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b=SbZrc6fY9T7SORAuDT6PDiTXud4pOI2WQgqyRUHxS1owfay8JbDu77kpqkUIyE5Qh Oh/B9UCKJB+lIoWnFfCJkNEXaGRYxN0dEk0VKyaLHGsvqket/qFnMn389yq3EgAvVW Ac7vuiezT5Fcq1vU3a5HDDsmi0t0TNco9o6PtXxg= Date: Thu, 17 Apr 2025 13:40:06 -0700 From: Andrew Morton To: Mykyta Yatsenko Cc: linux-mm@kvack.org, rostedt@goodmis.org, mhiramat@kernel.org, andrii@kernel.org, kernel-team@meta.com, linux-kernel@vger.kernel.org, Mykyta Yatsenko Subject: Re: [PATCH mm] maccess: fix strncpy_from_user_nofault empty string handling Message-Id: <20250417134006.60e0d6b3fc963bcbec1255b0@linux-foundation.org> In-Reply-To: <20250417152808.722409-1-mykyta.yatsenko5@gmail.com> References: <20250417152808.722409-1-mykyta.yatsenko5@gmail.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: C7BDB40002 X-Stat-Signature: 8n89c1rp45ejr7sdgx8pac9q6zkczbuw X-Rspam-User: X-HE-Tag: 1744922408-618651 X-HE-Meta: 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 0B0a7cm9 gWEzcZMph/cO7HV82nHkSaZ0Tvl6h2l4z/MjaL82j9/qMHuJKA8FR0iJqYvISajt2XddBIdUNQNqJ5uvZtN1BSReogY+nLEKbjpQvk/EP402T/Fw/ozBclq3v2J5/BXn8zLhmY5yY7UqPYJvRyPk01e1esI4/Lntl5V72IFXdtKyB+br8Fvdi1PDWeuFDAC7WCi0+aa8iBtttNyBFhg64OCOKV4M4CIEmZfnBUkmjTS/+ldu6D1I/0HRdAInruFf+k/lmryx8Syk0A38VF3k5l9RQsGtwliIf3t/aMDOzWbJL4xMOy9hnUHcMbC42/i/s+eDVKW/mHID9pVE0TFZn11pNpvCcJezWblDMAOFbW43ZehaxjEfMhms8UQO/5LEZhAYmgpr0+nuOXto= 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 Thu, 17 Apr 2025 16:28:08 +0100 Mykyta Yatsenko wrote: > strncpy_from_user_nofault should return the length of the copied string > including the trailing NUL, but if the argument unsafe_addr points to > an empty string ({'\0'}), the return value is 0. > > This happens as strncpy_from_user copies terminal symbol into dst > and returns 0 (as expected), but strncpy_from_user_nofault does not > modify ret as it is not equal to count and not greater than 0, so 0 is > returned, which contradicts the contract. Looks right, I think. But why do strncpy_from_user() and strncpy_from_user_nofault() have different interfaces? /** * strncpy_from_user: - Copy a NUL terminated string from userspace. * ... * On success, returns the length of the string (not including the trailing * NUL). /** * strncpy_from_user_nofault: - Copy a NUL terminated string from unsafe user * address. * ... * On success, returns the length of the string INCLUDING the trailing NUL. This is surprising and I'm wondering what led us to do this?