From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-180.mta1.migadu.com (out-180.mta1.migadu.com [95.215.58.180]) (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 091522E5D3C for ; Mon, 25 Aug 2025 11:19:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.180 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756120793; cv=none; b=MP/e6tQlGX1sX/bdwvrfZNqogoN9egA1BkKaNW4JcsYhUb18hAb5jvK0UW+wJ2UeNgYmiBPiFGXBxrNSPA4bf2kBcQ2IVqc8EPGu8z9jXQoV22i3r2wkIu69cthzvdi24vsWDmAwUL5f3pDTceEvI2zFx3DAub1r+Esj5HBUlOo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756120793; c=relaxed/simple; bh=abeEpDYL1qgnMqEOopqpv+UkECheT5tVxOCyUApnwH4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=p5tzxxz0KmW7z0Kkk7GWfiEgFYyBbpYwWrSf+2vCm7pY4w4chE3X5lRR8+Ji9FY8UU5FqELc14ka+qrwFZsAr9I2Ax0F+V3006bJPNUaTxHDT8uBWrBZku5QtI2f1yJOwLnWAWTpc8DzgRTQ+R77RVGn83dU7ovAamQtWzw9RQw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=cLHA4cDC; arc=none smtp.client-ip=95.215.58.180 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="cLHA4cDC" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1756120779; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Q3B0wNYJZR8rgNNF1zxTd6HIMaR7QIKKvouK3UYKEvM=; b=cLHA4cDCqEAbAsT55G9hBCiMO1/WtVQIBYsSTb84tnvkWYDt+dgExmtKFnm2O38aUL9xfW ER0biyvRqLkhNXP5M1jkfBh2cOWrtr+8H0Ug0cP7L7DkeUcA00lTg/xa3H5vrnCipnwmV5 HleeHsELwNfr8Whj9pmjWnyURQcormc= Date: Mon, 25 Aug 2025 19:19:19 +0800 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH] atomic: Specify natural alignment for atomic_t Content-Language: en-US To: Finn Thain Cc: akpm@linux-foundation.org, geert@linux-m68k.org, linux-kernel@vger.kernel.org, mhiramat@kernel.org, oak@helsinkinet.fi, peterz@infradead.org, stable@vger.kernel.org, will@kernel.org, Lance Yang , linux-m68k@lists.linux-m68k.org References: <7d9554bfe2412ed9427bf71ce38a376e06eb9ec4.1756087385.git.fthain@linux-m68k.org> <20250825032743.80641-1-ioworker0@gmail.com> <96ae7afc-c882-4c3d-9dea-3e2ae2789caf@linux.dev> <5a44c60b-650a-1f8a-d5cb-abf9f0716817@linux-m68k.org> <4e7e7292-338d-4a57-84ec-ae7427f6ad7c@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT Thanks for digging deeper! On 2025/8/25 18:49, Finn Thain wrote: > > [Belated Cc linux-m68k...] > > On Mon, 25 Aug 2025, Lance Yang wrote: > >> >> On 2025/8/25 14:17, Finn Thain wrote: >>> >>> On Mon, 25 Aug 2025, Lance Yang wrote: >>> >>>> >>>> What if we squash the runtime check fix into your patch? >>> >>> Did my patch not solve the problem? >> >> Hmm... it should solve the problem for natural alignment, which is a >> critical fix. >> >> But it cannot solve the problem of forced misalignment from drivers >> using #pragma pack(1). The runtime warning will still trigger in those >> cases. >> >> I built a simple test module on a kernel with your patch applied: >> >> ``` >> #include >> #include >> >> struct __attribute__((packed)) test_container { >> char padding[49]; >> struct mutex io_lock; >> }; >> >> static int __init alignment_init(void) >> { >> struct test_container cont; >> pr_info("io_lock address offset mod 4: %lu\n", (unsigned long)&cont.io_lock % 4); >> return 0; >> } >> >> static void __exit alignment_exit(void) >> { >> pr_info("Module unloaded\n"); >> } >> >> module_init(alignment_init); >> module_exit(alignment_exit); >> MODULE_LICENSE("GPL"); >> MODULE_AUTHOR("x"); >> MODULE_DESCRIPTION("x"); >> ``` >> >> Result from dmesg: >> [Mon Aug 25 15:44:50 2025] io_lock address offset mod 4: 1 >> > > Thanks for sending code to illustrate your point. Unfortunately, I was not > able to reproduce your results. Tested on x86, your test module shows no > misalignment: > > [131840.349042] io_lock address offset mod 4: 0 > > Tested on m68k I also get 0, given the patch at the top of this thread: > > [ 0.400000] io_lock address offset mod 4: 0 > >> >> As we can see, a packed struct can still force the entire mutex object >> to an unaligned address. With an address like this, the WARN_ON_ONCE can >> still be triggered. > > I don't think so. But there is something unexpected going on here -- the > output from pahole appears to say io_lock is at offset 49, which seems to > contradict the printk() output above. Interesting! That contradiction is the key. It seems we're seeing different compiler behaviors. I'm on GCC 14.2.0 (Debian 14.2.0-19), and it appears to be strictly respecting the packed attribute. So let's print something more: ``` #include #include struct __attribute__((packed)) test_container { char padding[49]; struct mutex io_lock; }; static int __init alignment_init(void) { struct test_container cont; pr_info("Container base address : %px\n", &cont); pr_info("io_lock member address : %px\n", &cont.io_lock); pr_info("io_lock address offset mod 4: %lu\n", (unsigned long)&cont.io_lock % 4); return 0; } static void __exit alignment_exit(void) { pr_info("Module unloaded\n"); } module_init(alignment_init); module_exit(alignment_exit); MODULE_LICENSE("GPL"); MODULE_AUTHOR("x"); MODULE_DESCRIPTION("x"); ``` Result from dmesg: ``` [Mon Aug 25 19:15:33 2025] Container base address : ff1100063570f840 [Mon Aug 25 19:15:33 2025] io_lock member address : ff1100063570f871 [Mon Aug 25 19:15:33 2025] io_lock address offset mod 4: 1 ``` io_lock is exactly base + 49, resulting in misalignment. Seems like your compiler is cleverly re-aligning the whole struct on the stack, but we can't rely on that behavior, as it's not guaranteed across all compilers or versions. wdyt?