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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3c70f238640sm527339f8f.26.2025.08.22.11.53.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 22 Aug 2025 11:53:08 -0700 (PDT) Date: Fri, 22 Aug 2025 19:53:03 +0100 From: David Laight To: Linus Torvalds Cc: Christophe Leroy , Michael Ellerman , Nicholas Piggin , Madhavan Srinivasan , Alexander Viro , Christian Brauner , Jan Kara , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Darren Hart , Davidlohr Bueso , Andre Almeida , Andrew Morton , Dave Hansen , Daniel Borkmann , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-block@vger.kernel.org Subject: Re: [PATCH v2 02/10] uaccess: Add speculation barrier to copy_from_user_iter() Message-ID: <20250822195303.0d9fb6eb@pumpkin> In-Reply-To: References: <82b9c88e63a6f1f5926e39471364168b345d84cc.1755854833.git.christophe.leroy@csgroup.eu> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 569A5180010 X-Rspam-User: X-Stat-Signature: kbd7z54dpjoth34fxn94ygqjm9hu1a7u X-Rspamd-Server: rspam09 X-HE-Tag: 1755888790-473210 X-HE-Meta: U2FsdGVkX1+nGjLCr/aLg0HonmOZg+oSSdlPDLtbqeTIXfm9A8KnKx3uMsASzpxfAW148qMaLAwLpZGJ0MeFFBr8XfNImB0uU+Z39KniJOckAyCxZaqsEXkE5ktyXOUD8gxijm2dVkFhuzAr7D+JBZq+Qn4N/gjC+a31FaSjNXBWLGbw+hotWP266a0jiiebC0KIb6SVaAAS0NMeiCXxdkvDMu0tEr5qFzKaZjoAIDJ3jzuwZbQkE792QWLhGya+vtyuOzvc1ED/xVte2zDPE807h+xpkEdExL0Dr1RW3BFVJAxk5utJjgh25UrZI3kqcW2fi2Jf9I7XUHCF3jSEja+ppeMymDvFQdiUxAKMwXNnMgmrU0OLfVwGjaHnn4HDNqZcGAuzetjnefMWfnW3UxG5Gul7R2h3wreTVG9B5AnAJmixTtOaWXO7jqZ07VTAnYU3MSpSFYCTWIvs7bKQPYI2dZpf///NujUBg2Fz6fqJIxoSlBxC0djw2DhMwda5cAvO9YHvoOgqoWJHJgXAO7mTph7fRKEy+ZU9ukvbxQ0G2fl/BXgdT3RsFS4SliCx6o3LuonTl/VbQEaqWnrZM814etQbj3FRTIqeY5Hb9B2zehVUDMbcSkQrAYLwyKgzPoz55RjtzYusQe7rNjC7KNoWOd08AQAlCBNjjcD2rpbXb9z3QSOh6s8ZIDpPZZHstki/DpnvkcudV9gLrkqmqSEpAe8dsSc+0hfs17nKojoHFRrsuWSe2k2wS9s61Pn3I3qFuDTrqUO9Ac/RPe/5u4F79egMuP3Zu8fpB+jB5ANjaVeRZfSKfhmOPEZ0qoa4nEsQXKbODXhXYd+7aXwcKJ10i/jmWQRM2YjcVBtuvZMJcWphyhVwA2qnpRlNvlNGAwo5s7rqHe2LLgjn6oxtxshF+mKMynxcxiTbaq0jdAFEEi3EkKYxJkw2Ty7epXDnkZCgiNxaJZqPS+0dwEJ 7gPaGwUZ 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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 Fri, 22 Aug 2025 09:46:37 -0400 Linus Torvalds wrote: > On Fri, 22 Aug 2025 at 05:58, Christophe Leroy > wrote: > > > > The results of "access_ok()" can be mis-speculated. The result is that > > you can end speculatively: > > > > if (access_ok(from, size)) > > // Right here > > I actually think that we should probably just make access_ok() itself do this. You'd need to re-introduce the read/write parameter. And you'd want it to be compile time. Although going through the code changing them to read_access_ok() and write_access_ok() would probably leave you with a lot fewer calls. > We don't have *that* many users since we have been de-emphasizing the > "check ahead of time" model, and any that are performance-critical can > these days be turned into masked addresses. Or aim to allocate a guard page on all archs, support 'masked' access on all of them, and then just delete access_ok(). That'll make it look less ugly. Perhaps not this week though :-) David > > As it is, now we're in the situation that careful places - like > _inline_copy_from_user(), and with your patch copy_from_user_iter() - > do maybe wethis by hand and are ugly as a result, and lazy and > probably incorrect places don't do it at all. > > That said, I don't object to this patch and maybe we should do that > access_ok() change later and independently of any powerpc work. > > Linus