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

[Joe Perches <joe@perches.com>]

On Sun, 2020-06-28 at 20:06 +0200, Bartosz Golaszewski wrote:
> On Sun, Jun 28, 2020 at 7:37 PM Joe Perches <joe@perches.com> 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
> > ---
> >  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.

Maybe:
---
 mm/slab_common.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index a143a8c8f874..06c2714ab8c9 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1073,6 +1073,9 @@ 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
@@ -1084,6 +1087,14 @@ void *krealloc(const void *p, size_t new_size, gfp_t flags)
 {
 	void *ret;
 
+	if (unlikely(is_kernel_rodata((unsigned long)p))) {
+		if (!(flags & __GFP_NOWARN)) {
+			printk(KERN_DEFAULT "invalid use of krealloc with const rodata\n");
+			dump_stack();
+		}
+		return NULL;
+	}
+
 	if (unlikely(!new_size)) {
 		kfree(p);
 		return ZERO_SIZE_PTR;