Commit 77473d1a authored Mar 16, 2023 by David Vernet Committed by Alexei Starovoitov Mar 16, 2023

bpf: Free struct bpf_cpumask in call_rcu handler



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: 516f4d33 ("bpf: Enable cpumasks to be queried and used as kptrs")
Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230316054028.88924-2-void@manifault.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>

parent 6cb9430b

kernel/bpf/cpumask.c

+14 −5

Original line number	Diff line number	Diff line
		@@ -9,6 +9,7 @@
		/**
		* struct bpf_cpumask - refcounted BPF cpumask wrapper structure
		* @cpumask: The actual cpumask embedded in the struct.
		* @rcu: The RCU head used to free the cpumask with RCU safety.
		* @usage: Object reference counter. When the refcount goes to 0, the
		* memory is released back to the BPF allocator, which provides
		* RCU safety.
		@@ -24,6 +25,7 @@
		*/
		struct bpf_cpumask {
		cpumask_t cpumask;
		struct rcu_head rcu;
		refcount_t usage;
		};

		@@ -108,6 +110,16 @@ __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 +133,8 @@ __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);
		}

		/**