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

Joe Perches
2020-06-28T18:20:50Z

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;