linux-kernel - Re: [Bug report] hash_name() may cross page boundary and trigger sleep in RCU context

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Message-ID: <20251209013021.2537-1-xieyuanbin1@huawei.com>
Date: Tue, 9 Dec 2025 09:30:21 +0800
From: Xie Yuanbin <xieyuanbin1@...wei.com>
To: <linux@...linux.org.uk>
CC: <akpm@...ux-foundation.org>, <brauner@...nel.org>,
	<catalin.marinas@....com>, <hch@....de>, <jack@...e.com>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<pangliyuan1@...wei.com>, <torvalds@...ux-foundation.org>,
	<viro@...iv.linux.org.uk>, <wangkefeng.wang@...wei.com>, <will@...nel.org>,
	<wozizhi@...weicloud.com>, <xieyuanbin1@...wei.com>, <yangerkun@...wei.com>
Subject: Re: [Bug report] hash_name() may cross page boundary and trigger sleep in RCU context

On Mon, 8 Dec 2025 15:43:56 +0000, Russell King wrote:
> On Mon, Dec 08, 2025 at 09:18:42PM +0800, Xie Yuanbin wrote:
>> I had indeed been lacking in consideration regarding do_alignment()
>> before, so thank you for reply. But, may I ask that, is there a scenario
>> where user-mode access to kernel addresses causes an alignment fault
>> (do_alignment())?
>
> If you mean, won't permission errors be detected first, then no.
> Alignment is one of the first things that is checked if alignment
> faults are enabled.
>
> So yes, if userspace attempts an unaigned load of a kernel address,
> and the CPU does not support / have enabled unaigned load support,
> then we will get a data abort with the FSR indicating an alignment
> fault. So do_alignment() wil be entered.
>
> Whether branch predictor handling needs to happen in this path is
> a separate question, but as it's highly likely we'll take an
> exception anyway and userspace is doing Bad Stuff, I feel it's
> better to be over-cautious.

Thanks for your reply. I know it now, and thank you.