaluation is unconditional. Unlike a regular
  81  *   conditional function pointer call:
  82  *
  83  *     if (my_func_ptr)
  84  *         my_func_ptr(arg1)
  85  *
  86  *   where the argument evaludation also depends on the pointer value.
  87  *
  88  *   When calling a static_call that can be NULL, use:
  89  *
  90  *     static_call_cond(name)(arg1);
  91  *
  92  *   which will include the required value tests to avoid NULL-pointer
  93  *   dereferences.
  94  */
  95
  96 #include <linux/types.h>
  97 #include <linux/cpu.h>
  98 #include <linux/static_call_types.h>
  99
 100 #ifdef CONFIG_HAVE_STATIC_CALL
 101 #include <asm/static_call.h>
 102
 103 /*
 104  * Either @site or @tramp can be NULL.
 105  */
 106 extern void arch_static_call_transform(void *site, void *tramp, void *func, bool tail);
 107
 108 #define STATIC_CALL_TRAMP_ADDR(name) &STATIC_CALL_TRAMP(name)
 109
 110 #else
 111 #define STATIC_CALL_TRAMP_ADDR(name) NULL
 112 #endif
 113
 114 #define static_call_update(name, func)                                  \
 115 ({                                                                      \
 116         BUILD_BUG_ON(!__same_type(*(func), STATIC_CALL_TRAMP(name)));   \
 117         __static_call_update(&STATIC_CALL_KEY(name),                    \
 118                              STATIC_CALL_TRAMP_ADDR(name), func);       \
 119 })
 120
 121 #ifdef CONFIG_HAVE_STATIC_CALL_INLINE
 122
 123 extern int __init static_call_init(void);
 124
 125 struct static_call_mod {
 126         struct static_call_mod *next;
 127         struct module *mod; /* for vmlinux, mod == NULL */
 128         struct static_call_site *sites;
 129 };
 130
 131 struct static_call_key {
 132         void *func;
 133         union {
 134                 /* bit 0: 0 = mods, 1 = sites */
 135                 unsigned long type;
 136                 struct static_call_mod *mods;
 137                 struct static_call_site *sites;
 138         };
 139 };
 140
 141 /* For finding the key associated with a trampoline */
 142 struct static_call_tramp_key {
 143         s32 tramp;
 144         s32 key;
 145 };
 146
 147 extern void __static_call_update(struct static_call_key *key, void *tramp, void *func);
 148 extern int static_call_mod_init(struct module *mod);
 149 extern int static_call_text_reserved(void *start, void *end);
 150
 151 #define DEFINE_STATIC_CALL(name, _func)                                 \
 152         DECLARE_STATIC_CALL(name, _func);                               \
 153         struct static_call_key STATIC_CALL_KEY(name) = {                \
 154                 .func = _func,                                          \
 155                 .type = 1,                                              \
 156         };                                                              \
 157         ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
 158
 159 #define DEFINE_STATIC_CALL_NULL(name, _func)                            \
 160         DECLARE_STATIC_CALL(name, _func);                               \
 161         struct static_call_key STATIC_CALL_KEY(name) = {                \
 162                 .func = NULL,                                           \
 163                 .type = 1,                                              \
 164         };                                                              \
 165         ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
 166
 167 #define static_call_cond(name)  (void)__static_call(name)
 168
 169 #define EXPORT_STATIC_CALL(name)                                        \
 170         EXPORT_SYMBOL(STATIC_CALL_KEY(name));                           \
 171         EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
 172 #define EXPORT_STATIC_CALL_GPL(name)                                    \
 173         EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name));                       \
 174         EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
 175
 176 /* Leave the key unexported, so modules can't change static call targets: */
 177 #define EXPORT_STATIC_CALL_TRAMP(name)                                  \
 178         EXPORT_SYMBOL(STATIC_CALL_TRAMP(name));                         \
 179         ARCH_ADD_TRAMP_KEY(name)
 180 #define EXPORT_STATIC_CALL_TRAMP_GPL(name)                              \
 181         EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name));                     \
 182         ARCH_ADD_TRAMP_KEY(name)
 183
 184 #elif defined(CONFIG_HAVE_STATIC_CALL)
 185
 186 static inline int static_call_init(void) { return 0; }
 187
 188 struct static_call_key {
 189         void *func;
 190 };
 191
 192 #define DEFINE_STATIC_CALL(name, _func)                                 \
 193         DECLARE_STATIC_CALL(name, _func);                               \
 194         struct static_call_key STATIC_CALL_KEY(name) = {                \
 195                 .func = _func,                                          \
 196         };                                                              \
 197         ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
 198
 199 #define DEFINE_STATIC_CALL_NULL(name, _func)                            \
 200         DECLARE_STATIC_CALL(name, _func);                               \
 201         struct static_call_key STATIC_CALL_KEY(name) = {                \
 202                 .func = NULL,                                           \
 203         };                                                              \
 204         ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
 205
 206 #define static_call_cond(name)  (void)__static_call(name)
 207
 208 static inline
 209 void __static_call_update(struct static_call_key *key, void *tramp, void *func)
 210 {
 211         cpus_read_lock();
 212         WRITE_ONCE(key->func, func);
 213         arch_static_call_transform(NULL, tramp, func, false);
 214         cpus_read_unlock();
 215 }
 216
 217 static inline int static_call_text_reserved(void *start, void *end)
 218 {
 219         return 0;
 220 }
 221
 222 #define EXPORT_STATIC_CALL(name)                                        \
 223         EXPORT_SYMBOL(STATIC_CALL_KEY(name));                           \
 224         EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
 225 #define EXPORT_STATIC_CALL_GPL(name)                                    \
 226         EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name));                       \
 227         EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
 228
 229 /* Leave the key unexported, so modules can't change static call targets: */
 230 #define EXPORT_STATIC_CALL_TRAMP(name)                                  \
 231         EXPORT_SYMBOL(STATIC_CALL_TRAMP(name))
 232 #define EXPORT_STATIC_CALL_TRAMP_GPL(name)                              \
 233         EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
 234
 235 #else /* Generic implementation */
 236
 237 static inline int static_call_init(void) { return 0; }
 238
 239 struct static_call_key {
 240         void *func;
 241 };
 242
 243 #define DEFINE_STATIC_CALL(name, _func)                                 \
 244         DECLARE_STATIC_CALL(name, _func);                               \
 245         struct static_call_key STATIC_CALL_KEY(name) = {                \
 246                 .func = _func,                                          \
 247         }
 248
 249 #define DEFINE_STATIC_CALL_NULL(name, _func)                            \
 250         DECLARE_STATIC_CALL(name, _func);                               \
 251         struct static_call_key STATIC_CALL_KEY(name) = {                \
 252                 .func = NULL,                                           \
 253         }
 254
 255 static inline void __static_call_nop(void) { }
 256
 257 /*
 258  * This horrific hack takes care of two things:
 259  *
 260  *  - it ensures the compiler will only load the function pointer ONCE,
 261  *    which avoids a reload race.
 262  *
 263  *  - it ensures the argument evaluation is unconditional, similar
 264  *    to the HAVE_STATIC_CALL variant.
 265  *
 266  * Sadly current GCC/Clang (10 for both) do not optimize this properly
 267  * and will emit an indirect call for the NULL case :-(
 268  */
 269 #define __static_call_cond(name)                                        \
 270 ({                                                                      \
 271         void *func = READ_ONCE(STATIC_CALL_KEY(name).func);             \
 272         if (!func)                                                      \
 273                 func = &__static_call_nop;                              \
 274         (typeof(STATIC_CALL_TRAMP(name))*)func;                         \
 275 })
 276
 277 #define static_call_cond(name)  (void)__static_call_cond(name)
 278
 279 static inline
 280 void __static_call_update(struct static_call_key *key, void *tramp, void *func)
 281 {
 282         WRITE_ONCE(key->func, func);
 283 }
 284
 285 static inline int static_call_text_reserved(void *start, void *end)
 286 {
 287         return 0;
 288 }
 289
 290 #define EXPORT_STATIC_CALL(name)        EXPORT_SYMBOL(STATIC_CALL_KEY(name))
 291 #define EXPORT_STATIC_CALL_GPL(name)    EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name))
 292
 293 #endif /* CONFIG_HAVE_STATIC_CALL */
 294
 295 #endif /* _LINUX_STATIC_CALL_H */