Re: [PATCH v4 2/6] treewide: use prandom_u32_max() when possible

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Oct 07, 2022 at 12:01:03PM -0600, Jason A. Donenfeld wrote:
> Rather than incurring a division or requesting too many random bytes for
> the given range, use the prandom_u32_max() function, which only takes
> the minimum required bytes from the RNG and avoids divisions.
> 
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Reviewed-by: KP Singh <kpsingh@kernel.org>
> Reviewed-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com> # for drbd
> Reviewed-by: Jan Kara <jack@suse.cz> # for ext2, ext4, and sbitmap
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---

<snip, skip to the xfs part>

> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index e2bdf089c0a3..6261599bb389 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -1520,7 +1520,7 @@ xfs_alloc_ag_vextent_lastblock(
>  
>  #ifdef DEBUG
>  	/* Randomly don't execute the first algorithm. */
> -	if (prandom_u32() & 1)
> +	if (prandom_u32_max(2))

I wonder if these usecases (picking 0 or 1 randomly) ought to have a
trivial wrapper to make it more obvious that we want boolean semantics:

static inline bool prandom_bool(void)
{
	return prandom_u32_max(2);
}

	if (prandom_bool())
		use_crazy_algorithm(...);

But this translation change looks correct to me, so for the XFS parts:
Acked-by: Darrick J. Wong <djwong@kernel.org>

--D


>  		return 0;
>  #endif
>  
> diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
> index 6cdfd64bc56b..7838b31126e2 100644
> --- a/fs/xfs/libxfs/xfs_ialloc.c
> +++ b/fs/xfs/libxfs/xfs_ialloc.c
> @@ -636,7 +636,7 @@ xfs_ialloc_ag_alloc(
>  	/* randomly do sparse inode allocations */
>  	if (xfs_has_sparseinodes(tp->t_mountp) &&
>  	    igeo->ialloc_min_blks < igeo->ialloc_blks)
> -		do_sparse = prandom_u32() & 1;
> +		do_sparse = prandom_u32_max(2);
>  #endif
>  
>  	/*
> diff --git a/include/linux/nodemask.h b/include/linux/nodemask.h
> index 4b71a96190a8..66ee9b4b7925 100644
> --- a/include/linux/nodemask.h
> +++ b/include/linux/nodemask.h
> @@ -509,7 +509,7 @@ static inline int node_random(const nodemask_t *maskp)
>  	w = nodes_weight(*maskp);
>  	if (w)
>  		bit = bitmap_ord_to_pos(maskp->bits,
> -			get_random_int() % w, MAX_NUMNODES);
> +			prandom_u32_max(w), MAX_NUMNODES);
>  	return bit;
>  #else
>  	return 0;
> diff --git a/lib/cmdline_kunit.c b/lib/cmdline_kunit.c
> index e6a31c927b06..a72a2c16066e 100644
> --- a/lib/cmdline_kunit.c
> +++ b/lib/cmdline_kunit.c
> @@ -76,7 +76,7 @@ static void cmdline_test_lead_int(struct kunit *test)
>  		int rc = cmdline_test_values[i];
>  		int offset;
>  
> -		sprintf(in, "%u%s", prandom_u32_max(256), str);
> +		sprintf(in, "%u%s", get_random_int() % 256, str);
>  		/* Only first '-' after the number will advance the pointer */
>  		offset = strlen(in) - strlen(str) + !!(rc == 2);
>  		cmdline_do_one_test(test, in, rc, offset);
> @@ -94,7 +94,7 @@ static void cmdline_test_tail_int(struct kunit *test)
>  		int rc = strcmp(str, "") ? (strcmp(str, "-") ? 0 : 1) : 1;
>  		int offset;
>  
> -		sprintf(in, "%s%u", str, prandom_u32_max(256));
> +		sprintf(in, "%s%u", str, get_random_int() % 256);
>  		/*
>  		 * Only first and leading '-' not followed by integer
>  		 * will advance the pointer.
> diff --git a/lib/kobject.c b/lib/kobject.c
> index 5f0e71ab292c..a0b2dbfcfa23 100644
> --- a/lib/kobject.c
> +++ b/lib/kobject.c
> @@ -694,7 +694,7 @@ static void kobject_release(struct kref *kref)
>  {
>  	struct kobject *kobj = container_of(kref, struct kobject, kref);
>  #ifdef CONFIG_DEBUG_KOBJECT_RELEASE
> -	unsigned long delay = HZ + HZ * (get_random_int() & 0x3);
> +	unsigned long delay = HZ + HZ * prandom_u32_max(4);
>  	pr_info("kobject: '%s' (%p): %s, parent %p (delayed %ld)\n",
>  		 kobject_name(kobj), kobj, __func__, kobj->parent, delay);
>  	INIT_DELAYED_WORK(&kobj->release, kobject_delayed_cleanup);
> diff --git a/lib/reed_solomon/test_rslib.c b/lib/reed_solomon/test_rslib.c
> index 6faf9c9a6215..4d241bdc88aa 100644
> --- a/lib/reed_solomon/test_rslib.c
> +++ b/lib/reed_solomon/test_rslib.c
> @@ -199,7 +199,7 @@ static int get_rcw_we(struct rs_control *rs, struct wspace *ws,
>  
>  		derrlocs[i] = errloc;
>  
> -		if (ewsc && (prandom_u32() & 1)) {
> +		if (ewsc && prandom_u32_max(2)) {
>  			/* Erasure with the symbol intact */
>  			errlocs[errloc] = 2;
>  		} else {
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index c4f04edf3ee9..ef0661504561 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -21,7 +21,7 @@ static int init_alloc_hint(struct sbitmap *sb, gfp_t flags)
>  		int i;
>  
>  		for_each_possible_cpu(i)
> -			*per_cpu_ptr(sb->alloc_hint, i) = prandom_u32() % depth;
> +			*per_cpu_ptr(sb->alloc_hint, i) = prandom_u32_max(depth);
>  	}
>  	return 0;
>  }
> diff --git a/lib/test_hexdump.c b/lib/test_hexdump.c
> index 0927f44cd478..41a0321f641a 100644
> --- a/lib/test_hexdump.c
> +++ b/lib/test_hexdump.c
> @@ -208,7 +208,7 @@ static void __init test_hexdump_overflow(size_t buflen, size_t len,
>  static void __init test_hexdump_overflow_set(size_t buflen, bool ascii)
>  {
>  	unsigned int i = 0;
> -	int rs = (prandom_u32_max(2) + 1) * 16;
> +	int rs = prandom_u32_max(2) + 1 * 16;
>  
>  	do {
>  		int gs = 1 << i;
> diff --git a/lib/test_vmalloc.c b/lib/test_vmalloc.c
> index 4f2f2d1bac56..56ffaa8dd3f6 100644
> --- a/lib/test_vmalloc.c
> +++ b/lib/test_vmalloc.c
> @@ -151,9 +151,7 @@ static int random_size_alloc_test(void)
>  	int i;
>  
>  	for (i = 0; i < test_loop_count; i++) {
> -		n = prandom_u32();
> -		n = (n % 100) + 1;
> -
> +		n = prandom_u32_max(n % 100) + 1;
>  		p = vmalloc(n * PAGE_SIZE);
>  
>  		if (!p)
> @@ -293,16 +291,12 @@ pcpu_alloc_test(void)
>  		return -1;
>  
>  	for (i = 0; i < 35000; i++) {
> -		unsigned int r;
> -
> -		r = prandom_u32();
> -		size = (r % (PAGE_SIZE / 4)) + 1;
> +		size = prandom_u32_max(PAGE_SIZE / 4) + 1;
>  
>  		/*
>  		 * Maximum PAGE_SIZE
>  		 */
> -		r = prandom_u32();
> -		align = 1 << ((r % 11) + 1);
> +		align = 1 << (prandom_u32_max(11) + 1);
>  
>  		pcpu[i] = __alloc_percpu(size, align);
>  		if (!pcpu[i])
> @@ -393,14 +387,11 @@ static struct test_driver {
>  
>  static void shuffle_array(int *arr, int n)
>  {
> -	unsigned int rnd;
>  	int i, j;
>  
>  	for (i = n - 1; i > 0; i--)  {
> -		rnd = prandom_u32();
> -
>  		/* Cut the range. */
> -		j = rnd % i;
> +		j = prandom_u32_max(i);
>  
>  		/* Swap indexes. */
>  		swap(arr[i], arr[j]);
> diff --git a/net/core/pktgen.c b/net/core/pktgen.c
> index a13ee452429e..5ca4f953034c 100644
> --- a/net/core/pktgen.c
> +++ b/net/core/pktgen.c
> @@ -2469,11 +2469,11 @@ static void mod_cur_headers(struct pktgen_dev *pkt_dev)
>  	}
>  
>  	if ((pkt_dev->flags & F_VID_RND) && (pkt_dev->vlan_id != 0xffff)) {
> -		pkt_dev->vlan_id = prandom_u32() & (4096 - 1);
> +		pkt_dev->vlan_id = prandom_u32_max(4096);
>  	}
>  
>  	if ((pkt_dev->flags & F_SVID_RND) && (pkt_dev->svlan_id != 0xffff)) {
> -		pkt_dev->svlan_id = prandom_u32() & (4096 - 1);
> +		pkt_dev->svlan_id = prandom_u32_max(4096);
>  	}
>  
>  	if (pkt_dev->udp_src_min < pkt_dev->udp_src_max) {
> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> index b9d995b5ce24..9dc070f2018e 100644
> --- a/net/ipv4/inet_hashtables.c
> +++ b/net/ipv4/inet_hashtables.c
> @@ -794,7 +794,7 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
>  	 * on low contention the randomness is maximal and on high contention
>  	 * it may be inexistent.
>  	 */
> -	i = max_t(int, i, (prandom_u32() & 7) * 2);
> +	i = max_t(int, i, prandom_u32_max(8) * 2);
>  	WRITE_ONCE(table_perturb[index], READ_ONCE(table_perturb[index]) + i + 2);
>  
>  	/* Head lock still held and bh's disabled */
> diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
> index c3c693b51c94..f075a9fb5ccc 100644
> --- a/net/sunrpc/cache.c
> +++ b/net/sunrpc/cache.c
> @@ -677,7 +677,7 @@ static void cache_limit_defers(void)
>  
>  	/* Consider removing either the first or the last */
>  	if (cache_defer_cnt > DFR_MAX) {
> -		if (prandom_u32() & 1)
> +		if (prandom_u32_max(2))
>  			discard = list_entry(cache_defer_list.next,
>  					     struct cache_deferred_req, recent);
>  		else
> diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
> index e976007f4fd0..c2caee703d2c 100644
> --- a/net/sunrpc/xprtsock.c
> +++ b/net/sunrpc/xprtsock.c
> @@ -1619,7 +1619,7 @@ static int xs_get_random_port(void)
>  	if (max < min)
>  		return -EADDRINUSE;
>  	range = max - min + 1;
> -	rand = (unsigned short) prandom_u32() % range;
> +	rand = (unsigned short) prandom_u32_max(range);
>  	return rand + min;
>  }
>  
> -- 
> 2.37.3
>