[PATCH bpf-next v4 1/3] selftests/bpf: Test non-sleepable kfunc context

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

A sleepable kfunc call can be rejected because the program itself is
not sleepable or because a sleepable program is inside an active
critical section.

Exercise the two causes separately. Call a sleepable kfunc directly
from a non-sleepable tracing program, then call it from a sleepable
tracing program inside an RCU read-side critical section. Assert both
the legacy verifier messages and the structured reasons.

Link: https://lore.kernel.org/bpf/2e42a1a2bf45f4d2aba7495bdc9f147558055740e2f3c8b9dae255f6c57fc13c@mail.kernel.org/
Link: https://lore.kernel.org/r/20260924170646.2366016-6-memxor@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../selftests/bpf/progs/preempt_lock.c        | 26 +++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/preempt_lock.c b/tools/testing/selftests/bpf/progs/preempt_lock.c
index 81c459435680..955391da326a 100644
--- a/tools/testing/selftests/bpf/progs/preempt_lock.c
+++ b/tools/testing/selftests/bpf/progs/preempt_lock.c
@@ -6,6 +6,8 @@
 #include "bpf_experimental.h"
 
 extern int bpf_copy_from_user_str(void *dst, u32 dst__sz, const void *unsafe_ptr__ign, u64 flags) __weak __ksym;
+extern void bpf_rcu_read_lock(void) __ksym;
+extern void bpf_rcu_read_unlock(void) __ksym;
 
 SEC("?tc")
 __failure __msg("BPF_EXIT instruction in main prog cannot be used inside bpf_preempt_disable-ed region")
@@ -179,6 +181,30 @@ int preempt_sleepable_kfunc(void *ctx)
 	return 0;
 }
 
+SEC("?fentry/" SYS_PREFIX "sys_getpgid")
+__failure __msg("program must be sleepable to call sleepable kfunc bpf_copy_from_user_str")
+__msg("cannot be used in non-sleepable program")
+int non_sleepable_kfunc(void *ctx)
+{
+	u32 data;
+
+	bpf_copy_from_user_str(&data, sizeof(data), NULL, 0);
+	return 0;
+}
+
+SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
+__failure __msg("kernel func bpf_copy_from_user_str is sleepable within rcu_read_lock region")
+__msg("cannot be used in RCU read lock region")
+int sleepable_kfunc_in_rcu(void *ctx)
+{
+	u32 data;
+
+	bpf_rcu_read_lock();
+	bpf_copy_from_user_str(&data, sizeof(data), NULL, 0);
+	bpf_rcu_read_unlock();
+	return 0;
+}
+
 int __noinline preempt_global_subprog(void)
 {
 	preempt_balance_subprog();
-- 
2.53.0