Re: [PATCH] mm: util: update the kerneldoc for kstrdup_const()

David Hildenbrand
2020-06-30T08:57:29Z

On 29.06.20 21:21, Joe Perches wrote:
> On Mon, 2020-06-29 at 12:54 +0200, David Hildenbrand wrote:
>> On 28.06.20 19:37, Joe Perches wrote:
>>> On Sun, 2020-06-28 at 17:25 +0200, Bartosz Golaszewski wrote:
>>>> From: Bartosz Golaszewski <bgolaszewski@baylibre.com>
>>>>
>>>> Memory allocated with kstrdup_const() must not be passed to regular
>>>> krealloc() as it is not aware of the possibility of the chunk residing
>>>> in .rodata. Since there are no potential users of krealloc_const()
>>>> at the moment, let's just update the doc to make it explicit.
>>>
>>> Another option would be to return NULL if it's
>>> used from krealloc with a pointer into rodata
> []
>>> diff --git a/mm/slab_common.c b/mm/slab_common.c
> []
>>> @@ -1683,6 +1683,9 @@ static __always_inline void *__do_krealloc(const void *p, size_t new_size,
>>> * @new_size: how many bytes of memory are required.
>>> * @flags: the type of memory to allocate.
>>> *
>>> + * If the object pointed to is in rodata (likely from kstrdup_const)
>>> + * %NULL is returned.
>>> + *
> []
>> Won't we have similar issues if somebody would do a kfree() instead of a
>> kfree_const()? So I think the original patch makes sense.
>
> Which is why I also suggested making kfree work for
> more types of memory freeing earlier this month.
>
> https://lore.kernel.org/lkml/573b3fbd5927c643920e1364230c296b23e7584d.camel@perches.com/

I'm curious, do we see a lot of BUGs that resulted from wrong usage such
that we need runtime checks for such things that the code can just
statically specify (If I use kstrdup_const() for allocation I can just
use kfree_const() for freeing - no runtime checks needed).

IOW, what's the real benefit that is worth spending extra runtime cycles?

--
Thanks,

David / dhildenb