Re: [PATCH bpf-next v1 09/19] bpf: add PTR_TO_BTF_ID_OR_NULL support

[Yonghong Song <yhs@fb.com>]

On 4/29/20 1:46 PM, Andrii Nakryiko wrote:
> On Mon, Apr 27, 2020 at 1:13 PM Yonghong Song <yhs@fb.com> wrote:
>>
>> Add bpf_reg_type PTR_TO_BTF_ID_OR_NULL support.
>> For tracing/iter program, the bpf program context
>> definition, e.g., for previous bpf_map target, looks like
>>    struct bpf_iter_bpf_map {
>>      struct bpf_dump_meta *meta;
>>      struct bpf_map *map;
>>    };
>>
>> The kernel guarantees that meta is not NULL, but
>> map pointer maybe NULL. The NULL map indicates that all
>> objects have been traversed, so bpf program can take
>> proper action, e.g., do final aggregation and/or send
>> final report to user space.
>>
>> Add btf_id_or_null_non0_off to prog->aux structure, to
>> indicate that for tracing programs, if the context access
>> offset is not 0, set to PTR_TO_BTF_ID_OR_NULL instead of
>> PTR_TO_BTF_ID. This bit is set for tracing/iter program.
>>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
>> ---
>>   include/linux/bpf.h   |  2 ++
>>   kernel/bpf/btf.c      |  5 ++++-
>>   kernel/bpf/verifier.c | 19 ++++++++++++++-----
>>   3 files changed, 20 insertions(+), 6 deletions(-)
>>
> 
> [...]
> 
>>
>>   static bool reg_may_point_to_spin_lock(const struct bpf_reg_state *reg)
>> @@ -410,7 +411,8 @@ static bool reg_type_may_be_refcounted_or_null(enum bpf_reg_type type)
>>          return type == PTR_TO_SOCKET ||
>>                  type == PTR_TO_SOCKET_OR_NULL ||
>>                  type == PTR_TO_TCP_SOCK ||
>> -               type == PTR_TO_TCP_SOCK_OR_NULL;
>> +               type == PTR_TO_TCP_SOCK_OR_NULL ||
>> +               type == PTR_TO_BTF_ID_OR_NULL;
> 
> BTF_ID is not considered to be refcounted for the purpose of verifier,
> unless I'm missing something?

You are correct. PTR_TO_BTF_ID is not there is a clear sign
PTR_TO_BTF_ID_OR_NULL should not be there either.

> 
>>   }
>>
>>   static bool arg_type_may_be_refcounted(enum bpf_arg_type type)
>> @@ -462,6 +464,7 @@ static const char * const reg_type_str[] = {
>>          [PTR_TO_TP_BUFFER]      = "tp_buffer",
>>          [PTR_TO_XDP_SOCK]       = "xdp_sock",
>>          [PTR_TO_BTF_ID]         = "ptr_",
>> +       [PTR_TO_BTF_ID_OR_NULL] = "ptr_or_null_",
>>   };
>>
> 
> [...]
>