Re: [PATCH v3 3/4] devres: provide devm_kstrdup_const()

[Kees Cook <keescook@chromium.org>]

On Mon, Sep 24, 2018 at 3:11 AM, Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> Provide a resource managed version of kstrdup_const(). This variant
> internally calls devm_kstrdup() on pointers that are outside of
> .rodata section and returns the string as is otherwise.
>
> Also provide a corresponding version of devm_kfree().
>
> Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
> Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---
>  drivers/base/devres.c  | 38 ++++++++++++++++++++++++++++++++++++++
>  include/linux/device.h |  3 +++
>  2 files changed, 41 insertions(+)
>
> diff --git a/drivers/base/devres.c b/drivers/base/devres.c
> index 438c91a43508..48185d57bc5b 100644
> --- a/drivers/base/devres.c
> +++ b/drivers/base/devres.c
> @@ -11,6 +11,8 @@
>  #include <linux/slab.h>
>  #include <linux/percpu.h>
>
> +#include <asm/sections.h>
> +
>  #include "base.h"
>
>  struct devres_node {
> @@ -822,6 +824,28 @@ char *devm_kstrdup(struct device *dev, const char *s, gfp_t gfp)
>  }
>  EXPORT_SYMBOL_GPL(devm_kstrdup);
>
> +/**
> + * devm_kstrdup_const - resource managed conditional string duplication
> + * @dev: device for which to duplicate the string
> + * @s: the string to duplicate
> + * @gfp: the GFP mask used in the kmalloc() call when allocating memory
> + *
> + * Strings allocated by devm_kstrdup_const will be automatically freed when
> + * the associated device is detached.
> + *
> + * RETURNS:
> + * Source string if it is in .rodata section otherwise it falls back to
> + * devm_kstrdup.
> + */
> +const char *devm_kstrdup_const(struct device *dev, const char *s, gfp_t gfp)
> +{
> +       if (is_kernel_rodata((unsigned long)s))
> +               return s;
> +
> +       return devm_kstrdup(dev, s, gfp);
> +}
> +EXPORT_SYMBOL(devm_kstrdup_const);
> +
>  /**
>   * devm_kvasprintf - Allocate resource managed space and format a string
>   *                  into that.
> @@ -895,6 +919,20 @@ void devm_kfree(struct device *dev, const void *p)
>  }
>  EXPORT_SYMBOL_GPL(devm_kfree);
>
> +/**
> + * devm_kfree_const - Resource managed conditional kfree
> + * @dev: device this memory belongs to
> + * @p: memory to free
> + *
> + * Function calls devm_kfree only if @p is not in .rodata section.
> + */
> +void devm_kfree_const(struct device *dev, const void *p)
> +{
> +       if (!is_kernel_rodata((unsigned long)p))
> +               devm_kfree(dev, p);
> +}
> +EXPORT_SYMBOL(devm_kfree_const);
> +
>  /**
>   * devm_kmemdup - Resource-managed kmemdup
>   * @dev: Device this memory belongs to
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 33f7cb271fbb..79ccc6eb0975 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -693,7 +693,10 @@ static inline void *devm_kcalloc(struct device *dev,
>         return devm_kmalloc_array(dev, n, size, flags | __GFP_ZERO);
>  }
>  extern void devm_kfree(struct device *dev, const void *p);
> +extern void devm_kfree_const(struct device *dev, const void *p);

With devm_kfree and devm_kfree_const both taking "const", how are
devm_kstrdup_const() and devm_kfree_const() going to be correctly
paired at compile time? (i.e. I wasn't expecting the prototype change
to devm_kfree())

-Kees

>  extern char *devm_kstrdup(struct device *dev, const char *s, gfp_t gfp) __malloc;
> +extern const char *devm_kstrdup_const(struct device *dev,
> +                                     const char *s, gfp_t gfp);
>  extern void *devm_kmemdup(struct device *dev, const void *src, size_t len,
>                           gfp_t gfp);
>
> --
> 2.18.0
>



-- 
Kees Cook
Pixel Security