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

[Peter Zijlstra]

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>