From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2AA0C37CD42; Tue, 2 Jun 2026 09:19:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780391995; cv=none; b=eFTRU8XZrFi+ve75hZXrwb6ptSllfRq6VqUFBX+9flJxDtV5ll4dHmwu/uBGkwxxbDQ3uHx4k0I0X9QGWY/FIw+LK9vMxi0+QnFbYKqiZ/QgPA6JZrZb2aKQbgjl19RP3vzIAwopehpGb10Sf3xY6Cl68qdAnwrAqbBs1O9tv7I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780391995; c=relaxed/simple; bh=hP67JeNpGsd91bjLjWBH123nmHtq1cwSYW0NQJk+JFM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QLQkncqxA8Q66XD1FoNuGAw0/32OaNQCCP4KoQ7xTMaImatW3ZOUWiQ8JsboBm8R+H4L44T9ZesQMpvv9N0Yr1XP9eD7/Ra6n/O2kcoxEdvVzUJxogfpUUKaKse+H2rSKBj6OWJ19LMpQ1cMw5ilZ8mARu95OFnjN9H5QWZAvks= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BNYD5hoE; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="BNYD5hoE" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CE0B21F00893; Tue, 2 Jun 2026 09:19:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780391993; bh=7gHvCsWNZjI8QmY3pCjcEfCTWb0bSVxI5a5CZALDQ+8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=BNYD5hoE4FSkqlFHssT/btTtpvouwiuIF4MEjbmsn4TMfPtjYlNicclO9M+lvr+0v yNdTG3Ome8V2lS8+sBYp2FQ/DkeSrsZ2HBVCu8RmdlGabr08yV/uGavdTnmZomBEtF fw7Ix+1G6aR2iKF/AKvWu2yYqgQl+ttmyXHQVp3KkVn2Bm6LEWdL+qPLle9KA+DkHV CkLoKBF5O3fgfytYtGMQLxVFSDhfaf6B/cRmOyWbz0AoVRjXCoIknBPNEOlqzTaIMC xpmGc68YUY04wbl7XxQMYinkVjPuKxKpgbCbrDndFrxGbD8PIjym13hv/7FX69TCMd APp+1CXI8dgHw== Message-ID: Date: Tue, 2 Jun 2026 11:19:49 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: don't allow empty relative nodemask in mpol_relative_nodemask() To: Gregory Price Cc: Yury Norov , Joshua Hahn , Andrew Morton , Zi Yan , Matthew Brost , Rakie Kim , Byungchul Park , Ying Huang , Alistair Popple , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Farhad Alemi , Waiman Long , Rasmus Villemoes , cgroups@vger.kernel.org References: <20260528124133.c88c27b11a8ea0ef05e494f7@linux-foundation.org> <20260529152616.2308736-1-joshua.hahnjy@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/2/26 10:44, Gregory Price wrote: > On Mon, Jun 01, 2026 at 04:32:25PM +0200, David Hildenbrand (Arm) wrote: >>> >>> Hi Joshua. >>> >>> Indeed, quick and dirty shot. >>> >>> The problem is that nodes_fold() can't work with the sz == 0. In >>> other words, folding to a 0-bit bitmap is an error. We don't check >>> that on bitmaps level because it's an internal helper, and it's a >>> caller's responsibility to validate the parameters. >>> >>> nodes_onto(), or more specifically bitmap_onto(), is a different >>> story. In case of empty relmap, the function actually clears all the >>> bits in dst and returns. >> >> It's very weird that mpol_new_nodemask() (->create() callback) disallows empty >> nodemasks, but mpol_rebind_nodemask() (->rebind() callback) would allow empty >> nodemasks. >> > > Was this actually observed? > > mpol_rebind_nodemask() happens when cgroup.cpuset changes, and > cgroup.cpuset cannot be empty. > > cpuset only changes with sysfs twiddles or offlining. In either case, > cpuset *guarantees* that cpuset.mems will never be empty. > > So... is this an observed bug or just a statically discovered > "bug" that can't actually be reached? According to the report [1] syzkaller can trigger it. There is no reproducer, though. [1] https://lore.kernel.org/all/CA+0ovCgxbZkXa+OU8w3s84R3KNPNxxRfmsNR-udh+afQBbGNmw@mail.gmail.com/ -- Cheers, David