From: "Jason A. Donenfeld" Jason@zx2c4.com
commit f5bda35fba615ace70a656d4700423fa6c9bebee upstream.
Since crng_ready() is only false briefly during initialization and then forever after becomes true, we don't need to evaluate it after, making it a prime candidate for a static branch.
One complication, however, is that it changes state in a particular call to credit_init_bits(), which might be made from atomic context, which means we must kick off a workqueue to change the static key. Further complicating things, credit_init_bits() may be called sufficiently early on in system initialization such that system_wq is NULL.
Fortunately, there exists the nice function execute_in_process_context(), which will immediately execute the function if !in_interrupt(), and otherwise defer it to a workqueue. During early init, before workqueues are available, in_interrupt() is always false, because interrupts haven't even been enabled yet, which means the function in that case executes immediately. Later on, after workqueues are available, in_interrupt() might be true, but in that case, the work is queued in system_wq and all goes well.
Cc: Theodore Ts'o tytso@mit.edu Cc: Sultan Alsawaf sultan@kerneltoast.com Reviewed-by: Dominik Brodowski linux@dominikbrodowski.net Signed-off-by: Jason A. Donenfeld Jason@zx2c4.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/char/random.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-)
--- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -77,8 +77,9 @@ static enum { CRNG_EMPTY = 0, /* Little to no entropy collected */ CRNG_EARLY = 1, /* At least POOL_EARLY_BITS collected */ CRNG_READY = 2 /* Fully initialized with POOL_READY_BITS collected */ -} crng_init = CRNG_EMPTY; -#define crng_ready() (likely(crng_init >= CRNG_READY)) +} crng_init __read_mostly = CRNG_EMPTY; +static DEFINE_STATIC_KEY_FALSE(crng_is_ready); +#define crng_ready() (static_branch_likely(&crng_is_ready) || crng_init >= CRNG_READY) /* Various types of waiters for crng_init->CRNG_READY transition. */ static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait); static struct fasync_struct *fasync; @@ -108,6 +109,11 @@ bool rng_is_initialized(void) } EXPORT_SYMBOL(rng_is_initialized);
+static void crng_set_ready(struct work_struct *work) +{ + static_branch_enable(&crng_is_ready); +} + /* Used by wait_for_random_bytes(), and considered an entropy collector, below. */ static void try_to_generate_entropy(void);
@@ -267,7 +273,7 @@ static void crng_reseed(void) ++next_gen; WRITE_ONCE(base_crng.generation, next_gen); WRITE_ONCE(base_crng.birth, jiffies); - if (!crng_ready()) + if (!static_branch_likely(&crng_is_ready)) crng_init = CRNG_READY; spin_unlock_irqrestore(&base_crng.lock, flags); memzero_explicit(key, sizeof(key)); @@ -785,6 +791,7 @@ static void extract_entropy(void *buf, s
static void credit_init_bits(size_t nbits) { + static struct execute_work set_ready; unsigned int new, orig, add; unsigned long flags;
@@ -800,6 +807,7 @@ static void credit_init_bits(size_t nbit
if (orig < POOL_READY_BITS && new >= POOL_READY_BITS) { crng_reseed(); /* Sets crng_init to CRNG_READY under base_crng.lock. */ + execute_in_process_context(crng_set_ready, &set_ready); process_random_ready_list(); wake_up_interruptible(&crng_init_wait); kill_fasync(&fasync, SIGIO, POLL_IN); @@ -1309,7 +1317,7 @@ SYSCALL_DEFINE3(getrandom, char __user * if (count > INT_MAX) count = INT_MAX;
- if (!(flags & GRND_INSECURE) && !crng_ready()) { + if (!crng_ready() && !(flags & GRND_INSECURE)) { int ret;
if (flags & GRND_NONBLOCK)