bpf: Reject bpf_timer for PREEMPT_RT

JIRA: https://issues.redhat.com/browse/RHEL-83826

commit e25ddfb
Author: Leon Hwang <leon.hwang@linux.dev>
Date:   Wed Sep 10 20:57:39 2025 +0800

    bpf: Reject bpf_timer for PREEMPT_RT

    When enable CONFIG_PREEMPT_RT, the kernel will warn when run timer
    selftests by './test_progs -t timer':

    BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48

    In order to avoid such warning, reject bpf_timer in verifier when
    PREEMPT_RT is enabled.

    Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
    Link: https://lore.kernel.org/r/20250910125740.52172-2-leon.hwang@linux.dev
    Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Signed-off-by: Gregory Bell <grbell@redhat.com>

Author: Gbell26

kernel/bpf/verifier.c

@@ -7763,6 +7763,10 @@ static int process_timer_func(struct bpf_verifier_env *env, int regno,
 		verbose(env, "verifier bug. Two map pointers in a timer helper\n");
 		return -EFAULT;
 	}
+	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		verbose(env, "bpf_timer cannot be used for PREEMPT_RT.\n");
+		return -EOPNOTSUPP;
+	}
 	meta->map_uid = reg->map_uid;
 	meta->map_ptr = map;
 	return 0;