bpf: Remove bpf_cgroup_kptr_get() kfunc

Now that bpf_cgroup_acquire() is KF_RCU | KF_RET_NULL,
bpf_cgroup_kptr_get() is redundant. Let's remove it, and update
selftests to instead use bpf_cgroup_acquire() where appropriate. The
next patch will update the BPF documentation to not mention
bpf_cgroup_kptr_get().

Signed-off-by: David Vernet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/helpers.c

···
 }
 
 /**
- * bpf_cgroup_kptr_get - Acquire a reference on a struct cgroup kptr. A cgroup
- * kptr acquired by this kfunc which is not subsequently stored in a map, must
- * be released by calling bpf_cgroup_release().
- * @cgrpp: A pointer to a cgroup kptr on which a reference is being acquired.
- */
-__bpf_kfunc struct cgroup *bpf_cgroup_kptr_get(struct cgroup **cgrpp)
-{
-	struct cgroup *cgrp;
-
-	rcu_read_lock();
-	/* Another context could remove the cgroup from the map and release it
-	 * at any time, including after we've done the lookup above. This is
-	 * safe because we're in an RCU read region, so the cgroup is
-	 * guaranteed to remain valid until at least the rcu_read_unlock()
-	 * below.
-	 */
-	cgrp = READ_ONCE(*cgrpp);
-
-	if (cgrp && !cgroup_tryget(cgrp))
-		/* If the cgroup had been removed from the map and freed as
-		 * described above, cgroup_tryget() will return false. The
-		 * cgroup will be freed at some point after the current RCU gp
-		 * has ended, so just return NULL to the user.
-		 */
-		cgrp = NULL;
-	rcu_read_unlock();
-
-	return cgrp;
-}
-
-/**
  * bpf_cgroup_release - Release the reference acquired on a cgroup.
  * If this kfunc is invoked in an RCU read region, the cgroup is guaranteed to
  * not be freed until the current grace period has ended, even if its refcount
···
 
 #ifdef CONFIG_CGROUPS
 BTF_ID_FLAGS(func, bpf_cgroup_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
-BTF_ID_FLAGS(func, bpf_cgroup_kptr_get, KF_ACQUIRE | KF_KPTR_GET | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_cgroup_release, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_cgroup_ancestor, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_cgroup_from_id, KF_ACQUIRE | KF_RET_NULL)

tools/testing/selftests/bpf/progs/cgrp_kfunc_common.h

···
 } __cgrps_kfunc_map SEC(".maps");
 
 struct cgroup *bpf_cgroup_acquire(struct cgroup *p) __ksym;
-struct cgroup *bpf_cgroup_kptr_get(struct cgroup **pp) __ksym;
 void bpf_cgroup_release(struct cgroup *p) __ksym;
 struct cgroup *bpf_cgroup_ancestor(struct cgroup *cgrp, int level) __ksym;
 struct cgroup *bpf_cgroup_from_id(u64 cgid) __ksym;
+void bpf_rcu_read_lock(void) __ksym;
+void bpf_rcu_read_unlock(void) __ksym;
 
 static inline struct __cgrps_kfunc_map_value *cgrps_kfunc_map_value_lookup(struct cgroup *cgrp)
 {

tools/testing/selftests/bpf/progs/cgrp_kfunc_failure.c

···
 }
 
 SEC("tp_btf/cgroup_mkdir")
-__failure __msg("arg#0 expected pointer to map value")
-int BPF_PROG(cgrp_kfunc_get_non_kptr_param, struct cgroup *cgrp, const char *path)
-{
-	struct cgroup *kptr;
-
-	/* Cannot use bpf_cgroup_kptr_get() on a non-kptr, even on a valid cgroup. */
-	kptr = bpf_cgroup_kptr_get(&cgrp);
-	if (!kptr)
-		return 0;
-
-	bpf_cgroup_release(kptr);
-
-	return 0;
-}
-
-SEC("tp_btf/cgroup_mkdir")
-__failure __msg("arg#0 expected pointer to map value")
-int BPF_PROG(cgrp_kfunc_get_non_kptr_acquired, struct cgroup *cgrp, const char *path)
-{
-	struct cgroup *kptr, *acquired;
-
-	acquired = bpf_cgroup_acquire(cgrp);
-	if (!acquired)
-		return 0;
-
-	/* Cannot use bpf_cgroup_kptr_get() on a non-map-value, even if the kptr was acquired. */
-	kptr = bpf_cgroup_kptr_get(&acquired);
-	bpf_cgroup_release(acquired);
-	if (!kptr)
-		return 0;
-
-	bpf_cgroup_release(kptr);
-
-	return 0;
-}
-
-SEC("tp_btf/cgroup_mkdir")
-__failure __msg("arg#0 expected pointer to map value")
-int BPF_PROG(cgrp_kfunc_get_null, struct cgroup *cgrp, const char *path)
-{
-	struct cgroup *kptr;
-
-	/* Cannot use bpf_cgroup_kptr_get() on a NULL pointer. */
-	kptr = bpf_cgroup_kptr_get(NULL);
-	if (!kptr)
-		return 0;
-
-	bpf_cgroup_release(kptr);
-
-	return 0;
-}
-
-SEC("tp_btf/cgroup_mkdir")
 __failure __msg("Unreleased reference")
 int BPF_PROG(cgrp_kfunc_xchg_unreleased, struct cgroup *cgrp, const char *path)
 {
···
 }
 
 SEC("tp_btf/cgroup_mkdir")
-__failure __msg("Unreleased reference")
-int BPF_PROG(cgrp_kfunc_get_unreleased, struct cgroup *cgrp, const char *path)
+__failure __msg("must be referenced or trusted")
+int BPF_PROG(cgrp_kfunc_rcu_get_release, struct cgroup *cgrp, const char *path)
 {
 	struct cgroup *kptr;
 	struct __cgrps_kfunc_map_value *v;
···
 	if (!v)
 		return 0;
 
-	kptr = bpf_cgroup_kptr_get(&v->cgrp);
-	if (!kptr)
-		return 0;
-
-	/* Kptr acquired above is never released. */
+	bpf_rcu_read_lock();
+	kptr = v->cgrp;
+	if (kptr)
+		/* Can't release a cgroup kptr stored in a map. */
+		bpf_cgroup_release(kptr);
+	bpf_rcu_read_unlock();
 
 	return 0;
 }

tools/testing/selftests/bpf/progs/cgrp_kfunc_success.c

···
 		return 0;
 	}
 
-	kptr = bpf_cgroup_kptr_get(&v->cgrp);
-	if (!kptr) {
+	bpf_rcu_read_lock();
+	kptr = v->cgrp;
+	if (!kptr)
 		err = 3;
-		return 0;
-	}
-
-	bpf_cgroup_release(kptr);
+	bpf_rcu_read_unlock();
 
 	return 0;
 }