bpf: Fix BPF_PROG_QUERY last field check

While working on the ebpf-go [0] library integration for bpf_mprog and tcx,
Lorenz noticed that two subsequent BPF_PROG_QUERY requests currently fail. A
typical workflow is to first gather the bpf_mprog count without passing program/
link arrays, followed by the second request which contains the actual array
pointers.

The initial call populates count and revision fields. The second call gets
rejected due to a BPF_PROG_QUERY_LAST_FIELD bug which should point to
query.revision instead of query.link_attach_flags since the former is really
the last member.

It was not noticed in libbpf as bpf_prog_query_opts() always calls bpf(2) with
an on-stack bpf_attr that is memset() each time (and therefore query.revision
was reset to zero).

  [0] https://ebpf-go.dev

Fixes: e420bed ("bpf: Add fd-based tcx multi-prog infra with link support")
Reported-by: Lorenz Bauer <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

Author: borkmann

kernel/bpf/syscall.c

@@ -3913,7 +3913,7 @@ static int bpf_prog_detach(const union bpf_attr *attr)
 	return ret;
 }
 
-#define BPF_PROG_QUERY_LAST_FIELD query.link_attach_flags
+#define BPF_PROG_QUERY_LAST_FIELD query.revision
 
 static int bpf_prog_query(const union bpf_attr *attr,
 			  union bpf_attr __user *uattr)