Re: [PATCH v3 01/11] driver: platform: Add helper for safer setting of driver_override

[Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxxxxx>]

On 28/02/2022 11:51, Abel Vesa wrote:
> On 22-02-27 14:52:04, Krzysztof Kozlowski 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)
>>
>> Provide a helper which clearly documents the usage of driver_override.
>> This will allow later to reuse the helper and reduce amount of
>> duplicated code.
>>
>> Convert the platform driver to use new helper and make the
>> driver_override field const char (it is not modified by the core).
>>
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxxxxx>
>> ---
>>  drivers/base/driver.c           | 51 +++++++++++++++++++++++++++++++++
>>  drivers/base/platform.c         | 28 +++---------------
>>  include/linux/device/driver.h   |  2 ++
>>  include/linux/platform_device.h |  7 ++++-
>>  4 files changed, 63 insertions(+), 25 deletions(-)
>>
>> diff --git a/drivers/base/driver.c b/drivers/base/driver.c
>> index 8c0d33e182fd..353750b0bbc5 100644
>> --- a/drivers/base/driver.c
>> +++ b/drivers/base/driver.c
>> @@ -30,6 +30,57 @@ 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.
>> + * @s: NUL terminated string, new driver name to force a match, pass empty
>> + *     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)
> 
> TBH, I think it would make more sense to have this generic
> driver_set_override receive only the dev and the string. And then,
> each bus type will have their own implementation that handle things
> their own way. This would allow all the drivers that will use this to
> do something like this:
> 
> 	ret = driver_set_override(&pdev->dev, "override_string");
> 
> I think it would look more cleaner.
> 

The interface in general is not for the drivers. Drivers use it in
exceptions (few cases in entire kernel) but many times they actually do
not need to.

Adding a dedicated driver_set_override() brings intention that such
usage is welcomed... but it's not. :)


Best regards,
Krzysztof