bpf: drop KF_ACQUIRE flag on BPF kfunc bpf_get_root_mem_cgroup()

With the BPF verifier now treating pointers to struct types returned
from BPF kfuncs as implicitly trusted by default, there is no need for
bpf_get_root_mem_cgroup() to be annotated with the KF_ACQUIRE flag.

bpf_get_root_mem_cgroup() does not acquire any references, but rather
simply returns a NULL pointer or a pointer to a struct mem_cgroup
object that is valid for the entire lifetime of the kernel.

This simplifies BPF programs using this kfunc by removing the
requirement to pair the call with bpf_put_mem_cgroup().

Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260113083949.2502978-2-mattbobrowski@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
index 716df49d76477d135a55874a5989968a3b645653..f95cd5d16f4cd93acd9a53e6d5e62c55b17b86f1 100644 (file)
--- a/mm/bpf_memcontrol.c
+++ b/mm/bpf_memcontrol.c
@@ -13,12 +13,12 @@ __bpf_kfunc_start_defs();
 /**
  * bpf_get_root_mem_cgroup - Returns a pointer to the root memory cgroup
  *
- * The function has KF_ACQUIRE semantics, even though the root memory
- * cgroup is never destroyed after being created and doesn't require
- * reference counting. And it's perfectly safe to pass it to
- * bpf_put_mem_cgroup()
- *
- * Return: A pointer to the root memory cgroup.
+ * Return: A pointer to the root memory cgroup. Notably, the pointer to the
+ * returned struct mem_cgroup is trusted by default, so it's perfectably
+ * acceptable to also pass this pointer into other BPF kfuncs (e.g.,
+ * bpf_mem_cgroup_usage()). Additionally, this BPF kfunc does not make use of
+ * KF_ACQUIRE semantics, so there's no requirement for the BPF program to call
+ * bpf_put_mem_cgroup() on the pointer returned by this BPF kfunc.
  */
 __bpf_kfunc struct mem_cgroup *bpf_get_root_mem_cgroup(void)
 {
@@ -162,7 +162,7 @@ __bpf_kfunc void bpf_mem_cgroup_flush_stats(struct mem_cgroup *memcg)
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(bpf_memcontrol_kfuncs)
-BTF_ID_FLAGS(func, bpf_get_root_mem_cgroup, KF_ACQUIRE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_get_root_mem_cgroup, KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_get_mem_cgroup, KF_ACQUIRE | KF_RET_NULL | KF_RCU)
 BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE)