Re: [PATCH v2 bpf-next 4/4] selftests/bpf: add test for bpf_iter_task_vma

[Song Liu <songliubraving@fb.com>]

> On Dec 16, 2020, at 10:18 AM, Yonghong Song <yhs@fb.com> wrote:
> 

[...]

>> +
>> +	err = bpf_iter_task_vma__load(skel);
>> +	if (CHECK(err, "bpf_iter_task_vma__load", "skeleton load failed\n"))
>> +		goto out;
>> +
>> +	do_dummy_read(skel->progs.proc_maps);
> 
> This do_dummy_read() is not needed, right?

do_dummy_read() helped me got bug in earlier version. I am planning to 
change the following to do smaller reads, then do_dummy_read() is no longer
needed. 

[...]

> 
>> +
>> +SEC("iter.s/task_vma") int proc_maps(struct bpf_iter__task_vma *ctx)
>> +{
>> +	struct __vm_area_struct *vma = ctx->vma;
>> +	struct seq_file *seq = ctx->meta->seq;
>> +	struct task_struct *task = ctx->task;
>> +	struct file *file = ctx->file;
>> +	char perm_str[] = "----";
>> +
>> +	if (task == (void *)0 || vma == (void *)0 || task->pid != pid)
> 
> I suppose kernel already filtered all non-group-leader tasks, so here
> we can have task->tgid != pid?

Yeah, that works. 

> 
>> +		return 0;
> 
> Using /proc system, user typically do cat /proc/pid/maps. How can we
> have a similar user experience with vma_iter here? One way to do this
> is:
>   - We still have this bpf program, filtering based on user pid,
>   - normal bpftool iter pin command pid the program to say /sys/fs/bpf/task_vma
>   - since "pid" is in a map, user can use bpftool to update "pid"
>     with the target pid.
>   - "cat /sys/fs/bpf/task_vma" will work.
> 
> One thing here is pid and d_path_buf are global (map) variables, so
> if two users are trying to do "cat /sys/fs/bpf/task_vma" at the same
> time, there will be interferences and it will not work.
> 
> One possible way is during BPF_ITER_CREATE, we duplicate all program
> maps. But this is unnecessary as in most cases, the bpf_iter is not
> pinned and private to applications.
> 
> Any other ideas?

Maybe we can use task local storage for pid and d_path_buf? 

To make it more practical, we probably want in kernel filtering based 
on pid. IOW, let user specify which task to iterate. 

Thanks,
Song