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SubjectRe: [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@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

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