[PATCH 2/2] bpftool: Fix regression of "bpftool cgroup tree" EINVAL on older kernels

[Mehmet Basaran]

From: YiFei Zhu <zhuyifei at google.com>

BugLink: https://bugs.launchpad.net/bugs/2119603

[ Upstream commit 43745d11bfd9683abdf08ad7a5cc403d6a9ffd15 ]

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 at google.com>
Reported-by: Minh-Anh Nguyen <minhanhdn at google.com>
Signed-off-by: YiFei Zhu <zhuyifei at google.com>
Signed-off-by: Andrii Nakryiko <andrii at kernel.org>
Acked-by: Quentin Monnet <qmo at kernel.org>
Link: https://lore.kernel.org/bpf/20250428211536.1651456-1-zhuyifei@google.com
Signed-off-by: Sasha Levin <sashal at kernel.org>
Signed-off-by: Manuel Diewald <manuel.diewald at canonical.com>
Signed-off-by: Mehmet Basaran <mehmet.basaran at canonical.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 955bea7b1220..4189c9d74fb0 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) {
-- 
2.43.0