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authorAlexei Starovoitov <ast@kernel.org>2023-03-03 07:14:43 +0300
committerDaniel Borkmann <daniel@iogearbox.net>2023-03-03 19:42:20 +0300
commit20c09d92faeefb8536f705d3a4629e0dc314c8a1 (patch)
treef68b08f9877e6905e1bbcaf90ec85d52bc83cc8e /Documentation/bpf/kfuncs.rst
parent8d093b4e95a2a16a2cfcd36869b348a17112fabe (diff)
downloadlinux-20c09d92faeefb8536f705d3a4629e0dc314c8a1.tar.xz
bpf: Introduce kptr_rcu.
The life time of certain kernel structures like 'struct cgroup' is protected by RCU. Hence it's safe to dereference them directly from __kptr tagged pointers in bpf maps. The resulting pointer is MEM_RCU and can be passed to kfuncs that expect KF_RCU. Derefrence of other kptr-s returns PTR_UNTRUSTED. For example: struct map_value { struct cgroup __kptr *cgrp; }; SEC("tp_btf/cgroup_mkdir") int BPF_PROG(test_cgrp_get_ancestors, struct cgroup *cgrp_arg, const char *path) { struct cgroup *cg, *cg2; cg = bpf_cgroup_acquire(cgrp_arg); // cg is PTR_TRUSTED and ref_obj_id > 0 bpf_kptr_xchg(&v->cgrp, cg); cg2 = v->cgrp; // This is new feature introduced by this patch. // cg2 is PTR_MAYBE_NULL | MEM_RCU. // When cg2 != NULL, it's a valid cgroup, but its percpu_ref could be zero if (cg2) bpf_cgroup_ancestor(cg2, level); // safe to do. } Signed-off-by: Alexei Starovoitov <ast@kernel.org> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Acked-by: Tejun Heo <tj@kernel.org> Acked-by: David Vernet <void@manifault.com> Link: https://lore.kernel.org/bpf/20230303041446.3630-4-alexei.starovoitov@gmail.com
Diffstat (limited to 'Documentation/bpf/kfuncs.rst')
-rw-r--r--Documentation/bpf/kfuncs.rst12
1 files changed, 7 insertions, 5 deletions
diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index b5d9b0d446bc..69eccf6f98ef 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -249,11 +249,13 @@ added later.
2.4.8 KF_RCU flag
-----------------
-The KF_RCU flag is used for kfuncs which have a rcu ptr as its argument.
-When used together with KF_ACQUIRE, it indicates the kfunc should have a
-single argument which must be a trusted argument or a MEM_RCU pointer.
-The argument may have reference count of 0 and the kfunc must take this
-into consideration.
+The KF_RCU flag is a weaker version of KF_TRUSTED_ARGS. The kfuncs marked with
+KF_RCU expect either PTR_TRUSTED or MEM_RCU arguments. The verifier guarantees
+that the objects are valid and there is no use-after-free. The pointers are not
+NULL, but the object's refcount could have reached zero. The kfuncs need to
+consider doing refcnt != 0 check, especially when returning a KF_ACQUIRE
+pointer. Note as well that a KF_ACQUIRE kfunc that is KF_RCU should very likely
+also be KF_RET_NULL.
.. _KF_deprecated_flag: