From 027bdec89364e94753a2211ca87800c765fbd75d Mon Sep 17 00:00:00 2001 From: David Vernet Date: Wed, 25 Jan 2023 08:38:16 -0600 Subject: [PATCH] bpf/docs: Document the nocast aliasing behavior of ___init When comparing BTF IDs for pointers being passed to kfunc arguments, the verifier will allow pointer types that are equivalent according to the C standard. For example, for: struct bpf_cpumask { cpumask_t cpumask; refcount_t usage; }; The verifier will allow a struct bpf_cpumask * to be passed to a kfunc that takes a const struct cpumask * (cpumask_t is a typedef of struct cpumask). The exception to this rule is if a type is suffixed with ___init, such as: struct nf_conn___init { struct nf_conn ct; }; The verifier will _not_ allow a struct nf_conn___init * to be passed to a kfunc that expects a struct nf_conn *. This patch documents this behavior in the kfuncs documentation page. Signed-off-by: David Vernet Link: https://lore.kernel.org/r/20230125143816.721952-8-void@manifault.com Signed-off-by: Alexei Starovoitov --- Documentation/bpf/kfuncs.rst | 43 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 43 insertions(+) diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst index 560f4ed..1a68322 100644 --- a/Documentation/bpf/kfuncs.rst +++ b/Documentation/bpf/kfuncs.rst @@ -247,6 +247,49 @@ type. An example is shown below:: } late_initcall(init_subsystem); +2.6 Specifying no-cast aliases with ___init +-------------------------------------------- + +The verifier will always enforce that the BTF type of a pointer passed to a +kfunc by a BPF program, matches the type of pointer specified in the kfunc +definition. The verifier, does, however, allow types that are equivalent +according to the C standard to be passed to the same kfunc arg, even if their +BTF_IDs differ. + +For example, for the following type definition: + +.. code-block:: c + + struct bpf_cpumask { + cpumask_t cpumask; + refcount_t usage; + }; + +The verifier would allow a ``struct bpf_cpumask *`` to be passed to a kfunc +taking a ``cpumask_t *`` (which is a typedef of ``struct cpumask *``). For +instance, both ``struct cpumask *`` and ``struct bpf_cpmuask *`` can be passed +to bpf_cpumask_test_cpu(). + +In some cases, this type-aliasing behavior is not desired. ``struct +nf_conn___init`` is one such example: + +.. code-block:: c + + struct nf_conn___init { + struct nf_conn ct; + }; + +The C standard would consider these types to be equivalent, but it would not +always be safe to pass either type to a trusted kfunc. ``struct +nf_conn___init`` represents an allocated ``struct nf_conn`` object that has +*not yet been initialized*, so it would therefore be unsafe to pass a ``struct +nf_conn___init *`` to a kfunc that's expecting a fully initialized ``struct +nf_conn *`` (e.g. ``bpf_ct_change_timeout()``). + +In order to accommodate such requirements, the verifier will enforce strict +PTR_TO_BTF_ID type matching if two types have the exact same name, with one +being suffixed with ``___init``. + 3. Core kfuncs ============== -- 2.7.4