On Wed, Jan 13, 2021 at 08:23:15PM +0000, Mark Brown wrote:
> On Wed, Jan 13, 2021 at 01:33:13PM -0600, Josh Poimboeuf wrote:
>
> > I think it's worth mentioning a little more about objtool. There are a
> > few passing mentions of objtool's generation of metadata (i.e. ORC), but
> > objtool has another relevant purpose: stack validation. That's
> > particularly important when it comes to frame pointers.
>
> > For some architectures like x86_64 and arm64 (but not powerpc/s390),
> > it's far too easy for a human to write asm and/or inline asm which
> > violates frame pointer protocol, silently causing the violater's callee
> > to get skipped in the unwind. Such architectures need objtool
> > implemented for CONFIG_STACK_VALIDATION.
>
> This basically boils down to just adding a statement saying "you may
> need to depend on objtool" I think?
Right, but maybe it would be a short paragraph or two.
> > > +There are several ways an architecture may identify kernel code which is deemed
> > > +unreliable to unwind from, e.g.
>
> > > +* Using metadata created by objtool, with such code annotated with
> > > + SYM_CODE_{START,END} or STACKFRAME_NON_STANDARD().
>
> > I'm not sure why SYM_CODE_{START,END} is mentioned here, but it doesn't
> > necessarily mean the code is unreliable, and objtool doesn't treat it as
> > such. Its mention can probably be removed unless there was some other
> > point I'm missing.
>
> I was reading that as being a thing that the architecture could possibly
> do, especially as a first step - it does seem like a reasonable thing to
> consider using anyway. I guess you could also use it the other way
> around and do additional checks for things that are supposed to be
> regular functions that you relax for SYM_CODE() sections.
Makes sense, but we have to be careful not to imply that objtool already
does something like that :-)
--
Josh
