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 6582535B632 for ; Fri, 17 Jul 2026 08:14:51 +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=1784276095; cv=none; b=hXudJMv9iu2M/ijGOERzS+BydHaJY7oQVGCZW0qollbO7VJq8WlsUZAFEFet0s+XCqRSbx0FW9x2fKHU7CrEUAKd7Od0o3GqEKy4fpjf/XBPRh0CSHrnyfRxV8MrgHTeY8pqKQ7jWv9nzdSn5jpUwXefLkbUczVKwtP3VNyCyhs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784276095; c=relaxed/simple; bh=7RZXV7prFOsk3VsnrP8J7mUkJM4ZlRaoUEYjRO/l4XA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=FoyR2YgSunsm1r0N1CtxW1964vP4z8NsA6Ab4HrOnnypob9CK4vM55ZQEJswVJm9pis0AUs7Zy5LKKXTfxCmZiBRR5hHMkuTDo3zleKMp2SQ9rVkfs2FGPFE1JBcZPU9uaOhoZYVNsD0a45ySWq7gq94GSVXewJtOUXfZnUmYC4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Qhm9+ReC; 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="Qhm9+ReC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 645A21F000E9; Fri, 17 Jul 2026 08:14:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784276088; bh=qbD5NQoBCvfd6Qk0CnSTm5qNQka/bIEh8/JvSOMdPQY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Qhm9+ReCkTzUw2FD4qqM68qbCnkwaHH0297utJdRXiZaiNvTFofltA+Owz930xcmj c8HS5+mKfyCwmmSj6cgpVKhpBj7GxhpOYSAPI6aJXU2JY9Q0qnll4LZWF1TChfa9Dd 2hlIpPK4EvYfL5hOyUbpMWaPrqN3224XLSeRyaw+xVk0KWFJrQSxpRLB0q2eer3OTj dF6tzyNbCCZJ8dw3xMdr9KbPru+aNRMagDqV3iIXxwruC8r3W7prmpbtKamf6FyIch lab0aSQm/MDZWkBsgNqcz781DtESVxJGiiOUeA5qUExHFsOnICRczGWlBob/dPInim GfWBnwZGoeVVw== Message-ID: Date: Fri, 17 Jul 2026 10:14:44 +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] fork: Honor task_struct's declared alignment To: Karl Mehltretter , Ingo Molnar , Peter Zijlstra , Andrew Morton Cc: Kees Cook , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Harry Yoo (Oracle)" References: <20260710123957.31774-1-kmehltretter@gmail.com> From: "Vlastimil Babka (SUSE)" Content-Language: en-US Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/10/26 14:39, Karl Mehltretter wrote: > Since commit cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant > size"), struct task_struct is declared __attribute__((aligned(64))) on > all architectures. > > But fork_init() still sets the task_struct slab cache's alignment to > > align = max(L1_CACHE_BYTES, ARCH_MIN_TASKALIGN) > > which is smaller than 64 on architectures whose cache lines are below > 64 bytes: e.g. 32 on ARMv5. > > In practice plain SLUB happens to hand out 64-byte-aligned objects > anyway. With CONFIG_SLUB_DEBUG_ON the red-zone padding shifts objects > to the requested alignment. > > With CONFIG_UBSAN_ALIGNMENT=y a boot on QEMU versatilepb (ARM926EJ-S, > v7.2-rc2, gcc 13.3) floods the console with reports like: > > UBSAN: misaligned-access in include/linux/sched.h:2087:9 > member access within misaligned address c295d7e0 for type 'struct task_struct' > which requires 64 byte alignment > CPU: 0 UID: 0 PID: 15 Comm: pr/ttyAMA-1 Not tainted 7.2.0-rc2 #1 VOLUNTARY > > Set the slab alignment to at least the type's declared alignment. > > Fixes: cb7ca40a3882 ("x86/fpu: Make task_struct::thread constant size") > Assisted-by: Claude:claude-fable-5 > Signed-off-by: Karl Mehltretter LGTM. Additionally I think you can replace the manual L1_CACHE_BYTES handling with a SLAB_HWCACHE_ALIGN flag. That will apply cache_line_size(). > --- > kernel/fork.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/kernel/fork.c b/kernel/fork.c > index f0e2e131a9a5..b85cf1343329 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -857,7 +857,8 @@ void __init fork_init(void) > #ifndef ARCH_MIN_TASKALIGN > #define ARCH_MIN_TASKALIGN 0 > #endif > - int align = max_t(int, L1_CACHE_BYTES, ARCH_MIN_TASKALIGN); > + int align = max3(L1_CACHE_BYTES, ARCH_MIN_TASKALIGN, > + __alignof__(struct task_struct)); > unsigned long useroffset, usersize; > > /* create a slab on which task_structs can be allocated */