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

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Message-ID: <20191021134759.GG1800@hirez.programming.kicks-ass.net>
Date:   Mon, 21 Oct 2019 15:47:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     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>,
        Josh Poimboeuf <jpoimboe@...hat.com>, linux@...musvillemoes.dk,
        cyphar@...har.com, keescook@...omium.org
Subject: Re: linux-next: Tree for Oct 18 (objtool)

On Mon, Oct 21, 2019 at 03:19:48PM +0200, Christian Brauner wrote:
> On Mon, Oct 21, 2019 at 03:11:49PM +0200, Peter Zijlstra wrote:

> > By popular request; here's that alternative. Completely untested :-)
> 
> Am I not getting some mails? :)

You're not on the 'right' IRC channels :-)

> I prefer this one as it allows us to avoid working around this in
> usercopy.c. Should especially make if this potentially helps in other
> cases as well?

That was Josh's argument too.

Personally I think GCC is being a moron here, because with value range
analysis it should be able to prove the shift-UB cannot happen (the <
sizeof(unsigned long) conditions on both), but alas, it emits the UBSAN
calls anyway.