Re: Commit 0d989ac2c90b broke my x86-64 build.

From: Josh Poimboeuf
Date: Mon Oct 25 2021 - 14:59:55 EST


On Mon, Oct 25, 2021 at 04:56:37PM +0200, Peter Zijlstra wrote:
> On Mon, Oct 25, 2021 at 07:46:56AM -0700, Josh Poimboeuf wrote:
> > On Mon, Oct 25, 2021 at 11:04:33AM +0200, Peter Zijlstra wrote:
> > > On Sun, Oct 24, 2021 at 09:51:45PM -0500, Rob Landley wrote:
> > > > > Unfortunately I think CONFIG_STACK_VALIDATION is no longer optional on
> > > > > x86-64 these days, because of static calls and retpolines.
> > > >
> > > > Does it need stack validation, or just a frame unwinder?
> > >
> > > static_calls rely on objtool to find all "call __SCT*" instructions and
> > > write their location in a .static_call_sites section.
> > >
> > > The having of static calls is not optional on x86_64, and I have zero
> > > interest in trying to work out what not having static_call() does, or to
> > > maintain that option.
> >
> > What I meant was, make STATIC_CALL_INLINE optional. Then it would use
> > out-of-line static calls which should just work, no?
>
> Yeah, I suppose so... I think we're then missing a STACK_VALIDATION
> dependency for KCOV. We rely on objtool to nop out those
> __sanitizer_cov_* calls.
>
> I had really hoped to just make objtool an unconditional part of x86_64.

I was hoping the opposite ;-) Not everybody wants the extra build
overhead, object size, complexity, warnings, etc. And it should be
pretty easy to make it optional anyway.

Plus it's a good idea to make the dependencies more explicit. We've
already been looking at modularizing, like creating a new CONFIG_OBJTOOL
option and splitting stack validation out from some of the other
features. This could be a nice extension of that.

Which reminds me, I'm still thinking we need to make the interface more
easily combinable, like:

objtool run \
[--validate] \
[--noinstr] \
[--retpoline] \
[--orc] \
[--mcount] \
[--static-call] \
[--kcov] \
[--frame-pointer] \
[--vmlinux] \
[--uaccess] \
[--module] \
[--no-unreachable] \
[--backup] \
[--stats] \
[--backtrace]

objtool dump \
[--orc] \
[--mcount] \
[--static-call] \
[--alternatives] \
[--whatever]

I can hopefully get to it one of these weeks...

--
Josh