linux-kernel - Re: [PATCH v2 34/39] objtool: Validate IBT assumptions

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Date:   Sun, 27 Feb 2022 14:20:55 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 34/39] objtool: Validate IBT assumptions

On Sun, Feb 27, 2022 at 06:00:03PM +0100, Peter Zijlstra wrote:
> > > @@ -3101,6 +3164,17 @@ static int validate_branch(struct objtoo
> > >  
> > >  		if (insn->hint) {
> > >  			state.cfi = *insn->cfi;
> > > +			if (ibt) {
> > > +				struct symbol *sym;
> > > +
> > > +				if (insn->cfi->type == UNWIND_HINT_TYPE_REGS_PARTIAL &&
> > > +				    (sym = find_symbol_by_offset(insn->sec, insn->offset)) &&
> > > +				    insn->type != INSN_ENDBR && !insn->noendbr) {
> > > +					WARN_FUNC("IRET_REGS hint without ENDBR: %s",
> > > +						  insn->sec, insn->offset,
> > > +						  sym->name);
> > > +				}
> > 
> > No need to print sym->name here, WARN_FUNC() already does it?
> 
> Almost; perhaps the change to make is to either introduce WARN_SYM or
> make WARN_FUNC also print !STT_FUNC symbols ?

In the case of no function, WARN_FUNC() falls back to printing sec+off.
Is that not good enough?

> > > +static void validate_ibt_insn(struct objtool_file *file, struct instruction *insn)
> > > +{
> > > +	struct reloc *reloc = insn_reloc(file, insn);
> > > +	struct instruction *target;
> > > +
> > > +	for (;;) {
> > > +		if (!reloc)
> > > +			return;
> > > +
> > > +		target = validate_ibt_reloc(file, reloc);
> > > +		if (target)
> > > +			validate_ibt_target(file, insn, target);
> > > +
> > > +		reloc = find_reloc_by_dest_range(file->elf, insn->sec, reloc->offset + 1,
> > > +						 (insn->offset + insn->len) - (reloc->offset + 1));
> > > +	}
> > 
> > I'm confused about what this loop is trying to do.  Why would an
> > instruction have more than one reloc?  It at least needs a comment.
> 
> Because there are some :/ 'mov' can have an immediate and a
> displacement, both needing a relocation.

<boom> mind blown.  How did I not know this?

-- 
Josh