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s=arc-20220608; d=hostedemail.com; t=1764152849; a=rsa-sha256; cv=none; b=ryJzSz6Dw7cBHYKFnoG8KCa/42WWX+To8zNsJ1Ta2oDygorCOtaBQGTf/dRWGPHZfyBeLO ZEsXtVp7U+kHUw7T5p/T1eRG9JzxCcKGVwSAynKSZgZdUBA9Bhqt7wjxIeCVwRdUxWzRsN yWb6czbPzBRXOosXHRx3G8Xh1psY/bs= ARC-Authentication-Results: i=1; imf20.hostedemail.com; dkim=none; spf=pass (imf20.hostedemail.com: domain of wozizhi@huaweicloud.com designates 45.249.212.56 as permitted sender) smtp.mailfrom=wozizhi@huaweicloud.com; dmarc=none Received: from mail.maildlp.com (unknown [172.19.163.235]) by dggsgout12.his.huawei.com (SkyGuard) with ESMTPS id 4dGbMP5mD0zKHMgr for ; Wed, 26 Nov 2025 18:26:41 +0800 (CST) Received: from mail02.huawei.com (unknown [10.116.40.112]) by mail.maildlp.com (Postfix) with ESMTP id 696011A0D59 for ; Wed, 26 Nov 2025 18:27:20 +0800 (CST) Received: from [10.174.176.88] (unknown [10.174.176.88]) by APP1 (Coremail) with SMTP id cCh0CgC3PkgH1iZpi0Q3CA--.23367S3; Wed, 26 Nov 2025 18:27:20 +0800 (CST) Message-ID: <33ab4aef-020e-49e7-8539-31bf78dac61a@huaweicloud.com> Date: Wed, 26 Nov 2025 18:27:19 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: Zizhi Wo Subject: Re: [Bug report] hash_name() may cross page boundary and trigger sleep in RCU context To: Zizhi Wo , jack@suse.com, brauner@kernel.org, hch@lst.de, akpm@linux-foundation.org, linux@armlinux.org.uk, torvalds@linux-foundation.org Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, yangerkun@huawei.com, wangkefeng.wang@huawei.com, pangliyuan1@huawei.com, xieyuanbin1@huawei.com References: <20251126090505.3057219-1-wozizhi@huaweicloud.com> In-Reply-To: <20251126090505.3057219-1-wozizhi@huaweicloud.com> Content-Type: text/plain; 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For example, when the > filename length is near the PAGE_SIZE boundary. This triggers a page fault, > which leads to a call to do_page_fault()->mmap_read_trylock(). If we can't > acquire the lock, we have to fall back to the mmap_read_lock() path, which > calls might_sleep(). This breaks RCU semantics because path lookup occurs > under an RCU read-side critical section. In linux-mainline, arm/arm64 > do_page_fault() still has this problem: > > lock_mm_and_find_vma->get_mmap_lock_carefully->mmap_read_lock_killable. > > And before commit bfcfaa77bdf0 ("vfs: use 'unsigned long' accesses for > dcache name comparison and hashing"), hash_name accessed the name byte by > byte. > > To prevent load_unaligned_zeropad() from accessing beyond the valid memory > region, we would need to intercept such cases beforehand? But doing so > would require replicating the internal logic of load_unaligned_zeropad(), > including handling endianness and constructing the correct value manually. > Given that load_unaligned_zeropad() is used in many places across the > kernel, we currently haven't found a good solution to address this cleanly. > > What would be the recommended way to handle this situation? Would > appreciate any feedback and guidance from the community. Thanks! > As a detail, we enabled CONFIG_DEBUG_PREEMPT and CONFIG_KFENCE, which allowed us to catch the potential out-of-bounds access. Thanks, Zizhi Wo