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

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Date:   Sun, 28 Jun 2020 10:37:29 -0700
From:   Joe Perches <joe@...ches.com>
To:     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 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;