Re: [RFC PATCH v1 1/4] Increase size of ucounts to atomic_long_t

[Christian Brauner <christian.brauner@ubuntu.com>]

On Mon, Nov 02, 2020 at 05:50:30PM +0100, Alexey Gladkov wrote:
> In order to be able to use ucounts for rlimits, the size must be increased.
> For example user_struct.mq_bytes (RLIMIT_MSGQUEUE) is unsigned long.

I don't have any issues with this change I just wonder what the exact
reason is. It's not immediately obvious to me.

> 
> Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
> ---
>  include/linux/user_namespace.h |  4 ++--
>  kernel/ucount.c                | 14 +++++++-------
>  2 files changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index 6ef1c7109fc4..fc75af812d73 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -86,7 +86,7 @@ struct user_namespace {
>  	struct ctl_table_header *sysctls;
>  #endif
>  	struct ucounts		*ucounts;
> -	int ucount_max[UCOUNT_COUNTS];
> +	long ucount_max[UCOUNT_COUNTS];
>  } __randomize_layout;
>  
>  struct ucounts {
> @@ -94,7 +94,7 @@ struct ucounts {
>  	struct user_namespace *ns;
>  	kuid_t uid;
>  	int count;
> -	atomic_t ucount[UCOUNT_COUNTS];
> +	atomic_long_t ucount[UCOUNT_COUNTS];
>  };
>  
>  extern struct user_namespace init_user_ns;
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 11b1596e2542..7b2bca8582ef 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -175,14 +175,14 @@ static void put_ucounts(struct ucounts *ucounts)
>  	kfree(ucounts);
>  }
>  
> -static inline bool atomic_inc_below(atomic_t *v, int u)
> +static inline bool atomic_long_inc_below(atomic_long_t *v, int u)
>  {
> -	int c, old;
> -	c = atomic_read(v);
> +	long c, old;
> +	c = atomic_long_read(v);
>  	for (;;) {
>  		if (unlikely(c >= u))
>  			return false;
> -		old = atomic_cmpxchg(v, c, c+1);
> +		old = atomic_long_cmpxchg(v, c, c+1);
>  		if (likely(old == c))
>  			return true;
>  		c = old;
> @@ -199,14 +199,14 @@ struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid,
>  		int max;
>  		tns = iter->ns;
>  		max = READ_ONCE(tns->ucount_max[type]);
> -		if (!atomic_inc_below(&iter->ucount[type], max))
> +		if (!atomic_long_inc_below(&iter->ucount[type], max))
>  			goto fail;
>  	}
>  	return ucounts;
>  fail:
>  	bad = iter;
>  	for (iter = ucounts; iter != bad; iter = iter->ns->ucounts)
> -		atomic_dec(&iter->ucount[type]);
> +		atomic_long_dec(&iter->ucount[type]);
>  
>  	put_ucounts(ucounts);
>  	return NULL;
> @@ -216,7 +216,7 @@ void dec_ucount(struct ucounts *ucounts, enum ucount_type type)
>  {
>  	struct ucounts *iter;
>  	for (iter = ucounts; iter; iter = iter->ns->ucounts) {
> -		int dec = atomic_dec_if_positive(&iter->ucount[type]);
> +		int dec = atomic_long_dec_if_positive(&iter->ucount[type]);
>  		WARN_ON_ONCE(dec < 0);
>  	}
>  	put_ucounts(ucounts);
> -- 
> 2.25.4
>