Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <CAHk-=wi4uO+Djqr4Jc1TnCofwxUTuXHtgkgwnVX86q06UGV6DA@mail.gmail.com>
 <6A09F721-0AD9-4B86-AB3E-563A1CF5ABDE@amacapital.net>
In-Reply-To: <6A09F721-0AD9-4B86-AB3E-563A1CF5ABDE@amacapital.net>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 21 Feb 2020 11:26:29 -0800
Message-ID: <CAHk-=whDHsbK3HTOpTF=ue_o04onRwTEaK_ZoJp_fjbqq4+=Jw@mail.gmail.com>
Subject: Re: [PATCH] mm/tlb: Fix use_mm() vs TLB invalidate
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>, Will Deacon <will@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Feb 21, 2020 at 11:22 AM Andy Lutomirski <luto@amacapital.net> wrot=
e:
>
> In this particular case, if we actually flub this, we are very likely to =
cause data corruption =E2=80=94 we=E2=80=99re about to do user access with =
the wrong mm.

So?

IF this ever triggers, it has presumably been around for decades.

Nobody noticed.

Adding a WARN_ON_ONCE() means that somebody will now notice. Good.

Adding a BUG_ON() will now possibly mean that the machine is dead, and
is not sending the bug-report to anybody.

It's not going to hit, of course. But if you are 100% sure of that,
then it shouldn't exist at all.

And if you're not 100% sure of it, then it's going to be in some very
unusual circumstances (possibly some odd driver subsystem that does
something completely wrong - wouldn't be the first time), and the last
thing you want to do is lose the information about it.

So BUG_ON() is basically ALWAYS 100% the wrong thing to do. The
argument that "there could be memory corruption" is complete and utter
garbage. See above why.

Stop it. Seriously.

                Linus