Now that tasks can be used as kfuncs, and the PTR_TRUSTED flag is
available for us to easily add basic acquire / get / release kfuncs, we
can do the same for cgroups. This patch set adds the following kfuncs
which enable using cgroups as kptrs:
struct cgroup *bpf_cgroup_acquire(struct cgroup *cgrp);
struct cgroup *bpf_cgroup_kptr_get(struct cgroup **cgrpp);
void bpf_cgroup_release(struct cgroup *cgrp);
A follow-on patch will add a selftest suite which validates these
kfuncs.
Signed-off-by: David Vernet <void@xxxxxxxxxxxxx>
---
kernel/bpf/helpers.c | 83 ++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 80 insertions(+), 3 deletions(-)
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 9e8a5557ea8d..420a4251cccc 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -4,6 +4,7 @@
#include <linux/bpf.h>
#include <linux/btf.h>
#include <linux/bpf-cgroup.h>
+#include <linux/cgroup.h>
#include <linux/rcupdate.h>
#include <linux/random.h>
#include <linux/smp.h>
@@ -1879,6 +1880,66 @@ void bpf_task_release(struct task_struct *p)
put_task_struct_rcu_user(p);
}
+#ifdef CONFIG_CGROUPS
+/**
+ * bpf_cgroup_acquire - Acquire a reference to a cgroup. A cgroup acquired by
+ * this kfunc which is not stored in a map as a kptr, must be released by
+ * calling bpf_cgroup_release().
+ * @cgrp: The cgroup on which a reference is being acquired.
+ */
+struct cgroup *bpf_cgroup_acquire(struct cgroup *cgrp)
+{
+ cgroup_get(cgrp);
+ return cgrp;
+}
+
+/**
+ * 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.
+ */
+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 struct 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
+ * drops to 0.
+ * @cgrp: The cgroup on which a reference is being released.
+ */
+void bpf_cgroup_release(struct cgroup *cgrp)
+{
+ if (!cgrp)
+ return;
+
+ cgroup_put(cgrp);
+}
+#endif /* CONFIG_CGROUPS */
+
void *bpf_cast_to_kern_ctx(void *obj)
{
return obj;
@@ -1904,6 +1965,12 @@ BTF_ID_FLAGS(func, bpf_list_pop_back, KF_ACQUIRE | KF_RET_NULL)
BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_TRUSTED_ARGS)
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)
+
+#ifdef CONFIG_CGROUPS
+BTF_ID_FLAGS(func, bpf_cgroup_acquire, KF_ACQUIRE | KF_TRUSTED_ARGS)
+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)
+#endif
BTF_SET8_END(generic_btf_ids)
static const struct btf_kfunc_id_set generic_kfunc_set = {
@@ -1915,6 +1982,10 @@ static const struct btf_kfunc_id_set generic_kfunc_set = {
BTF_ID_LIST(generic_dtor_ids)
BTF_ID(struct, task_struct)
BTF_ID(func, bpf_task_release)
+#ifdef CONFIG_CGROUPS
+BTF_ID(struct, cgroup)
+BTF_ID(func, bpf_cgroup_release)
+#endif
BTF_SET8_START(common_btf_ids)
BTF_ID_FLAGS(func, bpf_cast_to_kern_ctx)
@@ -1928,12 +1999,18 @@ static const struct btf_kfunc_id_set common_kfunc_set = {
static int __init kfunc_init(void)
{
- int ret;
+ int ret, idx = 0;
const struct btf_id_dtor_kfunc generic_dtors[] = {
{
- .btf_id = generic_dtor_ids[0],
- .kfunc_btf_id = generic_dtor_ids[1]
+ .btf_id = generic_dtor_ids[idx++],
+ .kfunc_btf_id = generic_dtor_ids[idx++]
},
+#ifdef CONFIG_CGROUPS
+ {
+ .btf_id = generic_dtor_ids[idx++],
+ .kfunc_btf_id = generic_dtor_ids[idx++]
+ },
+#endif
};
ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &generic_kfunc_set);
--
2.38.1
