random32: improvements to prandom_bytes

This patch addresses a couple of minor items, mostly addesssing
prandom_bytes(): 1) prandom_bytes{,_state}() should use size_t
for length arguments, 2) We can use put_unaligned() when filling
the array instead of open coding it [ perhaps some archs will
further benefit from their own arch specific implementation when
GCC cannot make up for it ], 3) Fix a typo, 4) Better use unsigned
int as type for getting the arch seed, 5) Make use of
prandom_u32_max() for timer slack.

Regarding the change to put_unaligned(), callers of prandom_bytes()
which internally invoke prandom_bytes_state(), don't bother as
they expect the array to be filled randomly and don't have any
control of the internal state what-so-ever (that's also why we
have periodic reseeding there, etc), so they really don't care.

Now for the direct callers of prandom_bytes_state(), which
are solely located in test cases for MTD devices, that is,
drivers/mtd/tests/{oobtest.c,pagetest.c,subpagetest.c}:

These tests basically fill a test write-vector through
prandom_bytes_state() with an a-priori defined seed each time
and write that to a MTD device. Later on, they set up a read-vector
and read back that blocks from the device. So in the verification
phase, the write-vector is being re-setup [ so same seed and
prandom_bytes_state() called ], and then memcmp()'ed against the
read-vector to check if the data is the same.

Akinobu, Lothar and I also tested this patch and it runs through
the 3 relevant MTD test cases w/o any errors on the nandsim device
(simulator for MTD devs) for x86_64, ppc64, ARM (i.MX28, i.MX53
and i.MX6):

# modprobe nandsim first_id_byte=0x20 second_id_byte=0xac \
third_id_byte=0x00 fourth_id_byte=0x15
# modprobe mtd_oobtest dev=0
# modprobe mtd_pagetest dev=0
# modprobe mtd_subpagetest dev=0

We also don't have any users depending directly on a particular
result of the PRNG (except the PRNG self-test itself), and that's
just fine as it e.g. allowed us easily to do things like upgrading
from taus88 to taus113.

Signed-off-by: Daniel Borkmann <[e-mail]>
Tested-by: Akinobu Mita <[e-mail]>
Tested-by: Lothar Waßmann <[e-mail]>
Cc: Hannes Frederic Sowa <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/random.h

···
 unsigned long randomize_range(unsigned long start, unsigned long end, unsigned long len);
 
 u32 prandom_u32(void);
-void prandom_bytes(void *buf, int nbytes);
+void prandom_bytes(void *buf, size_t nbytes);
 void prandom_seed(u32 seed);
 void prandom_reseed_late(void);
 
···
 };
 
 u32 prandom_u32_state(struct rnd_state *state);
-void prandom_bytes_state(struct rnd_state *state, void *buf, int nbytes);
+void prandom_bytes_state(struct rnd_state *state, void *buf, size_t nbytes);
 
 /**
  * prandom_u32_max - returns a pseudo-random number in interval [0, ep_ro)

lib/random32.c

···
 #include <linux/jiffies.h>
 #include <linux/random.h>
 #include <linux/sched.h>
+#include <asm/unaligned.h>
 
 #ifdef CONFIG_RANDOM32_SELFTEST
 static void __init prandom_state_selftest(void);
···
  *	This is used for pseudo-randomness with no outside seeding.
  *	For more random results, use prandom_bytes().
  */
-void prandom_bytes_state(struct rnd_state *state, void *buf, int bytes)
+void prandom_bytes_state(struct rnd_state *state, void *buf, size_t bytes)
 {
-	unsigned char *p = buf;
-	int i;
+	u8 *ptr = buf;
 
-	for (i = 0; i < round_down(bytes, sizeof(u32)); i += sizeof(u32)) {
-		u32 random = prandom_u32_state(state);
-		int j;
-
-		for (j = 0; j < sizeof(u32); j++) {
-			p[i + j] = random;
-			random >>= BITS_PER_BYTE;
-		}
+	while (bytes >= sizeof(u32)) {
+		put_unaligned(prandom_u32_state(state), (u32 *) ptr);
+		ptr += sizeof(u32);
+		bytes -= sizeof(u32);
 	}
-	if (i < bytes) {
-		u32 random = prandom_u32_state(state);
 
-		for (; i < bytes; i++) {
-			p[i] = random;
-			random >>= BITS_PER_BYTE;
-		}
+	if (bytes > 0) {
+		u32 rem = prandom_u32_state(state);
+		do {
+			*ptr++ = (u8) rem;
+			bytes--;
+			rem >>= BITS_PER_BYTE;
+		} while (bytes > 0);
 	}
 }
 EXPORT_SYMBOL(prandom_bytes_state);
···
  *	@buf: where to copy the pseudo-random bytes to
  *	@bytes: the requested number of bytes
  */
-void prandom_bytes(void *buf, int bytes)
+void prandom_bytes(void *buf, size_t bytes)
 {
 	struct rnd_state *state = &get_cpu_var(net_rand_state);
 
···
 
 static void prandom_warmup(struct rnd_state *state)
 {
-	/* Calling RNG ten times to satify recurrence condition */
+	/* Calling RNG ten times to satisfy recurrence condition */
 	prandom_u32_state(state);
 	prandom_u32_state(state);
 	prandom_u32_state(state);
···
 
 static u32 __extract_hwseed(void)
 {
-	u32 val = 0;
+	unsigned int val = 0;
 
 	(void)(arch_get_random_seed_int(&val) ||
 	       arch_get_random_int(&val));
···
 	prandom_seed(entropy);
 
 	/* reseed every ~60 seconds, in [40 .. 80) interval with slack */
-	expires = 40 + (prandom_u32() % 40);
+	expires = 40 + prandom_u32_max(40);
 	seed_timer.expires = jiffies + msecs_to_jiffies(expires * MSEC_PER_SEC);
 
 	add_timer(&seed_timer);