On Tue, May 19, 2020 at 5:04 PM Peter Zijlstra <pet...@infradead.org> wrote:
> +#ifdef CONFIG_DEBUG_ENTRY > /* Begin/end of an instrumentation safe region */ > -#define instrumentation_begin() ({ > \ > +#define instrumentation_begin() ({ \ > asm volatile("%c0:\n\t" \ > ".pushsection .discard.instr_begin\n\t" \ > ".long %c0b - .\n\t" \ > ".popsection\n\t" : : "i" (__COUNTER__)); \ > }) > > -#define instrumentation_end() ({ > \ > - asm volatile("%c0:\n\t" \ > +/* > + * Because instrumentation_{begin,end}() can nest, objtool validation > considers > + * _begin() a +1 and _end() a -1 and computes a sum over the instructions. > + * When the value is greater than 0, we consider instrumentation allowed. > + * > + * There is a problem with code like: > + * > + * noinstr void foo() > + * { > + * instrumentation_begin(); > + * ... > + * if (cond) { > + * instrumentation_begin(); > + * ... > + * instrumentation_end(); > + * } > + * bar(); > + * instrumentation_end(); > + * } > + * > + * If instrumentation_end() would be an empty label, like all the other > + * annotations, the inner _end(), which is at the end of a conditional block, > + * would land on the instruction after the block. > + * > + * If we then consider the sum of the !cond path, we'll see that the call to > + * bar() is with a 0-value, even though, we meant it to happen with a > positive > + * value. > + * > + * To avoid this, have _end() be a NOP instruction, this ensures it will be > + * part of the condition block and does not escape. > + */ > +#define instrumentation_end() ({ \ > + asm volatile("%c0: nop\n\t" \ > ".pushsection .discard.instr_end\n\t" \ > ".long %c0b - .\n\t" \ > ".popsection\n\t" : : "i" (__COUNTER__)); \ > }) Hello, I, who don't know how does the objtool handle it, am just curious. _begin() and _end() are symmetrical, which means if _end() (without nop) can escape, so can _begin() in a reverse way. For example: noinstr void foo() { instrumentation_begin(); do { instrumentation_begin(); ... instrumentation_end(); } while (cond); bar(); instrumentation_end(); } Here, the first _begin() can be "dragged" into the do-while block. Expectedly, objtool validation should not complain here. But objtool validation's not complaining means it can handle it magically correctly (by distinguishing how many _begin()s should be taken around the jmp target when jmp in a specific path), or handle it by not checking if all paths have the same count onto a jmp target (a little nervous to me), or other possible ways. Sorry for my curiosity. Thanks Lai.