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  * cs cs cs xorl %eax, %eax - a single 5 byte instruction that clears %[er]ax
  16  */
  17 static const u8 xor5rax[] = { 0x2e, 0x2e, 0x2e, 0x31, 0xc0 };
  18
  19 static const u8 retinsn[] = { RET_INSN_OPCODE, 0xcc, 0xcc, 0xcc, 0xcc };
  20
  21 static void __ref __static_call_transform(void *insn, enum insn_type type, void *func)
  22 {
  23         const void *emulate = NULL;
  24         int size = CALL_INSN_SIZE;
  25         const void *code;
  26
  27         switch (type) {
  28         case CALL:
  29                 code = text_gen_insn(CALL_INSN_OPCODE, insn, func);
  30                 if (func == &__static_call_return0) {
  31                         emulate = code;
  32                         code = &xor5rax;
  33                 }
  34
  35                 break;
  36
  37         case NOP:
  38                 code = x86_nops[5];
  39                 break;
  40
  41         case JMP:
  42                 code = text_gen_insn(JMP32_INSN_OPCODE, insn, func);
  43                 break;
  44
  45         case RET:
  46                 code = &retinsn;
  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, bool tramp)
  60 {
  61         u8 opcode = *(u8 *)insn;
  62
  63         if (tramp && memcmp(insn+5, "SCT", 3)) {
  64                 pr_err("trampoline signature fail");
  65                 BUG();
  66         }
  67
  68         if (tail) {
  69                 if (opcode == JMP32_INSN_OPCODE ||
  70                     opcode == RET_INSN_OPCODE)
  71                         return;
  72         } else {
  73                 if (opcode == CALL_INSN_OPCODE ||
  74                     !memcmp(insn, x86_nops[5], 5) ||
  75                     !memcmp(insn, xor5rax, 5))
  76                         return;
  77         }
  78
  79         /*
  80          * If we ever trigger this, our text is corrupt, we'll probably not live long.
  81          */
  82         pr_err("unexpected static_call insn opcode 0x%x at %pS\n", opcode, insn);
  83         BUG();
  84 }
  85
  86 static inline enum insn_type __sc_insn(bool null, bool tail)
  87 {
  88         /*
  89          * Encode the following table without branches:
  90          *
  91          *      tail    null    insn
  92          *      -----+-------+------
  93          *        0  |   0   |  CALL
  94          *        0  |   1   |  NOP
  95          *        1  |   0   |  JMP
  96          *        1  |   1   |  RET
  97          */
  98         return 2*tail + null;
  99 }
 100
 101 void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
 102 {
 103         mutex_lock(&text_mutex);
 104
 105         if (tramp) {
 106                 __static_call_validate(tramp, true, true);
 107                 __static_call_transform(tramp, __sc_insn(!func, true), func);
 108         }
 109
 110         if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site) {
 111                 __static_call_validate(site, tail, false);
 112                 __static_call_transform(site, __sc_insn(!func, tail), func);
 113         }
 114
 115         mutex_unlock(&text_mutex);
 116 }
 117 EXPORT_SYMBOL_GPL(arch_static_call_transform);