selftests/bpf: Add tests for bpf_throw lock leak from subprogs

Add test cases to ensure the verifier correctly rejects bpf_throw from
subprogs when RCU, preempt, or IRQ locks are held:

  * reject_subprog_rcu_lock_throw: subprog acquires bpf_rcu_read_lock and
    then calls bpf_throw
  * reject_subprog_throw_preempt_lock: always-throwing subprog called while
    caller holds bpf_preempt_disable
  * reject_subprog_throw_irq_lock: always-throwing subprog called while
    caller holds bpf_local_irq_save

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Ihor Solodrai <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Acked-by: Kumar Kartikeya Dwivedi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: theihor

tools/testing/selftests/bpf/progs/exceptions_fail.c

@@ -9,6 +9,10 @@
 
 extern void bpf_rcu_read_lock(void) __ksym;
 extern void bpf_rcu_read_unlock(void) __ksym;
+extern void bpf_preempt_disable(void) __ksym;
+extern void bpf_preempt_enable(void) __ksym;
+extern void bpf_local_irq_save(unsigned long *) __ksym;
+extern void bpf_local_irq_restore(unsigned long *) __ksym;
 
 #define private(name) SEC(".bss." #name) __hidden __attribute__((aligned(8)))
 
@@ -349,4 +353,47 @@ int reject_exception_throw_cb_diff(struct __sk_buff *ctx)
 	return 0;
 }
 
+__noinline static int always_throws(void)
+{
+	bpf_throw(0);
+	return 0;
+}
+
+__noinline static int rcu_lock_then_throw(void)
+{
+	bpf_rcu_read_lock();
+	bpf_throw(0);
+	return 0;
+}
+
+SEC("?tc")
+__failure __msg("bpf_throw cannot be used inside bpf_rcu_read_lock-ed region")
+int reject_subprog_rcu_lock_throw(void *ctx)
+{
+	rcu_lock_then_throw();
+	return 0;
+}
+
+SEC("?tc")
+__failure __msg("bpf_throw cannot be used inside bpf_preempt_disable-ed region")
+int reject_subprog_throw_preempt_lock(void *ctx)
+{
+	bpf_preempt_disable();
+	always_throws();
+	bpf_preempt_enable();
+	return 0;
+}
+
+SEC("?tc")
+__failure __msg("bpf_throw cannot be used inside bpf_local_irq_save-ed region")
+int reject_subprog_throw_irq_lock(void *ctx)
+{
+	unsigned long flags;
+
+	bpf_local_irq_save(&flags);
+	always_throws();
+	bpf_local_irq_restore(&flags);
+	return 0;
+}
+
 char _license[] SEC("license") = "GPL";