Commit e463b6de authored by Matt Bobrowski's avatar Matt Bobrowski Committed by Alexei Starovoitov
Browse files

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: default avatarMatt Bobrowski <mattbobrowski@google.com>
Acked-by: default avatarKumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260113083949.2502978-2-mattbobrowski@google.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent f8ade234

mm/bpf_memcontrol.c

+7 −7
Original line number Diff line number Diff line
@@ -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)