linux-kernel - Re: BUG_ON() in workingset_node_shadows

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Date:   Thu, 6 Oct 2016 15:29:04 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Willy Tarreau <w@....eu>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Antonio SJ Musumeci <trapexit@...wn.link>,
        Miklos Szeredi <miklos@...redi.hu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: BUG_ON() in workingset_node_shadows_dec() triggers

On Thu, Oct 6, 2016 at 3:07 PM, Kees Cook <keescook@...omium.org> wrote:
> The "cleanest" way to handle it seemed to be the lock-busting logic
> already built into BUG, so I moved to that.

Heh. The lock-busting logic in BUG() has always been broken. It's been
random hacks. It doesn't actually work in any general case, it just
occasionally happens to get things right. Mostly it tries to handle
the console locking (the whole "oops_in_progress" magic) so that if
you have a BUG_ON() in bad areas, at least you still end up getting
output.

But no, it's not reliable in any way, shape or form. That's really why
you want to continue after a BUG().

> By far the most problematic is "stop kernel execution from
> continuing", but that's currently the behavior that BUG depends on, so
> replacing BUG with anything needs to either fix the surrounding logic
> to fail sanely or we have the keep the feature.

Well, I'm not sure how much we actually end up depending on it,
considering that we now have two examples of BUG() implementations
that actually do _not_ depend on stopping execution: both the sound
subsystem and the XFS version of BUG_ON() end up not actually doing
the BUG() thing.

               Linus