[PATCH] random: cleanup fractional entropy shift constants

From: Jason A. Donenfeld
Date: Fri Jan 14 2022 - 10:33:16 EST

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Next in thread: David Laight: "RE: [PATCH] random: cleanup fractional entropy shift constants"
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The entropy estimator is calculated in terms of 1/8 bits, which means
there are various constants where things are shifted by 3. Move these
into our pool info enum with the other relevant constants, and normalize
the name a bit, prepending a POOL_ like the rest. While we're at it,
move an English assertion about sizes into a proper BUILD_BUG_ON so
that the compiler can ensure this invariant.

Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
drivers/char/random.c | 60 +++++++++++++++++++++----------------------
1 file changed, 29 insertions(+), 31 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 04b7efb034f7..1ca6d9c6d768 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -358,16 +358,6 @@

/* #define ADD_INTERRUPT_BENCH */

-/*
- * To allow fractional bits to be tracked, the entropy_count field is
- * denominated in units of 1/8th bits.
- *
- * 2*(ENTROPY_SHIFT + poolbitshift) must <= 31, or the multiply in
- * credit_entropy_bits() needs to be 64 bits wide.
- */
-#define ENTROPY_SHIFT 3
-#define ENTROPY_BITS() (input_pool.entropy_count >> ENTROPY_SHIFT)
-
/*
* If the entropy count falls under this number of bits, then we
* should wake up processes which are selecting or polling on write
@@ -425,8 +415,13 @@ enum poolinfo {
POOL_WORDMASK = POOL_WORDS - 1,
POOL_BYTES = POOL_WORDS * sizeof(u32),
POOL_BITS = POOL_BYTES * 8,
- POOL_BITSHIFT = ilog2(POOL_WORDS) + 5,
- POOL_FRACBITS = POOL_WORDS << (ENTROPY_SHIFT + 5),
+ POOL_BITSHIFT = ilog2(POOL_BITS),
+
+ /* To allow fractional bits to be tracked, the entropy_count field is
+ * denominated in units of 1/8th bits. */
+ POOL_ENTROPY_SHIFT = 3,
+#define POOL_ENTROPY_BITS() (input_pool.entropy_count >> POOL_ENTROPY_SHIFT)
+ POOL_FRACBITS = POOL_BITS << POOL_ENTROPY_SHIFT,

/* x^128 + x^104 + x^76 + x^51 +x^25 + x + 1 */
POOL_TAP1 = 104,
@@ -650,7 +645,10 @@ static void process_random_ready_list(void)
static void credit_entropy_bits(int nbits)
{
int entropy_count, entropy_bits, orig;
- int nfrac = nbits << ENTROPY_SHIFT;
+ int nfrac = nbits << POOL_ENTROPY_SHIFT;
+
+ /* Ensure that the multiplication can avoid being 64 bits wide. */
+ BUILD_BUG_ON(2 * (POOL_ENTROPY_SHIFT + POOL_BITSHIFT) > 31);

if (!nbits)
return;
@@ -683,17 +681,17 @@ static void credit_entropy_bits(int nbits)
* turns no matter how large nbits is.
*/
int pnfrac = nfrac;
- const int s = POOL_BITSHIFT + ENTROPY_SHIFT + 2;
+ const int s = POOL_BITSHIFT + POOL_ENTROPY_SHIFT + 2;
/* The +2 corresponds to the /4 in the denominator */

do {
- unsigned int anfrac = min(pnfrac, POOL_FRACBITS/2);
+ unsigned int anfrac = min(pnfrac, POOL_FRACBITS / 2);
unsigned int add =
- ((POOL_FRACBITS - entropy_count)*anfrac*3) >> s;
+ ((POOL_FRACBITS - entropy_count) * anfrac * 3) >> s;

entropy_count += add;
pnfrac -= anfrac;
- } while (unlikely(entropy_count < POOL_FRACBITS-2 && pnfrac));
+ } while (unlikely(entropy_count < POOL_FRACBITS - 2 && pnfrac));
}

if (WARN_ON(entropy_count < 0)) {
@@ -704,9 +702,9 @@ static void credit_entropy_bits(int nbits)
if (cmpxchg(&input_pool.entropy_count, orig, entropy_count) != orig)
goto retry;

- trace_credit_entropy_bits(nbits, entropy_count >> ENTROPY_SHIFT, _RET_IP_);
+ trace_credit_entropy_bits(nbits, entropy_count >> POOL_ENTROPY_SHIFT, _RET_IP_);

- entropy_bits = entropy_count >> ENTROPY_SHIFT;
+ entropy_bits = entropy_count >> POOL_ENTROPY_SHIFT;
if (crng_init < 2 && entropy_bits >= 128)
crng_reseed(&primary_crng, true);
}
@@ -1187,7 +1185,7 @@ void add_input_randomness(unsigned int type, unsigned int code,
last_value = value;
add_timer_randomness(&input_timer_state,
(type << 4) ^ code ^ (code >> 4) ^ value);
- trace_add_input_randomness(ENTROPY_BITS());
+ trace_add_input_randomness(POOL_ENTROPY_BITS());
}
EXPORT_SYMBOL_GPL(add_input_randomness);

@@ -1286,7 +1284,7 @@ void add_disk_randomness(struct gendisk *disk)
return;
/* first major is 1, so we get >= 0x200 here */
add_timer_randomness(disk->random, 0x100 + disk_devt(disk));
- trace_add_disk_randomness(disk_devt(disk), ENTROPY_BITS());
+ trace_add_disk_randomness(disk_devt(disk), POOL_ENTROPY_BITS());
}
EXPORT_SYMBOL_GPL(add_disk_randomness);
#endif
@@ -1313,7 +1311,7 @@ static size_t account(size_t nbytes, int min)
entropy_count = orig = READ_ONCE(input_pool.entropy_count);
ibytes = nbytes;
/* never pull more than available */
- have_bytes = entropy_count >> (ENTROPY_SHIFT + 3);
+ have_bytes = entropy_count >> (POOL_ENTROPY_SHIFT + 3);

if (have_bytes < 0)
have_bytes = 0;
@@ -1325,7 +1323,7 @@ static size_t account(size_t nbytes, int min)
pr_warn("negative entropy count: count %d\n", entropy_count);
entropy_count = 0;
}
- nfrac = ibytes << (ENTROPY_SHIFT + 3);
+ nfrac = ibytes << (POOL_ENTROPY_SHIFT + 3);
if ((size_t) entropy_count > nfrac)
entropy_count -= nfrac;
else
@@ -1335,7 +1333,7 @@ static size_t account(size_t nbytes, int min)
goto retry;

trace_debit_entropy(8 * ibytes);
- if (ibytes && ENTROPY_BITS() < random_write_wakeup_bits) {
+ if (ibytes && POOL_ENTROPY_BITS() < random_write_wakeup_bits) {
wake_up_interruptible(&random_write_wait);
kill_fasync(&fasync, SIGIO, POLL_OUT);
}
@@ -1423,7 +1421,7 @@ static ssize_t _extract_entropy(void *buf, size_t nbytes)
*/
static ssize_t extract_entropy(void *buf, size_t nbytes, int min)
{
- trace_extract_entropy(nbytes, ENTROPY_BITS(), _RET_IP_);
+ trace_extract_entropy(nbytes, POOL_ENTROPY_BITS(), _RET_IP_);
nbytes = account(nbytes, min);
return _extract_entropy(buf, nbytes);
}
@@ -1749,9 +1747,9 @@ urandom_read_nowarn(struct file *file, char __user *buf, size_t nbytes,
{
int ret;

- nbytes = min_t(size_t, nbytes, INT_MAX >> (ENTROPY_SHIFT + 3));
+ nbytes = min_t(size_t, nbytes, INT_MAX >> (POOL_ENTROPY_SHIFT + 3));
ret = extract_crng_user(buf, nbytes);
- trace_urandom_read(8 * nbytes, 0, ENTROPY_BITS());
+ trace_urandom_read(8 * nbytes, 0, POOL_ENTROPY_BITS());
return ret;
}

@@ -1791,7 +1789,7 @@ random_poll(struct file *file, poll_table * wait)
mask = 0;
if (crng_ready())
mask |= EPOLLIN | EPOLLRDNORM;
- if (ENTROPY_BITS() < random_write_wakeup_bits)
+ if (POOL_ENTROPY_BITS() < random_write_wakeup_bits)
mask |= EPOLLOUT | EPOLLWRNORM;
return mask;
}
@@ -1847,7 +1845,7 @@ static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
switch (cmd) {
case RNDGETENTCNT:
/* inherently racy, no point locking */
- ent_count = ENTROPY_BITS();
+ ent_count = POOL_ENTROPY_BITS();
if (put_user(ent_count, p))
return -EFAULT;
return 0;
@@ -2005,7 +2003,7 @@ static int proc_do_entropy(struct ctl_table *table, int write,
struct ctl_table fake_table;
int entropy_count;

- entropy_count = *(int *)table->data >> ENTROPY_SHIFT;
+ entropy_count = *(int *)table->data >> POOL_ENTROPY_SHIFT;

fake_table.data = &entropy_count;
fake_table.maxlen = sizeof(entropy_count);
@@ -2224,7 +2222,7 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
*/
wait_event_interruptible(random_write_wait,
!system_wq || kthread_should_stop() ||
- ENTROPY_BITS() <= random_write_wakeup_bits);
+ POOL_ENTROPY_BITS() <= random_write_wakeup_bits);
mix_pool_bytes(buffer, count);
credit_entropy_bits(entropy);
}
--
2.34.1

Next message: Geert Uytterhoeven: "Re: [PATCH RFC v1 1/3] bpf: move from sha1 to blake2s in tag calculation"
Previous message: Abel Vesa: "Re: [RESEND] dt-bindings: serial: fsl-lpuart: Add i.MX8DXL compatible"
In reply to: Jason A. Donenfeld: "[PATCH 7/7] random: de-duplicate INPUT_POOL constants"
Next in thread: David Laight: "RE: [PATCH] random: cleanup fractional entropy shift constants"
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