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

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Message-ID: <688a88d8-0b40-486b-abc7-cf7c4a5f68d6@paulmck-laptop>
Date: Tue, 15 Oct 2024 11:57:24 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Josh Poimboeuf <jpoimboe@...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 01:45:07PM -0500, Josh Poimboeuf wrote:
> 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.

And this fixes things for me, thank you!

							Thanx, Paul