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(p200300cbc712840050d0d1c190d6e2e0.dip0.t-ipconnect.de. [2003:cb:c712:8400:50d0:d1c1:90d6:e2e0]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38f259f8730sm17691713f8f.93.2025.02.19.01.15.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 19 Feb 2025 01:15:49 -0800 (PST) Message-ID: <41af4ffb-0383-4d00-9639-0bf16e1f5f37@redhat.com> Date: Wed, 19 Feb 2025 10:15:47 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/4] mm: permit guard regions for file-backed/shmem mappings To: Lorenzo Stoakes , Kalesh Singh Cc: Andrew Morton , Suren Baghdasaryan , "Liam R . Howlett" , Matthew Wilcox , Vlastimil Babka , "Paul E . McKenney" , Jann Horn , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Shuah Khan , linux-kselftest@vger.kernel.org, linux-api@vger.kernel.org, John Hubbard , Juan Yescas References: <45297010-a0a4-4a42-84e8-6f4764eab3b3@lucifer.local> From: David Hildenbrand 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: <45297010-a0a4-4a42-84e8-6f4764eab3b3@lucifer.local> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 8WlJ-0E7PL9BzVlCOvGmDOElnfaVXno54jEXb46qA08_1739956551 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 8099116000D X-Stat-Signature: exrwfz1k4sk5eg9u9xgjykqob6xurc7u X-HE-Tag: 1739956554-501825 X-HE-Meta: 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 fowlL4hP 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 19.02.25 10:03, Lorenzo Stoakes wrote: > On Wed, Feb 19, 2025 at 12:25:51AM -0800, Kalesh Singh wrote: >> On Thu, Feb 13, 2025 at 10:18 AM Lorenzo Stoakes >> wrote: >>> >>> The guard regions feature was initially implemented to support anonymous >>> mappings only, excluding shmem. >>> >>> This was done such as to introduce the feature carefully and incrementally >>> and to be conservative when considering the various caveats and corner >>> cases that are applicable to file-backed mappings but not to anonymous >>> ones. >>> >>> Now this feature has landed in 6.13, it is time to revisit this and to >>> extend this functionality to file-backed and shmem mappings. >>> >>> In order to make this maximally useful, and since one may map file-backed >>> mappings read-only (for instance ELF images), we also remove the >>> restriction on read-only mappings and permit the establishment of guard >>> regions in any non-hugetlb, non-mlock()'d mapping. >> >> Hi Lorenzo, >> >> Thank you for your work on this. > > You're welcome. > >> >> Have we thought about how guard regions are represented in /proc/*/[s]maps? > > This is off-topic here but... Yes, extensively. No they do not appear > there. > > I thought you had attended LPC and my talk where I mentioned this > purposefully as a drawback? > > I went out of my way to advertise this limitation at the LPC talk, in the > original series, etc. so it's a little disappointing that this is being > brought up so late, but nobody else has raised objections to this issue so > I think in general it's not a limitation that matters in practice. > >> >> In the field, I've found that many applications read the ranges from >> /proc/self/[s]maps to determine what they can access (usually related >> to obfuscation techniques). If they don't know of the guard regions it >> would cause them to crash; I think that we'll need similar entries to >> PROT_NONE (---p) for these, and generally to maintain consistency >> between the behavior and what is being said from /proc/*/[s]maps. > > No, we cannot have these, sorry. > > Firstly /proc/$pid/[s]maps describes VMAs. The entire purpose of this > feature is to avoid having to accumulate VMAs for regions which are not > intended to be accessible. > > Secondly, there is no practical means for this to be accomplished in > /proc/$pid/maps in _any_ way - as no metadata relating to a VMA indicates > they have guard regions. > > This is intentional, because setting such metadata is simply not practical > - why? Because when you try to split the VMA, how do you know which bit > gets the metadata and which doesn't? You can't without _reading page > tables_. > > /proc/$pid/smaps _does_ read page tables, but we can't start pretending > VMAs exist when they don't, this would be completely inaccurate, would > break assumptions for things like mremap (which require a single VMA) and > would be unworkable. > > The best that _could_ be achieved is to have a marker in /proc/$pid/smaps > saying 'hey this region has guard regions somewhere'. And then simply expose it in /proc/$pid/pagemap, which is a better interface for this pte-level information inside of VMAs. We should still have a spare bit for that purpose in the pagemap entries. -- Cheers, David / dhildenb