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

[Robin Murphy]

On 2022-02-23 14:22, Krzysztof Kozlowski wrote:
> 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@xxxxxxxxxx/
> > > > >
> > > > > 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.

But does anything actually need to modify a driver_override string? I 
wouldn't have thought so. I see at least two buses that *do* define 
theirs as const char *, but still assume to kfree() them.

> > 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?

Indeed, there's clearly some room for improvement still. And I'm not 
suggesting that these changes aren't already sensible as they are, just 
that the given justification seems a little unfair :)

Even kfree_const() can't help if someone has put their string in the 
middle of some larger block of kmalloc()ed memory, so perhaps 
encouraging a dedicated setter function rather than just exposing a raw 
string pointer is the ideal solution in the long term.

Cheers,
Robin.