linux-kernel - Re: [PATCH RESEND] bug: When !CONFIG_BUG, simplify WARN_ON

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Date:	Mon, 24 Feb 2014 13:16:05 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	David Howells <dhowells@...hat.com>,
	Josh Triplett <josh@...htriplett.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] bug: When !CONFIG_BUG, simplify WARN_ON_ONCE and
 family

> BUG() normally causes a fault and we print helpful messages before killing
> the task, and gcc knows we never continue because of the
> __builtin_unreachable() annotation.
> 
> If BUG() is defined as 'do { } while (0)' in the example above, we get
> a warning because the function may end without returning a number.
> If we define it to 'do { unreachable(); } while (0)', we don't get a
> warning, but we can get undefined behavior in the case we ever get to
> the end of the function.

That warning is the right thing though. In a lot of cases BUG(); is
followed by code that can lead to serious corruption and potentially
things like disk corruption following or security compromise.

We *should* be warning if you are stupid enough to build a kernel where
BUG() does not terminate.

While I agree defining it as do {} while(1); would be a lot smarter,
simply making it required that a platform provides an implementation of
BUG() would be even better. 

Alan
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