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Subject[PATCH 09/14] random: reserve entropy for nonblocking pool early on
While booting, our priority is to get the nonblocking pool (which
supplies /dev/urandom and the kernel's internal randomness
consumption) initialized soon. If someone reads from /dev/random,
let them wait until we've either done that, or have enough entropy
to serve them and also do that.

This adds a wrinkle to determining when we're ready for someone to
read from /dev/random, so factor that out.

At present most input goes directly to the nonblocking pool early
on anyway, but this puts us in a position to change that.

Signed-off-by: Greg Price <price@mit.edu>
---
drivers/char/random.c | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index f55365696..58e3e81d4 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -586,6 +586,17 @@ static void fast_mix(struct fast_pool *f, __u32 input[4])
f->count++;
}

+static int
+random_readable(int input_entropy_bits)
+{
+ /* We need enough bits to wake up for ... */
+ int thresh = random_read_wakeup_bits;
+ if (!nonblocking_pool.initialized)
+ /* ... that aren't reserved for the nonblocking pool. */
+ thresh += random_read_wakeup_bits;
+ return input_entropy_bits >= thresh;
+}
+
/*
* Credit (or debit) the entropy store with n bits of entropy.
* Use credit_entropy_bits_safe() if the value comes from userspace
@@ -669,7 +680,7 @@ retry:
int entropy_bits = entropy_count >> ENTROPY_SHIFT;

/* should we wake readers? */
- if (entropy_bits >= random_read_wakeup_bits) {
+ if (random_readable(entropy_bits)) {
wake_up_interruptible(&random_read_wait);
kill_fasync(&fasync, SIGIO, POLL_IN);
}
@@ -936,9 +947,12 @@ static void account_xfer(struct entropy_store *dest, int nbytes,
(dest->entropy_total+7) / 8);
}

- /* Reserve some for /dev/random's pool, unless we really need it. */
+ /* Reserve a reseed's worth for the nonblocking pool early on
+ * when we really need it; later, reserve some for /dev/random */
*reserved_bytes = 0;
- if (!dest->limit && dest->initialized)
+ if (dest == &blocking_pool && !nonblocking_pool.initialized)
+ *reserved_bytes = random_read_wakeup_bits / 8;
+ else if (dest == &nonblocking_pool && dest->initialized)
*reserved_bytes = 2 * (random_read_wakeup_bits / 8);
}

@@ -1329,8 +1343,7 @@ random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
return -EAGAIN;

wait_event_interruptible(random_read_wait,
- ENTROPY_BITS(&input_pool) >=
- random_read_wakeup_bits);
+ random_readable(ENTROPY_BITS(&input_pool)));
if (signal_pending(current))
return -ERESTARTSYS;
}
@@ -1361,7 +1374,7 @@ random_poll(struct file *file, poll_table * wait)
poll_wait(file, &random_read_wait, wait);
poll_wait(file, &random_write_wait, wait);
mask = 0;
- if (ENTROPY_BITS(&input_pool) >= random_read_wakeup_bits)
+ if (random_readable(ENTROPY_BITS(&input_pool)))
mask |= POLLIN | POLLRDNORM;
if (ENTROPY_BITS(&input_pool) < random_write_wakeup_bits)
mask |= POLLOUT | POLLWRNORM;
--
1.8.3.2


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