Re: [RFT PATCH 0/3] Fix kfree() of const memory on setting driver_override

[Robin Murphy <robin.murphy@arm.com>]

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.