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

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Date:   Wed, 23 Feb 2022 14:04:23 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...onical.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 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;

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.

Cheers,
Robin.