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

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Message-ID: <CAMRc=MciqbwHBPJf1w2m3xa6ZxbX7=Ca9ucEeyVaKa+FWPzu7A@mail.gmail.com>
Date:   Sun, 28 Jun 2020 20:06:09 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Joe Perches <joe@...ches.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH] mm: util: update the kerneldoc for kstrdup_const()

On Sun, Jun 28, 2020 at 7:37 PM Joe Perches <joe@...ches.com> 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
> ---
>  mm/slab_common.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 37d48a56431d..f8b49656171b 100644
> --- 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.
> + *
>   * The contents of the object pointed to are preserved up to the
>   * lesser of the new and old sizes.  If @p is %NULL, krealloc()
>   * behaves exactly like kmalloc().  If @new_size is 0 and @p is not a
> @@ -1694,6 +1697,9 @@ void *krealloc(const void *p, size_t new_size, gfp_t flags)
>  {
>         void *ret;
>
> +       if (unlikely(is_kernel_rodata((unsigned long)p)))
> +               return NULL;
> +
>         if (unlikely(!new_size)) {
>                 kfree(p);
>                 return ZERO_SIZE_PTR;
>
>
>

In that case we should probably add a WARN_ON() - otherwise the user
will be baffled by krealloc() failing.

Bartosz