linux-kernel - Re: [RFT PATCH 0/3] Fix kfree() of const memory on setting driver

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Message-ID: <636e5b92-8ed8-35a1-d6e9-516d5b35be91@canonical.com>
Date:   Wed, 23 Feb 2022 15:22:44 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Robin Murphy <robin.murphy@....com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Abel Vesa <abel.vesa@....com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        linux-clk@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-remoteproc@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, alsa-devel@...a-project.org
Subject: Re: [RFT PATCH 0/3] Fix kfree() of const memory on setting
 driver_override

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