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 ePXvv/bt 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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 Wed, Feb 19, 2025 at 10:15:47AM +0100, David Hildenbrand wrote: > 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. Ah yeah thanks David forgot about that! This is also a possibility if that'd solve your problems Kalesh? This bit will be fought over haha > > -- > Cheers, > > David / dhildenb >