linux-kernel - Re: linux-next: Tree for Mar 18 (objtool)

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Message-ID: <20200324164433.qusyu5h7ykx3f2bu@treble>
Date:   Tue, 24 Mar 2020 11:44:33 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: linux-next: Tree for Mar 18 (objtool)

On Fri, Mar 20, 2020 at 11:35:37AM -0700, Kees Cook wrote:
> On Fri, Mar 20, 2020 at 09:26:13AM +0100, Peter Zijlstra wrote:
> > On Thu, Mar 19, 2020 at 12:45:50PM -0500, Josh Poimboeuf wrote:
> > > > On Thu, Mar 19, 2020 at 12:33:31PM -0500, Josh Poimboeuf wrote:
> > 
> > > > > Actually I suspect it's the __builtin_unreachable() annotation which is
> > > > > making UBSAN add the __builtin_trap()...  because I don't see any double
> > > > > UD2s for WARNs.
> > 
> > > Actually, removing __builtin_unreachable() *does* make the extra UD2 go
> > > away -- I forgot I had some silly debug code.
> > 
> > LOL, check this:
> > 
> > "Built-in Function: void __builtin_unreachable (void)
> > 
> >     If control flow reaches the point of the __builtin_unreachable, the
> >     program is undefined. It is useful in situations where the compiler
> >     cannot deduce the unreachability of the code. "
> > 
> > Which, I bet, is what makes UBSAN insert that __builtin_trap().
> > 
> > What a friggin mess :/
> 
> What I'd like is to be able to specify to UBSAN what function to call
> for the trap. I'd prefer to specify a well-defined exception handler,
> but at present, UBSAN just inserts __builtin_trap().
> 
> Can't objtool be told to ignore a ud2 that lacks an execution path to it?

It can ignore unreachable UD2s, if we think that's the right fix.

I was hoping we could find a way to get rid of the double UD2s, but I
couldn't figure out a way to do that when I looked at it last week.

-- 
Josh