arch/x86/kernel/static_call.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/static_call.h>
   3 #include <linux/memory.h>
   4 #include <linux/bug.h>
   5 #include <asm/text-patching.h>
   6
   7 enum insn_type {
   8         CALL = 0, /* site call */
   9         NOP = 1,  /* site cond-call */
  10         JMP = 2,  /* tramp / site tail-call */
  11         RET = 3,  /* tramp / site cond-tail-call */
  12 };
  13
  14 /*
  15  * data16 data16 xorq %rax, %rax - a single 5 byte instruction that clears %rax
  16  * The REX.W cancels the effect of any data16.
  17  */
  18 static const u8 xor5rax[] = { 0x66, 0x66, 0x48, 0x31, 0xc0 };
  19
  20 static void __ref __static_call_transform(void *insn, enum insn_type type, void *func)
  21 {
  22         const void *emulate = NULL;
  23         int size = CALL_INSN_SIZE;
  24         const void *code;
  25
  26         switch (type) {
  27         case CALL:
  28                 code = text_gen_insn(CALL_INSN_OPCODE, insn, func);
  29                 if (func == &__static_call_return0) {
  30                         emulate = code;
  31                         code = &xor5rax;
  32                 }
  33
  34                 break;
  35
  36         case NOP:
  37                 code = x86_nops[5];
  38                 break;
  39
  40         case JMP:
  41                 code = text_gen_insn(JMP32_INSN_OPCODE, insn, func);
  42                 break;
  43
  44         case RET:
  45                 code = text_gen_insn(RET_INSN_OPCODE, insn, func);
  46                 size = RET_INSN_SIZE;
  47                 break;
  48         }
  49
  50         if (memcmp(insn, code, size) == 0)
  51                 return;
  52
  53         if (unlikely(system_state == SYSTEM_BOOTING))
  54                 return text_poke_early(insn, code, size);
  55
  56         text_poke_bp(insn, code, size, emulate);
  57 }
  58
  59 static void __static_call_validate(void *insn, bool tail)
  60 {
  61         u8 opcode = *(u8 *)insn;
  62
  63         if (tail) {
  64                 if (opcode == JMP32_INSN_OPCODE ||
  65                     opcode == RET_INSN_OPCODE)
  66                         return;
  67         } else {
  68                 if (opcode == CALL_INSN_OPCODE ||
  69                     !memcmp(insn, x86_nops[5], 5) ||
  70                     !memcmp(insn, xor5rax, 5))
  71                         return;
  72         }
  73
  74         /*
  75          * If we ever trigger this, our text is corrupt, we'll probably not live long.
  76          */
  77         WARN_ONCE(1, "unexpected static_call insn opcode 0x%x at %pS\n", opcode, insn);
  78 }
  79
  80 static inline enum insn_type __sc_insn(bool null, bool tail)
  81 {
  82         /*
  83          * Encode the following table without branches:
  84          *
  85          *      tail    null    insn
  86          *      -----+-------+------
  87          *        0  |   0   |  CALL
  88          *        0  |   1   |  NOP
  89          *        1  |   0   |  JMP
  90          *        1  |   1   |  RET
  91          */
  92         return 2*tail + null;
  93 }
  94
  95 void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
  96 {
  97         mutex_lock(&text_mutex);
  98
  99         if (tramp) {
 100                 __static_call_validate(tramp, true);
 101                 __static_call_transform(tramp, __sc_insn(!func, true), func);
 102         }
 103
 104         if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site) {
 105                 __static_call_validate(site, tail);
 106                 __static_call_transform(site, __sc_insn(!func, tail), func);
 107         }
 108
 109         mutex_unlock(&text_mutex);
 110 }
 111 EXPORT_SYMBOL_GPL(arch_static_call_transform);