Re: [PATCH] mm/slab: make __free(kfree) accept error pointers

[Matthew Wilcox <willy@infradead.org>]

On Sun, Apr 28, 2024 at 05:26:44PM +0300, Dan Carpenter wrote:
> Currently, if an automatically freed allocation is an error pointer that
> will lead to a crash.  An example of this is in wm831x_gpio_dbg_show().
> 
>    171	char *label __free(kfree) = gpiochip_dup_line_label(chip, i);
>    172	if (IS_ERR(label)) {
>    173		dev_err(wm831x->dev, "Failed to duplicate label\n");
>    174		continue;
>    175  }
> 
> The auto clean up function should check for error pointers as well,
> otherwise we're going to keep hitting issues like this.
> 
> Fixes: 54da6a092431 ("locking: Introduce __cleanup() based infrastructure")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
> ---
> Obviously, the fixes tag isn't very fair but it will tell the -stable
> tools how far to backport this.
> 
>  include/linux/slab.h  | 4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 4cc37ef22aae..5f5766219375 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -279,7 +279,7 @@ void kfree(const void *objp);
>  void kfree_sensitive(const void *objp);
>  size_t __ksize(const void *objp);
>  
> -DEFINE_FREE(kfree, void *, if (_T) kfree(_T))
> +DEFINE_FREE(kfree, void *, if (!IS_ERR_OR_NULL(_T)) kfree(_T))

Wait, why do we check 'if (_T)' at all?  kfree() already handles NULL
pointers just fine.  I wouldn't be averse to making it handle error
pointers either.

> -DEFINE_FREE(kvfree, void *, if (_T) kvfree(_T))
> +DEFINE_FREE(kvfree, void *, if (!IS_ERR_OR_NULL(_T)) kvfree(_T))

Ditto kvfree().  Fixing kfree() would fix both of these.