RE: [PATCH v5 1/3] refcount: Create unchecked atomic_t implementation

[Reshetova, Elena]

> 
> Many subsystems will not use refcount_t unless there is a way to build the
> kernel so that there is no regression in speed compared to atomic_t. This
> adds CONFIG_REFCOUNT_FULL to enable the full refcount_t implementation
> which has the validation but is slightly slower. When not enabled,
> refcount_t uses the basic unchecked atomic_t routines, which results in
> no code changes compared to just using atomic_t directly.
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  arch/Kconfig             |  9 +++++++++
>  include/linux/refcount.h | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  lib/refcount.c           |  3 +++
>  3 files changed, 56 insertions(+)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 6c00e5b00f8b..fba3bf186728 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -867,4 +867,13 @@ config STRICT_MODULE_RWX
>  config ARCH_WANT_RELAX_ORDER
>  	bool
> 
> +config REFCOUNT_FULL
> +	bool "Perform full reference count validation at the expense of
> speed"
> +	help
> +	  Enabling this switches the refcounting infrastructure from a fast
> +	  unchecked atomic_t implementation to a fully state checked
> +	  implementation, which can be slower but provides protections
> +	  against various use-after-free conditions that can be used in
> +	  security flaw exploits.
> +
>  source "kernel/gcov/Kconfig"
> diff --git a/include/linux/refcount.h b/include/linux/refcount.h
> index b34aa649d204..68ecb431dbab 100644
> --- a/include/linux/refcount.h
> +++ b/include/linux/refcount.h
> @@ -41,6 +41,7 @@ static inline unsigned int refcount_read(const refcount_t *r)
>  	return atomic_read(&r->refs);
>  }
> 
> +#ifdef CONFIG_REFCOUNT_FULL
>  extern __must_check bool refcount_add_not_zero(unsigned int i, refcount_t *r);
>  extern void refcount_add(unsigned int i, refcount_t *r);
> 
> @@ -52,6 +53,49 @@ extern void refcount_sub(unsigned int i, refcount_t *r);
> 
>  extern __must_check bool refcount_dec_and_test(refcount_t *r);
>  extern void refcount_dec(refcount_t *r);
> +#else
> +static inline __must_check bool refcount_add_not_zero(unsigned int i,
> +
> 	      refcount_t *r)
> +{
> +	return atomic_add_return(i, &r->refs) != 0;
> +}

Maybe atomic_add_unless(&r->refs, i, 0) in order to be consistent with the below inc_not_zero implementation?
> +
> +static inline void refcount_add(unsigned int i, refcount_t *r)
> +{
> +	atomic_add(i, &r->refs);
> +}
> +
> +static inline __must_check bool refcount_inc_not_zero(refcount_t *r)
> +{
> +	return atomic_add_unless(&r->refs, 1, 0);
> +}
> +
> +static inline void refcount_inc(refcount_t *r)
> +{
> +	atomic_inc(&r->refs);
> +}
> +
> +static inline __must_check bool refcount_sub_and_test(unsigned int i,
> +
> 	      refcount_t *r)
> +{
> +	return atomic_sub_return(i, &r->refs) == 0;
> +}

Any reason for not using atomic_sub_and_test() here?

> +
> +static inline void refcount_sub(unsigned int i, refcount_t *r)
> +{
> +	atomic_sub(i, &r->refs);
> +}
> +
> +static inline __must_check bool refcount_dec_and_test(refcount_t *r)
> +{
> +	return atomic_dec_return(&r->refs) == 0;
> +}

Same here: atomic_dec_and_test()?

Best Regards,
Elena.

> +
> +static inline void refcount_dec(refcount_t *r)
> +{
> +	atomic_dec(&r->refs);
> +}
> +#endif /* CONFIG_REFCOUNT_FULL */
> 
>  extern __must_check bool refcount_dec_if_one(refcount_t *r);
>  extern __must_check bool refcount_dec_not_one(refcount_t *r);
> diff --git a/lib/refcount.c b/lib/refcount.c
> index 9f906783987e..5d0582a9480c 100644
> --- a/lib/refcount.c
> +++ b/lib/refcount.c
> @@ -37,6 +37,8 @@
>  #include <linux/refcount.h>
>  #include <linux/bug.h>
> 
> +#ifdef CONFIG_REFCOUNT_FULL
> +
>  /**
>   * refcount_add_not_zero - add a value to a refcount unless it is 0
>   * @i: the value to add to the refcount
> @@ -225,6 +227,7 @@ void refcount_dec(refcount_t *r)
>  	WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit
> 0; leaking memory.\n");
>  }
>  EXPORT_SYMBOL(refcount_dec);
> +#endif /* CONFIG_REFCOUNT_FULL */
> 
>  /**
>   * refcount_dec_if_one - decrement a refcount if it is 1
> --
> 2.7.4