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 #include <linux/stringify.h>
   8
   9 /* Begin/end of an instrumentation safe region */
  10 #define __instrumentation_begin(c) ({                                   \
  11         asm volatile(__stringify(c) ": nop\n\t"                         \
  12                      ".pushsection .discard.instr_begin\n\t"            \
  13                      ".long " __stringify(c) "b - .\n\t"                \
  14                      ".popsection\n\t");                                \
  15 })
  16 #define instrumentation_begin() __instrumentation_begin(__COUNTER__)
  17
  18 /*
  19  * Because instrumentation_{begin,end}() can nest, objtool validation considers
  20  * _begin() a +1 and _end() a -1 and computes a sum over the instructions.
  21  * When the value is greater than 0, we consider instrumentation allowed.
  22  *
  23  * There is a problem with code like:
  24  *
  25  * noinstr void foo()
  26  * {
  27  *      instrumentation_begin();
  28  *      ...
  29  *      if (cond) {
  30  *              instrumentation_begin();
  31  *              ...
  32  *              instrumentation_end();
  33  *      }
  34  *      bar();
  35  *      instrumentation_end();
  36  * }
  37  *
  38  * If instrumentation_end() would be an empty label, like all the other
  39  * annotations, the inner _end(), which is at the end of a conditional block,
  40  * would land on the instruction after the block.
  41  *
  42  * If we then consider the sum of the !cond path, we'll see that the call to
  43  * bar() is with a 0-value, even though, we meant it to happen with a positive
  44  * value.
  45  *
  46  * To avoid this, have _end() be a NOP instruction, this ensures it will be
  47  * part of the condition block and does not escape.
  48  */
  49 #define __instrumentation_end(c) ({                                     \
  50         asm volatile(__stringify(c) ": nop\n\t"                         \
  51                      ".pushsection .discard.instr_end\n\t"              \
  52                      ".long " __stringify(c) "b - .\n\t"                \
  53                      ".popsection\n\t");                                \
  54 })
  55 #define instrumentation_end() __instrumentation_end(__COUNTER__)
  56 #else
  57 # define instrumentation_begin()        do { } while(0)
  58 # define instrumentation_end()          do { } while(0)
  59 #endif
  60
  61 #endif /* __LINUX_INSTRUMENTATION_H */