Re: Create inner maps dynamically from ebpf kernel prog program

[rainkin <rainkin1993@gmail.com>]

>
>
>
> On 6/21/21 6:12 AM, rainkin wrote:
> > Hi,
> >
> > My ebpf program is attched to kprobe/vfs_read, my use case is to store
> > information of each file (i.e., inode) of each process by using
> > map-in-map (e.g., outer map is a hash map where key is pid, value is a
> > inner map where key is inode, value is some stateful information I
> > want to store.
> > Thus I need to create a new inner map for a new coming inode.
> >
> > I know there exists local storage for task/inode, however, limited to
> > my kernel version (4.1x), those local storage cannot be used.
> >
> > I tried two methods:
> > 1. dynamically create a new inner in user-land ebpf program by
> > following this tutorial:
> > https://github.com/torvalds/linux/blob/master/samples/bpf/test_map_in_map_user.c
> > Then insert the new inner map into the outer map.
> > The limitation of this method:
> > It requires ebpf kernel program send a message to user-land program to
> > create a newly inner map.
> > And ebpf kernel programs might access the map before user-land program
> > finishes the job.
> >
> > 2. Thus, i prefer the second method: dynamically create inner maps in
> > the kernel ebpf program.
> > According to the discussion in the following thread, it seems that it
> > can be done by calling bpf_map_update_elem():
> > https://lore.kernel.org/bpf/878sdlpv92.fsf@toke.dk/T/#e9bac624324ffd3efb0c9f600426306e3a40ec
> > 7b5
> >> Creating a new map for map_in_map from bpf prog can be implemented.
> >> bpf_map_update_elem() is doing memory allocation for map elements. In such a case calling
> >> this helper on map_in_map can, in theory, create a new inner map and insert it into the outer map.
> >
> > However, when I call method to create a new inner, it return the error:
> > 64: (bf) r2 = r10
> > 65: (07) r2 += -144
> > 66: (bf) r3 = r10
> > 67: (07) r3 += -176
> > ; bpf_map_update_elem(&outer, &ino, &new_inner, BPF_ANY);
> > 68: (18) r1 = 0xffff8dfb7399e400
> > 70: (b7) r4 = 0
> > 71: (85) call bpf_map_update_elem#2
> > cannot pass map_type 13 into func bpf_map_update_elem#2
>
> This is expected based on current verifier implementation.
> In verifier check_map_func_compatibility() function, we have
>
>          case BPF_MAP_TYPE_ARRAY_OF_MAPS:
>          case BPF_MAP_TYPE_HASH_OF_MAPS:
>                  if (func_id != BPF_FUNC_map_lookup_elem)
>                          goto error;
>                  break;
>
> For array/hash map-in-map, the only supported helper
> is bpf_map_lookup_elem(). bpf_map_update_elem()
> is not supported yet.

Thanks for your answer!
If I understand correctly, the conclusion is that (at least for now)
*ebpf kernel program*
CAN only do lookup for array/hash map-in-map, and CANNOT do
add/update/delete for array/hash
map-in-map, and CANNOT create reguar hash/array maps dynamically.


>
> For your method #1, the bpf helper bpf_send_signal() or
> bpf_send_signal_thread() might help to send some info
> to user space, but I think they are not available in
> 4.x kernels.
>
> Maybe a single map with key (pid, inode) may work?
>
> >
> > new_inner is a structure of inner hashmap.
> >
> > Any suggestions?
> > Thanks,
> > Rainkin
> >

a single map with key (pid, inode) is ok for the above scenario, however,
when I want to cleanup all entries realted to a certain pid when a
process exits,
a single map is NOT ok. I need to go through all the keys of the
single map and delete keys related
to the certain pid.