linux-kernel - Re: [PATCH] bitmap: Fix return values to be unsigned

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Date:   Tue, 17 May 2022 08:49:38 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Kees Cook <keescook@...omium.org>,
        Yury Norov <yury.norov@...il.com>
Cc:     Christophe de Dinechin <dinechin@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] bitmap: Fix return values to be unsigned

On 17/05/2022 05.54, Kees Cook wrote:
> Both nodemask and bitmap routines had mixed return values that provided
> potentially signed results that could never happen. This was leading to
> the compiler getting confusing about the range of possible return values
> (it was thinking things could be negative where they could not be). Fix
> all the nodemask and bitmap routines that should be returning unsigned
> (or bool) values. Silences GCC 12 warnings:

So, for the bitmap functions themselves, makes sense, and then also for
the nodemask functions which are merely wrappers around the bitmap
functions (or wrappers around wrappers ...). But see below.

>  
>  #define first_node(src) __first_node(&(src))
> -static inline int __first_node(const nodemask_t *srcp)
> +static inline unsigned int __first_node(const nodemask_t *srcp)
>  {
> -	return min_t(int, MAX_NUMNODES, find_first_bit(srcp->bits, MAX_NUMNODES));
> +	return min_t(unsigned int, MAX_NUMNODES, find_first_bit(srcp->bits, MAX_NUMNODES));
>  }

Unrelated to the type change, but what's that min() doing there in the
first place? Doesn't find_first_bit() already return the nbits argument
if no "first bit" exists (i.e., the bitmap is empty)?

>  #define next_node(n, src) __next_node((n), &(src))
> -static inline int __next_node(int n, const nodemask_t *srcp)
> +static inline unsigned int __next_node(int n, const nodemask_t *srcp)
>  {
> -	return min_t(int,MAX_NUMNODES,find_next_bit(srcp->bits, MAX_NUMNODES, n+1));
> +	return min_t(unsigned int, MAX_NUMNODES, find_next_bit(srcp->bits, MAX_NUMNODES, n+1));
>  }

Same here and a few more places.

It seems to go all the way back to pre-git. Hm. Could be cleaned up
separately I guess.

>  
>  #if defined(CONFIG_NUMA) && (MAX_NUMNODES > 1)
> -extern int node_random(const nodemask_t *maskp);
> +extern unsigned int node_random(const nodemask_t *maskp);

So this one I'm not convinced about. It has a documented return value of
NUMA_NO_NODE aka -1 if the mask is empty. And since it's not a wrapper
around a corresponding bitmap_random() (which would presumably, did it
exist, use the "return nbits if empty" convention), there's no
compelling reason to make its return type unsigned.

>  
> @@ -18,9 +18,9 @@ EXPORT_SYMBOL(__next_node_in);
>   * Return the bit number of a random bit set in the nodemask.
>   * (returns NUMA_NO_NODE if nodemask is empty)
>   */
> -int node_random(const nodemask_t *maskp)
> +unsigned int node_random(const nodemask_t *maskp)
>  {
> -	int w, bit = NUMA_NO_NODE;
> +	unsigned int w, bit = NUMA_NO_NODE;
>  
>  	w = nodes_weight(*maskp);
>  	if (w)

Rasmus