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[188.155.181.108]) by smtp.gmail.com with ESMTPSA id hp7sm2475722ejc.144.2022.02.23.06.22.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 23 Feb 2022 06:22:45 -0800 (PST) Message-ID: <636e5b92-8ed8-35a1-d6e9-516d5b35be91@canonical.com> Date: Wed, 23 Feb 2022 15:22:44 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [RFT PATCH 0/3] Fix kfree() of const memory on setting driver_override Content-Language: en-US To: Robin Murphy , Rasmus Villemoes , Abel Vesa , Michael Turquette , Stephen Boyd , Shawn Guo , Sascha Hauer , Pengutronix Kernel Team , Fabio Estevam , NXP Linux Team , Bjorn Andersson , Mathieu Poirier , Andy Gross , Srinivas Kandagatla , linux-clk@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-remoteproc@vger.kernel.org, linux-arm-msm@vger.kernel.org, alsa-devel@alsa-project.org References: <20220222132707.266883-1-krzysztof.kozlowski@canonical.com> <708eabb1-7b35-d525-d4c3-451d4a3de84f@rasmusvillemoes.dk> <0442526f-b6d9-8868-ac1c-dd11a2d3b2ab@arm.com> From: Krzysztof Kozlowski In-Reply-To: <0442526f-b6d9-8868-ac1c-dd11a2d3b2ab@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 23/02/2022 15:04, Robin Murphy wrote: > On 2022-02-22 14:06, Krzysztof Kozlowski wrote: >> On 22/02/2022 14:51, Rasmus Villemoes wrote: >>> On 22/02/2022 14.27, Krzysztof Kozlowski wrote: >>>> Hi, >>>> >>>> Drivers still seem to use driver_override incorrectly. Perhaps my old >>>> patch makes sense now? >>>> https://lore.kernel.org/all/1550484960-2392-3-git-send-email-krzk@kernel.org/ >>>> >>>> Not tested - please review and test (e.g. by writing to dirver_override >>>> sysfs entry with KASAN enabled). >>> >>> Perhaps it would make sense to update the core code to release using >>> kfree_const(), allowing drivers to set the initial value with >>> kstrdup_const(). Drivers that currently use kstrdup() or kasprintf() >>> will continue to work [but if they kstrdup() a string literal they could >>> be changed to use kstrdup_const]. >> >> The core here means several buses, so the change would not be that >> small. However I don't see the reason why "driver_override" is special >> and should be freed with kfree_const() while most of other places don't >> use it. >> >> The driver_override field definition is here obvious: "char *", so any >> assignments of "const char *" are logically wrong (although GCC does not >> warn of this literal string const discarding). Adding kfree_const() is >> hiding the problem - someone did not read the definition of assigned field. > > That's not the issue, though, is it? If I take the struct > platform_device definition at face value, this should be perfectly valid: > > static char foo[] = "foo"; > pdev->driver_override = &foo; Yes, that's not the issue. It's rather about the interface. By convention we do not modify string literals but "char *driver_override" indicates that this is modifiable memory. I would argue that it even means that ownership is passed. Therefore passing string literal to "char *driver_override" is wrong from logical point of view. Plus, as you mentioned later, can lead to undefined behavior. > > And in fact that's effectively how the direct assignment form works > anyway - string literals are static arrays of type char (or wchar_t), > *not* const char, however trying to modify them is undefined behaviour. > > There's a big difference between "non-const" and "kfree()able", and > AFAICS there's no obvious clue that the latter is actually a requirement. Then maybe kfreeable should be made a requirement? Or at least clearly documented? Best regards, Krzysztof