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Subject[PATCH v5] lib/vsprintf: defer filling siphash key on RT
Date
On RT, we can't call get_random_bytes() from inside of the raw locks
that callers of vsprintf might take, because get_random_bytes() takes
normal spinlocks. So on those RT systems, defer the siphash key
generation to a worker.

We also do the deferal for CONFIG_PROVE_RAW_LOCK_NESTING systems, which
catches RT issues on non-RT. Branching on CONFIG_PROVE_RAW_LOCK_NESTING
is partly awful, as it basically defeats the purpose of lockdep. But in
this case, it really generates incorrect splats.

Also, avoid using a static_branch, as this isn't the fast path.

Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
Sebastian - again, feel free to take this and modify it as needed. Just
posting ideas... -Jason

lib/vsprintf.c | 36 +++++++++++++++++++++---------------
1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 3c1853a9d1c0..a2a61915eb6b 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -750,37 +750,43 @@ static int __init debug_boot_weak_hash_enable(char *str)
}
early_param("debug_boot_weak_hash", debug_boot_weak_hash_enable);

-static DEFINE_STATIC_KEY_FALSE(filled_random_ptr_key);
+static bool filled_ptr_key __read_mostly;
+static siphash_key_t ptr_key __read_mostly;

-static void enable_ptr_key_workfn(struct work_struct *work)
+static void fill_ptr_key_workfn(struct work_struct *work)
{
- static_branch_enable(&filled_random_ptr_key);
+ if (READ_ONCE(filled_ptr_key))
+ return;
+ get_random_bytes(&ptr_key, sizeof(ptr_key));
+ /* Pairs with smp_rmb() before reading ptr_key. */
+ smp_wmb();
+ WRITE_ONCE(filled_ptr_key, true);
}

/* Maps a pointer to a 32 bit unique identifier. */
static inline int __ptr_to_hashval(const void *ptr, unsigned long *hashval_out)
{
- static siphash_key_t ptr_key __read_mostly;
unsigned long hashval;

- if (!static_branch_likely(&filled_random_ptr_key)) {
- static bool filled = false;
+ if (!READ_ONCE(filled_ptr_key)) {
static DEFINE_SPINLOCK(filling);
- static DECLARE_WORK(enable_ptr_key_work, enable_ptr_key_workfn);
unsigned long flags;

- if (!system_unbound_wq || !rng_is_initialized() ||
- !spin_trylock_irqsave(&filling, flags))
+ if ((IS_ENABLED(CONFIG_PREEMPT_RT) || IS_ENABLED(CONFIG_PROVE_RAW_LOCK_NESTING)) &&
+ rng_is_initialized()) {
+ static DECLARE_WORK(fill_ptr_key_work, fill_ptr_key_workfn);
+ queue_work(system_unbound_wq, &fill_ptr_key_work);
return -EAGAIN;
-
- if (!filled) {
- get_random_bytes(&ptr_key, sizeof(ptr_key));
- queue_work(system_unbound_wq, &enable_ptr_key_work);
- filled = true;
}
+
+ if (!rng_is_initialized() || !spin_trylock_irqsave(&filling, flags))
+ return -EAGAIN;
+
+ fill_ptr_key_workfn(NULL);
spin_unlock_irqrestore(&filling, flags);
}
-
+ /* Pairs with smp_wmb() after writing ptr_key. */
+ smp_rmb();

#ifdef CONFIG_64BIT
hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
--
2.35.1
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