bpf: Add __uninit kfunc annotation

This patch adds __uninit as a kfunc annotation.

This will be useful for scenarios such as for example in dynptrs,
indicating whether the dynptr should be checked by the verifier as an
initialized or an uninitialized dynptr.

Without this annotation, the alternative would be needing to hard-code
in the verifier the specific kfunc to indicate that arg should be
treated as an uninitialized arg.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://lore.kernel.org/r/20230301154953.641654-7-joannelkoong@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index 226313747be5..9a78533d25ac 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -100,6 +100,23 @@ Hence, whenever a constant scalar argument is accepted by a kfunc which is not a
 size parameter, and the value of the constant matters for program safety, __k
 suffix should be used.
 
+2.2.2 __uninit Annotation
+--------------------
+
+This annotation is used to indicate that the argument will be treated as
+uninitialized.
+
+An example is given below::
+
+        __bpf_kfunc int bpf_dynptr_from_skb(..., struct bpf_dynptr_kern *ptr__uninit)
+        {
+        ...
+        }
+
+Here, the dynptr will be treated as an uninitialized dynptr. Without this
+annotation, the verifier will reject the program if the dynptr passed in is
+not initialized.
+
 .. _BPF_kfunc_nodef:
 
 2.3 Using an existing kernel function