Re: [RFC][PATCH 00/16] objtool: vmlinux.o and noinstr validation

[Masami Hiramatsu]

On Thu, 12 Mar 2020 17:23:37 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Thu, Mar 12, 2020 at 02:41:07PM +0100, Peter Zijlstra wrote:
> > Hi all,
> > 
> > These patches extend objtool to be able to run on vmlinux.o and validate
> > Thomas's proposed noinstr annotation:
> > 
> >   https://lkml.kernel.org/r/20200310170951.87c29e9c1cfbddd93ccd92b3@xxxxxxxxxx
> > 
> >  "That's why we want the sections and the annotation. If something calls
> >   out of a noinstr section into a regular text section and the call is not
> >   annotated at the call site, then objtool can complain and tell you. What
> >   Peter and I came up with looks like this:
> > 
> >   noinstr foo()
> > 	do_protected(); <- Safe because in the noinstr section
> > 	instr_begin();  <- Marks the begin of a safe region, ignored
> > 			   by objtool
> > 	do_stuff();     <- All good
> > 	instr_end();    <- End of the safe region. objtool starts
> > 			   looking again
> > 	do_other_stuff();  <- Unsafe because do_other_stuff() is
> > 			      not protected
> > 
> >   and:
> > 
> >   noinstr do_protected()
> > 	bar();          <- objtool will complain here
> >   "
> > 
> > It should be accompanied by something like the below; which you'll find in a
> > series by Thomas.
> > 
> 
> So one of the problem i've ran into while playing with this and Thomas'
> patches is that it is 'difficult' to deal with indirect function calls.
> 
> objtool basically gives up instantly.

Can we introduce a "safe-call" wrapper function instead of indirect
call, and if objtool found an indirect call without safe-call function,
it can make it an error?

static int __noinstr safe_indirect_callback(int (*fn)(...), real-args)
{
	if (!is_instr_text(fn))
		return -ERANGE;
	return fn(real-args)
}

BTW, out of curiously, if BUG*() or WARN*() cases happens in the noinstr
section, do we also need to move them (register dump, stack unwinding,
printk, console output, etc.) all into noinstr section?

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>