On Tue, Feb 21, 2017 at 03:35:32PM -0600, Josh Poimboeuf wrote: > The BUG() macro's use of __builtin_unreachable() via the unreachable() > macro tells gcc that the instruction is a dead end, and that it's safe > to assume the current code path will not execute past the previous > instruction. > > On x86, the BUG() macro is implemented with the 'ud2' instruction. When > objtool's branch analysis sees that instruction, it knows the current > code path has come to a dead end. > > Peter Zijlstra has been working on a patch to change the WARN macros to > use 'ud2'. That patch will break objtool's assumption that 'ud2' is > always a dead end. > > Generally it's best for objtool to avoid making those kinds of > assumptions anyway. The more ignorant it is of kernel code internals, > the better. > > So create a more generic way for objtool to detect dead ends by adding > an annotation to the unreachable() macro. The annotation stores a > pointer to the end of the unreachable code path in an '__unreachable' > section. Objtool can read that section to find the dead ends. >
Tested-by: Peter Zijlstra (Intel) <pet...@infradead.org>
