Re: [PATCH v4 01/11] driver: platform: Add helper for safer setting of driver_override

[Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>]

On 15/03/2022 18:59, Andy Shevchenko wrote:
> 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.

Sure (for this and later comments).

> 
>> 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(...);
>

Indeed, thanks.

Best regards,
Krzysztof