[PATCH bpf-next 1/5] bpf: Free struct bpf_cpumask in call_rcu handler

David Vernet
2023-03-16T01:41:32Z

The struct bpf_cpumask type uses the bpf_mem_cache_{alloc,free}() APIs
to allocate and free its cpumasks. The bpf_mem allocator may currently
immediately reuse some memory when its freed, without waiting for an RCU
read cycle to elapse. We want to be able to treat struct bpf_cpumask
objects as completely RCU safe.

This is necessary for two reasons:

1. bpf_cpumask_kptr_get() currently does an RCU-protected
refcnt_inc_not_zero(). This of course assumes that the underlying
memory is not reused, and is therefore unsafe in its current form.

2. We want to be able to get rid of bpf_cpumask_kptr_get() entirely, and
intead use the superior kptr RCU semantics now afforded by the
verifier.

This patch fixes (1), and enables (2), by making struct bpf_cpumask RCU
safe. A subsequent patch will update the verifier to allow struct
bpf_cpumask * pointers to be passed to KF_RCU kfuncs, and then a latter
patch will remove bpf_cpumask_kptr_get().

Fixes: 516f4d3397c9 ("bpf: Enable cpumasks to be queried and used as kptrs")
Signed-off-by: David Vernet <void@manifault.com>
---
kernel/bpf/cpumask.c | 18 +++++++++++++-----
1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c
index b6587ec40f1b..9ab462c5848a 100644
--- a/kernel/bpf/cpumask.c
+++ b/kernel/bpf/cpumask.c
@@ -24,6 +24,7 @@
*/
struct bpf_cpumask {
cpumask_t cpumask;
+ struct rcu_head rcu;
refcount_t usage;
};

@@ -108,6 +109,16 @@ __bpf_kfunc struct bpf_cpumask *bpf_cpumask_kptr_get(struct bpf_cpumask **cpumas
return cpumask;
}

+static void cpumask_free_cb(struct rcu_head *head)
+{
+ struct bpf_cpumask *cpumask;
+
+ cpumask = container_of(head, struct bpf_cpumask, rcu);
+ migrate_disable();
+ bpf_mem_cache_free(&bpf_cpumask_ma, cpumask);
+ migrate_enable();
+}
+
/**
* bpf_cpumask_release() - Release a previously acquired BPF cpumask.
* @cpumask: The cpumask being released.
@@ -121,11 +132,8 @@ __bpf_kfunc void bpf_cpumask_release(struct bpf_cpumask *cpumask)
if (!cpumask)
return;

- if (refcount_dec_and_test(&cpumask->usage)) {
- migrate_disable();
- bpf_mem_cache_free(&bpf_cpumask_ma, cpumask);
- migrate_enable();
- }
+ if (refcount_dec_and_test(&cpumask->usage))
+ call_rcu(&cpumask->rcu, cpumask_free_cb);
}

/**
--
2.39.0