On 1/22/24 1:22 PM, thinker.li@xxxxxxxxx wrote:
From: Kui-Feng Lee <thinker.li@xxxxxxxxx>
Allow passing a null pointer to the operators provided by a struct_ops
object. This is an RFC to collect feedbacks/opinions.
The previous discussions against v1 came to the conclusion that the
developer should did it in ".is_valid_access". However, recently, kCFI for
struct_ops has been landed. We found it is possible to provide a generic
way to annotate arguments by adding a suffix after argument names of stub
functions. So, this RFC is resent to present the new idea.
The function pointers that are passed to struct_ops operators (the function
pointers) are always considered reliable until now. They cannot be
null. However, in certain scenarios, it should be possible to pass null
pointers to these operators. For instance, sched_ext may pass a null
pointer in the struct task type to an operator that is provided by its
struct_ops objects.
The proposed solution here is to add PTR_MAYBE_NULL annotations to
arguments and create instances of struct bpf_ctx_arg_aux (arg_info) for
these arguments. These arg_infos will be installed at
prog->aux->ctx_arg_info and will be checked by the BPF verifier when
loading the programs. When a struct_ops program accesses arguments in the
ctx, the verifier will call btf_ctx_access() (through
bpf_verifier_ops->is_valid_access) to verify the access. btf_ctx_access()
will check arg_info and use the information of the matched arg_info to
properly set reg_type.
For nullable arguments, this patch sets an arg_info to label them with
PTR_TO_BTF_ID | PTR_TRUSTED | PTR_MAYBE_NULL. This enforces the verifier to
check programs and ensure that they properly check the pointer. The
programs should check if the pointer is null before reading/writing the
pointed memory.
The implementer of a struct_ops should annotate the arguments that can be
null. The implementer should define a stub function (empty) as a
placeholder for each defined operator. The name of a stub function should
be in the pattern "<st_op_type>_stub_<operator name>". For example, for
test_maybe_null of struct bpf_testmod_ops, it's stub function name should
be "bpf_testmod_ops_stub_test_maybe_null". You mark an argument nullable by
suffixing the argument name with "__nullable" at the stub function. Here
is the example in bpf_testmod.c.
Neat idea to reuse the cfi stub. Some high level comments.
bpf_struct_ops_desc_init is also collecting the details of each func_proto
member. Check if this "__nullable" collection can be done in the same loop.
Simplify the implementation of the member_arg_info allocations. There is no need
to compact everything in one continuous memory.
