Re: [PATCHv4 bpf-next 1/4] bpf: Mark vma objects as trusted for task_vma iter and find_vma callback

[Yonghong Song <yhs@meta.com>]

On 11/28/22 10:43 AM, Alexei Starovoitov wrote:
> On Mon, Nov 28, 2022 at 5:29 AM Jiri Olsa <jolsa@kernel.org> wrote:
>>
>> Marking following vma objects as trusted so they can be used
>> as arguments for kfunc function added in following changes:
>>
>>    - vma object argument in find_vma callback function
>>    - vma object in context of task_vma iterator program
>>
>> Both places lock vma object so it can't go away while running
>> the bpf program.
>>
>> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
>> ---
>>   kernel/bpf/task_iter.c | 2 +-
>>   kernel/bpf/verifier.c  | 2 +-
>>   2 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
>> index c2a2182ce570..cd67b3cadd91 100644
>> --- a/kernel/bpf/task_iter.c
>> +++ b/kernel/bpf/task_iter.c
>> @@ -755,7 +755,7 @@ static struct bpf_iter_reg task_vma_reg_info = {
>>                  { offsetof(struct bpf_iter__task_vma, task),
>>                    PTR_TO_BTF_ID_OR_NULL },
>>                  { offsetof(struct bpf_iter__task_vma, vma),
>> -                 PTR_TO_BTF_ID_OR_NULL },
>> +                 PTR_TO_BTF_ID_OR_NULL | PTR_TRUSTED },
> 
> Yonghong, Song,
> 
> Do you remember when task or vma is NULL here?
> Maybe we can do: if (!task || !vma) skip prog run
> in __task_vma_seq_show()
> and make both pointers as PTR_TO_BTF_ID | PTR_TRUSTED?

The 'NULL' is to indicate the last bpf prog run before iteration
ends. It is to provide an opportunity for bpf program to know
all regular iterations are done and the bpf program can do
end aggregation or print a footer if the prog link is cat'able.