Re: [PATCH 1/3] driver core: generalize driver_override in struct device

[Geert Uytterhoeven]

Hi Danilo,

On Mon, 2 Mar 2026 at 01:28, Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
> Currently, there are 12 busses (including platform and PCI) that
> duplicate the driver_override logic for their individual devices.
>
> All of them seem to be prone to the bug described in [1].
>
> While this could be solved for every bus individually using a separate
> lock, solving this in the driver-core generically results in less (and
> cleaner) changes overall.
>
> Thus, move driver_override to struct device, provide corresponding
> accessors for busses and handle locking with a separate lock internally.
>
> In particular, add device_set_driver_override(),
> device_has_driver_override(), device_match_driver_override() and a
> helper, DEVICE_ATTR_DRIVER_OVERRIDE(), to declare the corresponding
> sysfs store() and show() callbacks.
>
> Until all busses have migrated, keep driver_set_override() in place.
>
> Note that we can't use the device lock for the reasons described in [2].
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=220789 [1]
> Link: https://lore.kernel.org/driver-core/DGRGTIRHA62X.3RY09D9SOK77P@xxxxxxxxxx/ [2]
> Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>

Thanks for your patch!

> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -381,6 +381,66 @@ static void __exit deferred_probe_exit(void)
>  }
>  __exitcall(deferred_probe_exit);
>
> +int __device_set_driver_override(struct device *dev, const char *s, size_t len)
> +{
> +       const char *new, *old;
> +       char *cp;
> +
> +       if (!s)
> +               return -EINVAL;
> +
> +       /*
> +        * The stored value will be used in sysfs show callback (sysfs_emit()),
> +        * which has a length limit of PAGE_SIZE and adds a trailing newline.
> +        * Thus we can store one character less to avoid truncation during sysfs
> +        * show.
> +        */
> +       if (len >= (PAGE_SIZE - 1))
> +               return -EINVAL;
> +
> +       /*
> +        * Compute the real length of the string in case userspace sends us a
> +        * bunch of \0 characters like python likes to do.
> +        */
> +       len = strlen(s);
> +

The newline case below is is basically the same case as the empty
string.  Hence if you would move the newline check here:

    if (len) {
            cp = strnchr(s, len, '\n');
            if (cp)
                    len = cp - s;
    }

then the "cp != s" check below is no longer needed.

> +       if (!len) {
> +               /* Empty string passed - clear override */
> +               spin_lock(&dev->driver_override.lock);
> +               old = dev->driver_override.name;
> +               dev->driver_override.name = NULL;
> +               spin_unlock(&dev->driver_override.lock);
> +               kfree(old);
> +
> +               return 0;
> +       }

Also, this block can be eliminated completely...

> +
> +       cp = strnchr(s, len, '\n');
> +       if (cp)
> +               len = cp - s;
> +
> +       new = kstrndup(s, len, GFP_KERNEL);
> +       if (!new)
> +               return -ENOMEM;

... by pre-initializing new to NULL, and making the allocation of new
conditional on len being non-zero.

> +
> +       spin_lock(&dev->driver_override.lock);
> +       old = dev->driver_override.name;
> +       if (cp != s) {
> +               dev->driver_override.name = new;
> +               spin_unlock(&dev->driver_override.lock);
> +       } else {
> +               /* "\n" passed - clear override */
> +               dev->driver_override.name = NULL;
> +               spin_unlock(&dev->driver_override.lock);
> +
> +               kfree(new);
> +       }
> +       kfree(old);
> +
> +       return 0;
> +}
> +EXPORT_SYMBOL_GPL(__device_set_driver_override);
> +
>  /**
>   * device_is_bound() - Check if device is bound to a driver
>   * @dev: device to check

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds