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(p200300cbc7074f00a44a5ad6765a0635.dip0.t-ipconnect.de. [2003:cb:c707:4f00:a44a:5ad6:765a:635]) by smtp.gmail.com with ESMTPSA id d9-20020adf9c89000000b0033df46f70dbsm9450876wre.9.2024.03.12.08.33.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 12 Mar 2024 08:33:15 -0700 (PDT) Message-ID: <08905bcc-677d-4981-926d-7f407b2f6a4a@redhat.com> Date: Tue, 12 Mar 2024 16:33:14 +0100 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 06/24] fsverity: pass tree_blocksize to end_enable_verity() Content-Language: en-US From: David Hildenbrand To: "Darrick J. Wong" , Matthew Wilcox Cc: Andrey Albershteyn , fsverity@lists.linux.dev, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, chandan.babu@oracle.com, akpm@linux-foundation.org, linux-mm@kvack.org, Eric Biggers References: <20240304191046.157464-2-aalbersh@redhat.com> <20240304191046.157464-8-aalbersh@redhat.com> <20240305005242.GE17145@sol.localdomain> <20240306163000.GP1927156@frogsfrogsfrogs> <20240307220224.GA1799@sol.localdomain> <20240308034650.GK1927156@frogsfrogsfrogs> <20240308044017.GC8111@sol.localdomain> <20240311223815.GW1927156@frogsfrogsfrogs> <9927568e-9f36-4417-9d26-c8a05c220399@redhat.com> Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <9927568e-9f36-4417-9d26-c8a05c220399@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 12.03.24 16:13, David Hildenbrand wrote: > On 11.03.24 23:38, Darrick J. Wong wrote: >> [add willy and linux-mm] >> >> On Thu, Mar 07, 2024 at 08:40:17PM -0800, Eric Biggers wrote: >>> On Thu, Mar 07, 2024 at 07:46:50PM -0800, Darrick J. Wong wrote: >>>>> BTW, is xfs_repair planned to do anything about any such extra blocks? >>>> >>>> Sorry to answer your question with a question, but how much checking is >>>> $filesystem expected to do for merkle trees? >>>> >>>> In theory xfs_repair could learn how to interpret the verity descriptor, >>>> walk the merkle tree blocks, and even read the file data to confirm >>>> intactness. If the descriptor specifies the highest block address then >>>> we could certainly trim off excess blocks. But I don't know how much of >>>> libfsverity actually lets you do that; I haven't looked into that >>>> deeply. :/ >>>> >>>> For xfs_scrub I guess the job is theoretically simpler, since we only >>>> need to stream reads of the verity files through the page cache and let >>>> verity tell us if the file data are consistent. >>>> >>>> For both tools, if something finds errors in the merkle tree structure >>>> itself, do we turn off verity? Or do we do something nasty like >>>> truncate the file? >>> >>> As far as I know (I haven't been following btrfs-progs, but I'm familiar with >>> e2fsprogs and f2fs-tools), there isn't yet any precedent for fsck actually >>> validating the data of verity inodes against their Merkle trees. >>> >>> e2fsck does delete the verity metadata of inodes that don't have the verity flag >>> enabled. That handles cleaning up after a crash during FS_IOC_ENABLE_VERITY. >>> >>> I suppose that ideally, if an inode's verity metadata is invalid, then fsck >>> should delete that inode's verity metadata and remove the verity flag from the >>> inode. Checking for a missing or obviously corrupt fsverity_descriptor would be >>> fairly straightforward, but it probably wouldn't catch much compared to actually >>> validating the data against the Merkle tree. And actually validating the data >>> against the Merkle tree would be complex and expensive. Note, none of this >>> would work on files that are encrypted. >>> >>> Re: libfsverity, I think it would be possible to validate a Merkle tree using >>> libfsverity_compute_digest() and the callbacks that it supports. But that's not >>> quite what it was designed for. >>> >>>> Is there an ioctl or something that allows userspace to validate an >>>> entire file's contents? Sort of like what BLKVERIFY would have done for >>>> block devices, except that we might believe its answers? >>> >>> Just reading the whole file and seeing whether you get an error would do it. >>> >>> Though if you want to make sure it's really re-reading the on-disk data, it's >>> necessary to drop the file's pagecache first. >> >> I tried a straight pagecache read and it worked like a charm! >> >> But then I thought to myself, do I really want to waste memory bandwidth >> copying a bunch of data? No. I don't even want to incur system call >> overhead from reading a single byte every $pagesize bytes. >> >> So I created 2M mmap areas and read a byte every $pagesize bytes. That >> worked too, insofar as SIGBUSes are annoying to handle. But it's >> annoying to take signals like that. >> >> Then I started looking at madvise. MADV_POPULATE_READ looked exactly >> like what I wanted -- it prefaults in the pages, and "If populating >> fails, a SIGBUS signal is not generated; instead, an error is returned." >> > > Yes, these were the expected semantics :) > >> But then I tried rigging up a test to see if I could catch an EIO, and >> instead I had to SIGKILL the process! It looks filemap_fault returns >> VM_FAULT_RETRY to __xfs_filemap_fault, which propagates up through >> __do_fault -> do_read_fault -> do_fault -> handle_pte_fault -> >> handle_mm_fault -> faultin_page -> __get_user_pages. At faultin_pages, >> the VM_FAULT_RETRY is translated to -EBUSY. >> >> __get_user_pages squashes -EBUSY to 0, so faultin_vma_page_range returns >> that to madvise_populate. Unfortunately, madvise_populate increments >> its loop counter by the return value (still 0) so it runs in an >> infinite loop. The only way out is SIGKILL. > > That's certainly unexpected. One user I know is QEMU, which primarily > uses MADV_POPULATE_WRITE to prefault page tables. Prefaulting in QEMU is > primarily used with shmem/hugetlb, where I haven't heard of any such > endless loops. > >> >> So I don't know what the correct behavior is here, other than the >> infinite loop seems pretty suspect. Is it the correct behavior that >> madvise_populate returns EIO if __get_user_pages ever returns zero? >> That doesn't quite sound right if it's the case that a zero return could >> also happen if memory is tight. > > madvise_populate() ends up calling faultin_vma_page_range() in a loop. > That one calls __get_user_pages(). > > __get_user_pages() documents: "0 return value is possible when the fault > would need to be retried." > > So that's what the caller does. IIRC, there are cases where we really > have to retry (at least once) and will make progress, so treating "0" as > an error would be wrong. > > Staring at other __get_user_pages() users, __get_user_pages_locked() > documents: "Please note that this function, unlike __get_user_pages(), > will not return 0 for nr_pages > 0, unless FOLL_NOWAIT is used.". > > But there is some elaborate retry logic in there, whereby the retry will > set FOLL_TRIED->FAULT_FLAG_TRIED, and I think we'd fail on the second > retry attempt (there are cases where we retry more often, but that's > related to something else I believe). > > So maybe we need a similar retry logic in faultin_vma_page_range()? Or > make it use __get_user_pages_locked(), but I recall when I introduced > MADV_POPULATE_READ, there was a catch to it. I'm trying to figure out who will be setting the VM_FAULT_SIGBUS in the mmap()+access case you describe above. Staring at arch/x86/mm/fault.c:do_user_addr_fault(), I don't immediately see how we would transition from a VM_FAULT_RETRY loop to VM_FAULT_SIGBUS. Because VM_FAULT_SIGBUS would be required for that function to call do_sigbus(). -- Cheers, David / dhildenb