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From:   David Vernet <void@manifault.com>
To:     ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
        martin.lau@linux.dev
Cc:     kernel-team@fb.com, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, yhs@fb.com, song@kernel.org,
        john.fastabend@gmail.com, kpsingh@kernel.org, sdf@google.com,
        haoluo@google.com, jolsa@kernel.org, tj@kernel.org,
        memxor@gmail.com
Subject: [PATCH 1/2] bpf: Add kfuncs for storing struct task_struct * as a kptr
Date:   Fri, 30 Sep 2022 15:47:19 -0500
Message-Id: <20220930204720.3116493-2-void@manifault.com>
In-Reply-To: <20220930204720.3116493-1-void@manifault.com>
References: <20220930204720.3116493-1-void@manifault.com>
MIME-Version: 1.0
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Precedence: bulk

Now that BPF supports adding new kernel functions with kfuncs, and
storing kernel objects in maps with kptrs, we can add a set of kfuncs
which allow struct task_struct objects to be stored in maps as
referenced kptrs. The possible use-cases for doing this are plentiful.
During tracing, for example, it would be useful to be able to collect
some tasks that performed a certain operation, and then periodically
summarize who they are, which cgroup they're in, how much CPU time
they've spent, etc.

In order to enable this, this patch adds three new kfuncs:

struct task_struct *bpf_task_acquire(struct task_struct *p);
struct task_struct *bpf_task_kptr_get(struct task_struct **pp);
void bpf_task_release(struct task_struct *p);

A follow-on patch will add selftests validating these kfuncs.

Signed-off-by: David Vernet <void@manifault.com>
---
 kernel/bpf/helpers.c | 75 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 74 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index b069517a3da0..eb9950eaec35 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1711,9 +1711,82 @@ static const struct btf_kfunc_id_set tracing_kfunc_set = {
 	.set   = &tracing_btf_ids,
 };
 
+/**
+ * bpf_task_acquire - Acquire a reference to a task. A task acquired by this
+ * kfunc which is not stored in a map as a kptr, must be released by calling
+ * bpf_task_release().
+ */
+__used noinline
+struct task_struct *bpf_task_acquire(struct task_struct *p)
+{
+	refcount_inc(&p->rcu_users);
+	return p;
+}
+
+/**
+ * bpf_task_kptr_get - Acquire a reference on a struct task_struct * kptr. A
+ * task kptr acquired by this kfunc which is not stored in a map as a kptr,
+ * must be released by calling bpf_task_release().
+ */
+__used noinline
+struct task_struct *bpf_task_kptr_get(struct task_struct **pp)
+{
+	struct task_struct *p;
+
+	rcu_read_lock();
+	p = READ_ONCE(*pp);
+	if (p && !refcount_inc_not_zero(&p->rcu_users))
+		p = NULL;
+	rcu_read_unlock();
+
+	return p;
+}
+
+/**
+ * bpf_task_release - Release the reference acquired on a struct task_struct *.
+ * If this kfunc is invoked in an RCU read region, the task_struct is
+ * guaranteed to not be freed until the current grace period has ended, even if
+ * its refcount drops to 0.
+ */
+__used noinline void bpf_task_release(struct task_struct *p)
+{
+	if (!p)
+		return;
+
+	put_task_struct_rcu_user(p);
+}
+
+BTF_SET8_START(task_kfunc_btf_ids)
+BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE)
+BTF_ID_FLAGS(func, bpf_task_kptr_get, KF_ACQUIRE | KF_KPTR_GET | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_task_release, KF_RELEASE | KF_TRUSTED_ARGS)
+BTF_SET8_END(task_kfunc_btf_ids)
+
+static const struct btf_kfunc_id_set task_kfunc_set = {
+	.owner = THIS_MODULE,
+	.set   = &task_kfunc_btf_ids,
+};
+
+BTF_ID_LIST(task_dtor_kfunc_ids)
+BTF_ID(struct, task_struct)
+BTF_ID(func, bpf_task_release)
+
 static int __init kfunc_init(void)
 {
-	return register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &tracing_kfunc_set);
+	int ret;
+	const struct btf_id_dtor_kfunc helper_dtor_kfuncs[] = {
+		{
+			.btf_id       = task_dtor_kfunc_ids[0],
+			.kfunc_btf_id = task_dtor_kfunc_ids[1]
+		},
+	};
+
+	ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &tracing_kfunc_set);
+	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &task_kfunc_set);
+	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &task_kfunc_set);
+	return ret ?: register_btf_id_dtor_kfuncs(helper_dtor_kfuncs,
+						  ARRAY_SIZE(helper_dtor_kfuncs),
+						  THIS_MODULE);
 }
 
 late_initcall(kfunc_init);
-- 
2.37.3