Re: [PATCH v3 3/4] devres: provide devm_kstrdup_const()

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On 2018-09-27 01:13, Kees Cook wrote:
> On Mon, Sep 24, 2018 at 3:11 AM, Bartosz Golaszewski <brgl@bgdev.pl> wrote:
>> Provide a resource managed version of kstrdup_const(). This variant
>> internally calls devm_kstrdup() on pointers that are outside of
>> .rodata section and returns the string as is otherwise.
>>
>> Also provide a corresponding version of devm_kfree().
>>
>> +/**
>> + * devm_kfree_const - Resource managed conditional kfree
>> + * @dev: device this memory belongs to
>> + * @p: memory to free
>> + *
>> + * Function calls devm_kfree only if @p is not in .rodata section.
>> + */
>> +void devm_kfree_const(struct device *dev, const void *p)
>> +{
>> +       if (!is_kernel_rodata((unsigned long)p))
>> +               devm_kfree(dev, p);
>> +}
>> +EXPORT_SYMBOL(devm_kfree_const);
>> +
>>  /**
>>   * devm_kmemdup - Resource-managed kmemdup
>>   * @dev: Device this memory belongs to
>> diff --git a/include/linux/device.h b/include/linux/device.h
>> index 33f7cb271fbb..79ccc6eb0975 100644
>> --- a/include/linux/device.h
>> +++ b/include/linux/device.h
>> @@ -693,7 +693,10 @@ static inline void *devm_kcalloc(struct device *dev,
>>         return devm_kmalloc_array(dev, n, size, flags | __GFP_ZERO);
>>  }
>>  extern void devm_kfree(struct device *dev, const void *p);
>> +extern void devm_kfree_const(struct device *dev, const void *p);
> 
> With devm_kfree and devm_kfree_const both taking "const", how are
> devm_kstrdup_const() and devm_kfree_const() going to be correctly
> paired at compile time? (i.e. I wasn't expecting the prototype change
> to devm_kfree())

Just drop devm_kfree_const and teach devm_kfree to ignore
is_kernel_rodata(). That avoids the 50-100 bytes of overhead for adding
yet another EXPORT_SYMBOL and makes it easier to port drivers to
devm_kstrdup_const (and avoids the bugs Kees is worried about). devm
managed resources are almost never freed explicitly, so that single
extra comparison in devm_kfree shouldn't matter for performance.

Rasmus