Re: [PATCH] objtool: improve detection of BUG() and other dead ends

From: Peter Zijlstra
Date: Tue Feb 21 2017 - 16:43:42 EST

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) <peterz@xxxxxxxxxxxxx>