linux-kernel - Re: [PATCH] mm: util: update the kerneldoc for kstrdup

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Message-ID: <98163576-c98e-77f7-17a7-efd04dc2e86f@redhat.com>
Date:   Tue, 30 Jun 2020 10:57:17 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Joe Perches <joe@...ches.com>, Bartosz Golaszewski <brgl@...ev.pl>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH] mm: util: update the kerneldoc for kstrdup_const()

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@...libre.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