bpf/selftests: test_access_variable_array breaks due to sched_domain::span removal

[Venkat Rao Bagalkote]

Hi,


While running BPF selftests on current linux-next, I noticed that
test_access_variable_array fails to build due to reliance on
struct sched_domain::span, which is no longer appers to be BTF-visible 
after recent
scheduler refactoring.

The Build error I am seeing is:

progs/test_access_variable_array.c:14:13: error: no member named 'span' 
in 'struct sched_domain'  CLNG-BPF [test_progs] test_check_mtu.bpf.o

   14 |         span = sd->span[0];
      |                ~~  ^

Below is a proposed update to the test that switches from
sched_domain::span to sched_group::cpumask. This preserves the original
intent of validating variable-length array access via BTF while avoiding
reliance on removed scheduler internals.


diff --git 
a/tools/testing/selftests/bpf/progs/test_access_variable_array.c 
b/tools/testing/selftests/bpf/progs/test_access_variable_array.c
index 326b7d1f496a..c9f345ccde3c 100644
--- a/tools/testing/selftests/bpf/progs/test_access_variable_array.c
+++ b/tools/testing/selftests/bpf/progs/test_access_variable_array.c
@@ -4,14 +4,18 @@
 #include "vmlinux.h"
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>

-unsigned long span = 0;
+unsigned long cpumask0 = 0;

-SEC("fentry/sched_balance_rq")
-int BPF_PROG(fentry_fentry, int this_cpu, struct rq *this_rq,
-               struct sched_domain *sd)
+SEC("fentry/sched_balance_find_dst_group_cpu")
+int BPF_PROG(fentry_fentry, struct sched_group *sg, struct task_struct *p,
+               int this_cpu)
 {
-       span = sd->span[0];
+       unsigned long *mask;
+       /* Read pointer to variable-length CPU mask */
+       mask = BPF_CORE_READ(sg, cpumask);
+       cpumask0 = mask[0];

        return 0;
 }


I have tested this change, and it seems to be working as expected.


# ./test_progs -t access_variable_array
#1       access_variable_array:OK
Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED


Please note that this comes from my early days of working on upstream
kernel contributions, and I am still learning the BPF and scheduler
internals. My understanding here may be incomplete, so I wanted to
share this primarily to report the breakage I am seeing and propose a
possible direction for fixing the test.

I would appreciate any feedback on whether this is the right approach,
or if there is a more appropriate structure or hook to use for
preserving the variable-length array coverage in this selftest.


Regards,

Venkat.