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.
