Re: [PATCH bpf 1/2] bpf: Check timer_off for map_in_map only when map value have timer

[Hengqi Chen <hengqi.chen@gmail.com>]

Hi, Alexei:

On 2022/11/28 08:44, Alexei Starovoitov wrote:
> On Sat, Nov 26, 2022 at 2:54 AM Hengqi Chen <hengqi.chen@gmail.com> wrote:
>>
>> The timer_off value could be -EINVAL or -ENOENT when map value of
>> inner map is struct and contains no bpf_timer. The EINVAL case happens
>> when the map is created without BTF key/value info, map->timer_off
>> is set to -EINVAL in map_create(). The ENOENT case happens when
>> the map is created with BTF key/value info (e.g. from BPF skeleton),
>> map->timer_off is set to -ENOENT as what btf_find_timer() returns.
>> In bpf_map_meta_equal(), we expect timer_off to be equal even if
>> map value does not contains bpf_timer. This rejects map_in_map created
>> with BTF key/value info to be updated using inner map without BTF
>> key/value info in case inner map value is struct. This commit lifts
>> such restriction.
> 
> Sorry, but I prefer to label this issue as 'wont-fix'.
> Mixing BTF enabled and non-BTF inner maps is a corner case

We do have such usecase. The BPF progs and maps are pinned to bpffs
using BPF object file. And the map_in_map is updated by some other
process which don't have access to such BTF info.

> that is not worth fixing.

Is there a way to get this fixed for v5.x series only ?

> At some point we will require all programs and maps to contain BTF.
> It's necessary for introspection.

We don't care much about BTF for introspection. In production, we always
have a version field and some reserved fields in the map value for backward
compatibility. The interpretation of such map values are left to upper layer.

> The maps as blobs of data should not be used.
> Much so adding support for mixed use as inner maps.