bpf: Document BPF_F_QUERY_EFFECTIVE flag

Document BPF_F_QUERY_EFFECTIVE flag, mostly to clarify how it affects
attach_flags what may not be obvious and what may lead to confision.

Specifically attach_flags is returned only for target_fd but if programs
are inherited from an ancestor cgroup then returned attach_flags for
current cgroup may be confusing. For example, two effective programs of
same attach_type can be returned but w/o BPF_F_ALLOW_MULTI in
attach_flags.

Simple repro:
  # bpftool c s /sys/fs/cgroup/path/to/task
  ID       AttachType      AttachFlags     Name
  # bpftool c s /sys/fs/cgroup/path/to/task effective
  ID       AttachType      AttachFlags     Name
  95043    ingress                         tw_ipt_ingress
  95048    ingress                         tw_ingress

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200108014006.938363-1-rdna@fb.com

include/uapi/linux/bpf.h

@@ -359,7 +359,12 @@ enum bpf_attach_type {
 /* Enable memory-mapping BPF map */
 #define BPF_F_MMAPABLE		(1U << 10)
 
-/* flags for BPF_PROG_QUERY */
+/* Flags for BPF_PROG_QUERY. */
+
+/* Query effective (directly attached + inherited from ancestor cgroups)
+ * programs that will be executed for events within a cgroup.
+ * attach_flags with this flag are returned only for directly attached programs.
+ */
 #define BPF_F_QUERY_EFFECTIVE	(1U << 0)
 
 enum bpf_stack_build_id_status {

tools/include/uapi/linux/bpf.h

@@ -359,7 +359,12 @@ enum bpf_attach_type {
 /* Enable memory-mapping BPF map */
 #define BPF_F_MMAPABLE		(1U << 10)
 
-/* flags for BPF_PROG_QUERY */
+/* Flags for BPF_PROG_QUERY. */
+
+/* Query effective (directly attached + inherited from ancestor cgroups)
+ * programs that will be executed for events within a cgroup.
+ * attach_flags with this flag are returned only for directly attached programs.
+ */
 #define BPF_F_QUERY_EFFECTIVE	(1U << 0)
 
 enum bpf_stack_build_id_status {