include/linux/instrumentation.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef __LINUX_INSTRUMENTATION_H
   3 #define __LINUX_INSTRUMENTATION_H
   4
   5 #if defined(CONFIG_DEBUG_ENTRY) && defined(CONFIG_STACK_VALIDATION)
   6
   7 /* Begin/end of an instrumentation safe region */
   8 #define instrumentation_begin() ({                                      \
   9         asm volatile("%c0: nop\n\t"                                             \
  10                      ".pushsection .discard.instr_begin\n\t"            \
  11                      ".long %c0b - .\n\t"                               \
  12                      ".popsection\n\t" : : "i" (__COUNTER__));          \
  13 })
  14
  15 /*
  16  * Because instrumentation_{begin,end}() can nest, objtool validation considers
  17  * _begin() a +1 and _end() a -1 and computes a sum over the instructions.
  18  * When the value is greater than 0, we consider instrumentation allowed.
  19  *
  20  * There is a problem with code like:
  21  *
  22  * noinstr void foo()
  23  * {
  24  *      instrumentation_begin();
  25  *      ...
  26  *      if (cond) {
  27  *              instrumentation_begin();
  28  *              ...
  29  *              instrumentation_end();
  30  *      }
  31  *      bar();
  32  *      instrumentation_end();
  33  * }
  34  *
  35  * If instrumentation_end() would be an empty label, like all the other
  36  * annotations, the inner _end(), which is at the end of a conditional block,
  37  * would land on the instruction after the block.
  38  *
  39  * If we then consider the sum of the !cond path, we'll see that the call to
  40  * bar() is with a 0-value, even though, we meant it to happen with a positive
  41  * value.
  42  *
  43  * To avoid this, have _end() be a NOP instruction, this ensures it will be
  44  * part of the condition block and does not escape.
  45  */
  46 #define instrumentation_end() ({                                        \
  47         asm volatile("%c0: nop\n\t"                                     \
  48                      ".pushsection .discard.instr_end\n\t"              \
  49                      ".long %c0b - .\n\t"                               \
  50                      ".popsection\n\t" : : "i" (__COUNTER__));          \
  51 })
  52 #else
  53 # define instrumentation_begin()        do { } while(0)
  54 # define instrumentation_end()          do { } while(0)
  55 #endif
  56
  57 #endif /* __LINUX_INSTRUMENTATION_H */