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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-475c428c62fsm48753105e9.8.2025.10.22.07.20.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 22 Oct 2025 07:20:08 -0700 (PDT) Date: Wed, 22 Oct 2025 15:20:06 +0100 From: David Laight To: Thomas Gleixner Cc: LKML , Christophe Leroy , Mathieu Desnoyers , Andrew Cooper , Linus Torvalds , kernel test robot , Russell King , linux-arm-kernel@lists.infradead.org, x86@kernel.org, Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , linuxppc-dev@lists.ozlabs.org, Paul Walmsley , Palmer Dabbelt , linux-riscv@lists.infradead.org, Heiko Carstens , Christian Borntraeger , Sven Schnelle , linux-s390@vger.kernel.org, Julia Lawall , Nicolas Palix , Peter Zijlstra , Darren Hart , Davidlohr Bueso , =?UTF-8?B?QW5kcsOp?= Almeida , Alexander Viro , Christian Brauner , Jan Kara , linux-fsdevel@vger.kernel.org Subject: Re: [patch V4 07/12] uaccess: Provide scoped user access regions Message-ID: <20251022152006.4d461c8b@pumpkin> In-Reply-To: <20251022103112.294959046@linutronix.de> References: <20251022102427.400699796@linutronix.de> <20251022103112.294959046@linutronix.de> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 22 Oct 2025 14:49:10 +0200 (CEST) Thomas Gleixner wrote: > User space access regions are tedious and require similar code patterns all > over the place: > > if (!user_read_access_begin(from, sizeof(*from))) > return -EFAULT; > unsafe_get_user(val, from, Efault); > user_read_access_end(); > return 0; > Efault: > user_read_access_end(); > return -EFAULT; > > This got worse with the recent addition of masked user access, which > optimizes the speculation prevention: > > if (can_do_masked_user_access()) > from = masked_user_read_access_begin((from)); > else if (!user_read_access_begin(from, sizeof(*from))) > return -EFAULT; > unsafe_get_user(val, from, Efault); > user_read_access_end(); > return 0; > Efault: > user_read_access_end(); > return -EFAULT; > > There have been issues with using the wrong user_*_access_end() variant in > the error path and other typical Copy&Pasta problems, e.g. using the wrong > fault label in the user accessor which ends up using the wrong accesss end > variant. > > These patterns beg for scopes with automatic cleanup. The resulting outcome > is: > scoped_user_read_access(from, Efault) > unsafe_get_user(val, from, Efault); > return 0; > Efault: > return -EFAULT; > > The scope guarantees the proper cleanup for the access mode is invoked both > in the success and the failure (fault) path. > > The scoped_user_$MODE_access() macros are implemented as self terminating > nested for() loops. Thanks to Andrew Cooper for pointing me at them. The > scope can therefore be left with 'break', 'goto' and 'return'. Even > 'continue' "works" due to the self termination mechanism. I think that 'feature' should be marked as a 'bug', consider code like: for (; len >= sizeof (*uaddr); uaddr++; len -= sizeof (*uaddr)) { scoped_user_read_access(uaddr, Efault) { int frag_len; unsafe_get_user(frag_len, &uaddr->len, Efault); if (!frag_len) break; ... } ... } The expectation would be that the 'break' applies to the visible 'for' loop. But you need a 'goto' to escape from the visible loop. Someone who groks the static checkers might want to try to detect continue/break in those loops. David