[PATCH bpf] bpftool: Fix regression of "bpftool cgroup tree" EINVAL on older kernels

[YiFei Zhu <zhuyifei@google.com>]

If cgroup_has_attached_progs queries an attach type not supported
by the running kernel, due to the kernel being older than the bpftool
build, it would encounter an -EINVAL from BPF_PROG_QUERY syscall.

Prior to commit 98b303c9bf05 ("bpftool: Query only cgroup-related
attach types"), this EINVAL would be ignored by the function, allowing
the function to only consider supported attach types. The commit
changed so that, instead of querying all attach types, only attach
types from the array `cgroup_attach_types` is queried. The assumption
is that because these are only cgroup attach types, they should all
be supported. Unfortunately this assumption may be false when the
kernel is older than the bpftool build, where the attach types queried
by bpftool is not yet implemented in the kernel. This would result in
errors such as:

  $ bpftool cgroup tree
  CgroupPath
  ID       AttachType      AttachFlags     Name
  Error: can't query bpf programs attached to /sys/fs/cgroup: Invalid argument

This patch restores the logic of ignoring EINVAL from prior to that patch.

Fixes: 98b303c9bf05 ("bpftool: Query only cgroup-related attach types")
Reported-by: Sagarika Sharma <sharmasagarika@google.com>
Reported-by: Minh-Anh Nguyen <minhanhdn@google.com>
Signed-off-by: YiFei Zhu <zhuyifei@google.com>
---
 tools/bpf/bpftool/cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/bpf/bpftool/cgroup.c b/tools/bpf/bpftool/cgroup.c
index 93b139bfb9880..3f1d6be512151 100644
--- a/tools/bpf/bpftool/cgroup.c
+++ b/tools/bpf/bpftool/cgroup.c
@@ -221,7 +221,7 @@ static int cgroup_has_attached_progs(int cgroup_fd)
 	for (i = 0; i < ARRAY_SIZE(cgroup_attach_types); i++) {
 		int count = count_attached_bpf_progs(cgroup_fd, cgroup_attach_types[i]);
 
-		if (count < 0)
+		if (count < 0 && errno != EINVAL)
 			return -1;
 
 		if (count > 0) {
-- 
2.49.0.901.g37484f566f-goog