Allows user space to update/lookup map elements under bpf_spin_lock

[mehrdadrad]

• Added FLock constant (BPF_F_LOCK)
• Added three new map methods to support lookup with FLock:
  • LookupWithFlags
  • LookupBytesWithFlags
  • LookupAndDeleteWithFlags
• Added tests with test data.

[lmb]

This adds a lot of API for a pretty niche use case. Could you instead detect whether a map contains a spinlock at creation time and infer the need for BPF_F_LOCK? This will be somewhat complicated by LoadPinnedMap which should also work with spinlock maps.

Alternatively we should investigate a single new method LookupOpts(key, valueOut, opts). Opts could be:

struct Opts {
    Delete bool
    Lock bool
}

That would avoid so much new API as well.

[mehrdadrad]

@lmb re: detect whether a map contains a spinlock
I was thinking about it but why BPF_F_LOCK is an option which it MUST pass from user space through BPF command? why doesn't the kernel handle it by itself?
I just tried bpftool and looks we can detect a map with bpf_spin_lock through BTF:

[9] STRUCT 'hash_elem' size=8 vlen=2
        'cnt' type_id=2 bits_offset=0
        'lock' type_id=10 bits_offset=32
[10] STRUCT 'bpf_spin_lock' size=4 vlen=1
        'val' type_id=6 bits_offset=0

 
In meanwhile the libbpf added more APIs as below if we follow them:

• int bpf_map_lookup_and_delete_elem_flags(int fd, const void *key, void *value, __u64 flags)
• int bpf_map_lookup_elem_flags(int fd, const void *key, void *value, __u64 flags)

Your alternative option can be like below:
LookupOpts should be support:

• BPF_MAP_LOOKUP_AND_DELETE_ELEM
• BPF_MAP_LOOKUP_ELEM

LookupOptions struct { 
   Flags MapUpdateFlags 
   Delete bool
}
LookupOpts(key, valueOut interface{}, opts LookupOptions) error

The last thing, we can add at least two of APIs which I already did same as libbpf.

Any thoughts?

[lmb]

You're right, since it's possible to do a lookup without BPF_F_LOCK always adding it doesn't make sense. Let's scrap the BTF idea.

The best thing we can do (imo):

• LookupWithFlags
• LookupAndDeleteWithFlags

There should be a new type MapLookupFlags uint64 though, with a constant LookupLock.

cc @ti-mo

[lmb]

Can you squash your changes please?

[lmb]

Thanks!