Virtualization - Mar 2022 - [PATCH v4 01/11] driver: platform: Add helper for safer setting of driver_override

On Sat, Mar 12, 2022 at 5:16 PM Krzysztof Kozlowski
<krzysztof.kozlowski at canonical.com> 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)
>     (do_one_initcall) from [<c0f012c0>]
(kernel_init_freeable+0x3d0/0x4d8)
>     (kernel_init_freeable) from [<c0a7def0>] (kernel_init+0x8/0x114)
>     (kernel_init) from [<c01010b4>] (ret_from_fork+0x14/0x20)
I believe you may remove these three.
> Provide a helper which clearly documents the usage of driver_override.
> This will allow later to reuse the helper and reduce amount of
the amount
> duplicated code.
> Convert the platform driver to use new helper and make the
a new
> driver_override field const char (it is not modified by the core).
...
> +/**
> + * 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.
> + * @s: NUL terminated string, new driver name to force a match, pass empty
NUL-terminated? (44 vs 115 occurrences)
> + *     string to clear it
> + * @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 (!dev || !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;
> +
> +       new = kstrndup(s, len, GFP_KERNEL);
> +       if (!new)
> +               return -ENOMEM;
> +
> +       cp = strchr(new, '\n');
> +       if (cp)
> +               *cp = '\0';
AFAIU you may reduce memory footprint by

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

new = kstrndup(...);
> +       device_lock(dev);
> +       old = *override;
> +       if (cp != new) {
> +               *override = new;
> +       } else {
> +               kfree(new);
> +               *override = NULL;
> +       }
> +       device_unlock(dev);
> +
> +       kfree(old);
> +
> +       return 0;
> +}
-- 
With Best Regards,
Andy Shevchenko