linux-kernel - Re: [PATCH v3 26/26] objtool: Add STT

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Date:   Tue, 24 Mar 2020 23:34:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org, x86@...nel.org,
        mhiramat@...nel.org, mbenes@...e.cz, brgerst@...il.com
Subject: Re: [PATCH v3 26/26] objtool: Add STT_NOTYPE noinstr validation

On Tue, Mar 24, 2020 at 05:16:16PM -0500, Josh Poimboeuf wrote:
> On Tue, Mar 24, 2020 at 04:31:39PM +0100, Peter Zijlstra wrote:

> > +	if (state.noinstr) {
> > +		/*
> > +		 * In vmlinux mode we will not run validate_unwind_hints() by
> > +		 * default which means we'll not otherwise visit STT_NOTYPE
> > +		 * symbols.
> > +		 *
> > +		 * In case of --duplicate mode, insn->visited will avoid actual
> > +		 * duplicate work being done.
> > +		 */
> > +		list_for_each_entry(func, &sec->symbol_list, list) {
> > +			if (func->type != STT_NOTYPE)
> > +				continue;
> > +
> > +			warnings += validate_symbol(file, sec, func, &state);
> > +		}
> > +	}
> > +
> 
> I guess this is ok, but is there a valid reason why we don't just call
> validate_unwind_hints()?
> 
> It's also slightly concerning that validate_reachable_instructions()
> isn't called, I'm not 100% convinced all the code will get checked.

This will only end up running on .noinstr.text, while
validate_unwind_hints() will run on *everything*. That is, we're
purposely not checking everything.

It very much relies on the !vmlinux mode to do the unreachable things.