linux-kernel - Re: [BUG] -next objtool build failure (bisected)

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Message-ID: <20241015184507.hiepeqrjgcjssvoj@treble>
Date: Tue, 15 Oct 2024 13:45:07 -0500
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: dmitry.torokhov@...il.com, bentiss@...nel.org, sfr@...b.auug.org.au,
	linux-kernel@...r.kernel.org, linux-next@...r.kernel.org,
	linux-toolchains@...r.kernel.org
Subject: Re: [BUG] -next objtool build failure (bisected)

On Tue, Oct 15, 2024 at 11:10:11AM -0700, Paul E. McKenney wrote:
> Hello!
> 
> The next-20241011 release gets me build errors like the following:
> 
> 	vmlinux.o: warning: objtool: fetch_item() falls through to next function
> 
> Bisection leads me here:
> 
> 	61595012f280 ("HID: simplify code in fetch_item()")
> 
> This diff looks inoffensive to me, but I get this error on this commit
> and not on its predecessor.
> 
> This build failure happens on quite a few different kernel configurations.
> I bisected using this particular reproducer:
> 
> tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --buildonly --configs SRCU-T

The problem is likely this:

+	default:
+		unreachable();

The unreachable() annotation by itself produces undefined behavior, as
it will just fall off the edge of the function if the default case is
ever executed.

Better to do

	default:
		BUG();

which makes the behavior defined.

-- 
Josh