1 /* SPDX-License-Identifier: GPL-2.0 */
2 #ifndef __LINUX_INSTRUMENTATION_H
3 #define __LINUX_INSTRUMENTATION_H
4 
5 #ifdef CONFIG_NOINSTR_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" : : "i" (c));			\
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" : : "i" (c));			\
54 })
55 #define instrumentation_end() __instrumentation_end(__COUNTER__)
56 #else /* !CONFIG_NOINSTR_VALIDATION */
57 # define instrumentation_begin()	do { } while(0)
58 # define instrumentation_end()		do { } while(0)
59 #endif /* CONFIG_NOINSTR_VALIDATION */
60 
61 #endif /* __LINUX_INSTRUMENTATION_H */
62