Re: [PATCH v7 01/12] driver: platform: Add helper for safer setting of driver_override

["Rafael J. Wysocki" <rafael@xxxxxxxxxx>]

On Tue, Apr 19, 2022 at 1:34 PM Krzysztof Kozlowski
<krzysztof.kozlowski@xxxxxxxxxx> wrote:
>
> Several core drivers and buses expect that driver_override is a
> dynamically allocated memory thus later they can kfree() it.
>
> However such assumption is not documented, there were in the past and
> there are already users setting it to a string literal. This leads to
> kfree() of static memory during device release (e.g. in error paths or
> during unbind):
>
>     kernel BUG at ../mm/slub.c:3960!
>     Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
>     ...
>     (kfree) from [<c058da50>] (platform_device_release+0x88/0xb4)
>     (platform_device_release) from [<c0585be0>] (device_release+0x2c/0x90)
>     (device_release) from [<c0a69050>] (kobject_put+0xec/0x20c)
>     (kobject_put) from [<c0f2f120>] (exynos5_clk_probe+0x154/0x18c)
>     (exynos5_clk_probe) from [<c058de70>] (platform_drv_probe+0x6c/0xa4)
>     (platform_drv_probe) from [<c058b7ac>] (really_probe+0x280/0x414)
>     (really_probe) from [<c058baf4>] (driver_probe_device+0x78/0x1c4)
>     (driver_probe_device) from [<c0589854>] (bus_for_each_drv+0x74/0xb8)
>     (bus_for_each_drv) from [<c058b48c>] (__device_attach+0xd4/0x16c)
>     (__device_attach) from [<c058a638>] (bus_probe_device+0x88/0x90)
>     (bus_probe_device) from [<c05871fc>] (device_add+0x3dc/0x62c)
>     (device_add) from [<c075ff10>] (of_platform_device_create_pdata+0x94/0xbc)
>     (of_platform_device_create_pdata) from [<c07600ec>] (of_platform_bus_create+0x1a8/0x4fc)
>     (of_platform_bus_create) from [<c0760150>] (of_platform_bus_create+0x20c/0x4fc)
>     (of_platform_bus_create) from [<c07605f0>] (of_platform_populate+0x84/0x118)
>     (of_platform_populate) from [<c0f3c964>] (of_platform_default_populate_init+0xa0/0xb8)
>     (of_platform_default_populate_init) from [<c01031f8>] (do_one_initcall+0x8c/0x404)
>
> Provide a helper which clearly documents the usage of driver_override.
> This will allow later to reuse the helper and reduce the amount of
> duplicated code.
>
> Convert the platform driver to use a new helper and make the
> driver_override field const char (it is not modified by the core).
>
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx>
> ---
>  drivers/base/driver.c           | 69 +++++++++++++++++++++++++++++++++
>  drivers/base/platform.c         | 28 ++-----------
>  include/linux/device/driver.h   |  2 +
>  include/linux/platform_device.h |  6 ++-
>  4 files changed, 80 insertions(+), 25 deletions(-)
>
> diff --git a/drivers/base/driver.c b/drivers/base/driver.c
> index 8c0d33e182fd..1b9d47b10bd0 100644
> --- a/drivers/base/driver.c
> +++ b/drivers/base/driver.c
> @@ -30,6 +30,75 @@ static struct device *next_device(struct klist_iter *i)
>         return dev;
>  }
>
> +/**
> + * driver_set_override() - Helper to set or clear driver override.
> + * @dev: Device to change
> + * @override: Address of string to change (e.g. &device->driver_override);
> + *            The contents will be freed and hold newly allocated override.

I would stick to one-line description here and possibly expand them in
the body of the comment.

Regardless, I think that the series is an improvement, so please feel
free to add

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

to this patch and

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

to the other patches in the series.

> + * @s: NUL-terminated string, new driver name to force a match, pass empty
> + *     string to clear it ("" or "\n", where the latter is only for sysfs
> + *     interface).
> + * @len: length of @s
> + *
> + * Helper to set or clear driver override in a device, intended for the cases
> + * when the driver_override field is allocated by driver/bus code.
> + *
> + * Returns: 0 on success or a negative error code on failure.
> + */
> +int driver_set_override(struct device *dev, const char **override,
> +                       const char *s, size_t len)
> +{
> +       const char *new, *old;
> +       char *cp;
> +
> +       if (!override || !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;
> +
> +       if (!len) {
> +               /* Empty string passed - clear override */
> +               device_lock(dev);
> +               old = *override;
> +               *override = NULL;
> +               device_unlock(dev);
> +               kfree(old);
> +
> +               return 0;
> +       }
> +
> +       cp = strnchr(s, len, '\n');
> +       if (cp)
> +               len = cp - s;
> +
> +       new = kstrndup(s, len, GFP_KERNEL);
> +       if (!new)
> +               return -ENOMEM;
> +
> +       device_lock(dev);
> +       old = *override;
> +       if (cp != s) {
> +               *override = new;
> +       } else {
> +               /* "\n" passed - clear override */
> +               kfree(new);
> +               *override = NULL;
> +       }
> +       device_unlock(dev);
> +
> +       kfree(old);
> +
> +       return 0;
> +}
> +EXPORT_SYMBOL_GPL(driver_set_override);
> +
>  /**
>   * driver_for_each_device - Iterator for devices bound to a driver.
>   * @drv: Driver we're iterating.
> diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> index 8cc272fd5c99..b684157b7f2f 100644
> --- a/drivers/base/platform.c
> +++ b/drivers/base/platform.c
> @@ -1275,31 +1275,11 @@ static ssize_t driver_override_store(struct device *dev,
>                                      const char *buf, size_t count)
>  {
>         struct platform_device *pdev = to_platform_device(dev);
> -       char *driver_override, *old, *cp;
> -
> -       /* We need to keep extra room for a newline */
> -       if (count >= (PAGE_SIZE - 1))
> -               return -EINVAL;
> -
> -       driver_override = kstrndup(buf, count, GFP_KERNEL);
> -       if (!driver_override)
> -               return -ENOMEM;
> -
> -       cp = strchr(driver_override, '\n');
> -       if (cp)
> -               *cp = '\0';
> -
> -       device_lock(dev);
> -       old = pdev->driver_override;
> -       if (strlen(driver_override)) {
> -               pdev->driver_override = driver_override;
> -       } else {
> -               kfree(driver_override);
> -               pdev->driver_override = NULL;
> -       }
> -       device_unlock(dev);
> +       int ret;
>
> -       kfree(old);
> +       ret = driver_set_override(dev, &pdev->driver_override, buf, count);
> +       if (ret)
> +               return ret;
>
>         return count;
>  }
> diff --git a/include/linux/device/driver.h b/include/linux/device/driver.h
> index 15e7c5e15d62..700453017e1c 100644
> --- a/include/linux/device/driver.h
> +++ b/include/linux/device/driver.h
> @@ -151,6 +151,8 @@ extern int __must_check driver_create_file(struct device_driver *driver,
>  extern void driver_remove_file(struct device_driver *driver,
>                                const struct driver_attribute *attr);
>
> +int driver_set_override(struct device *dev, const char **override,
> +                       const char *s, size_t len);
>  extern int __must_check driver_for_each_device(struct device_driver *drv,
>                                                struct device *start,
>                                                void *data,
> diff --git a/include/linux/platform_device.h b/include/linux/platform_device.h
> index 7c96f169d274..582d83ed9a91 100644
> --- a/include/linux/platform_device.h
> +++ b/include/linux/platform_device.h
> @@ -31,7 +31,11 @@ struct platform_device {
>         struct resource *resource;
>
>         const struct platform_device_id *id_entry;
> -       char *driver_override; /* Driver name to force a match */
> +       /*
> +        * Driver name to force a match.  Do not set directly, because core
> +        * frees it.  Use driver_set_override() to set or clear it.
> +        */
> +       const char *driver_override;
>
>         /* MFD cell pointer */
>         struct mfd_cell *mfd_cell;
> --
> 2.32.0
>