Re: [PATCH v2 bpf-next 7/7] selftests/bpf: Add tests for rbtree API interaction in sleepable progs

[Yonghong Song <yonghong.song@linux.dev>]

On 8/21/23 12:33 PM, Dave Marchevsky wrote:
> Confirm that the following sleepable prog states fail verification:
>    * bpf_rcu_read_unlock before bpf_spin_unlock
>       * RCU CS will last at least as long as spin_lock CS

I think the reason is bpf_spin_lock() does not allow any functions
in spin lock region except some graph api kfunc's.

> 
> Also confirm that correct usage passes verification, specifically:
>    * Explicit use of bpf_rcu_read_{lock, unlock} in sleepable test prog
>    * Implied RCU CS due to spin_lock CS
> 
> None of the selftest progs actually attach to bpf_testmod's
> bpf_testmod_test_read.
> 
> Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
> ---
>   .../selftests/bpf/progs/refcounted_kptr.c     | 71 +++++++++++++++++++
>   .../bpf/progs/refcounted_kptr_fail.c          | 28 ++++++++
>   2 files changed, 99 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/progs/refcounted_kptr.c b/tools/testing/selftests/bpf/progs/refcounted_kptr.c
> index c55652fdc63a..893a4fdb4b6e 100644
[...]
> diff --git a/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c b/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
> index 0b09e5c915b1..1ef07f6ee580 100644
> --- a/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
> +++ b/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
> @@ -13,6 +13,9 @@ struct node_acquire {
>   	struct bpf_refcount refcount;
>   };
>   
> +extern void bpf_rcu_read_lock(void) __ksym;
> +extern void bpf_rcu_read_unlock(void) __ksym;
> +
>   #define private(name) SEC(".data." #name) __hidden __attribute__((aligned(8)))
>   private(A) struct bpf_spin_lock glock;
>   private(A) struct bpf_rb_root groot __contains(node_acquire, node);
> @@ -71,4 +74,29 @@ long rbtree_refcounted_node_ref_escapes_owning_input(void *ctx)
>   	return 0;
>   }
>   
> +SEC("?fentry.s/bpf_testmod_test_read")
> +__failure __msg("function calls are not allowed while holding a lock")
> +int BPF_PROG(rbtree_fail_sleepable_lock_across_rcu,
> +	     struct file *file, struct kobject *kobj,
> +	     struct bin_attribute *bin_attr, char *buf, loff_t off, size_t len)
> +{
> +	struct node_acquire *n;
> +
> +	n = bpf_obj_new(typeof(*n));
> +	if (!n)
> +		return 0;
> +
> +	/* spin_{lock,unlock} are in different RCU CS */
> +	bpf_rcu_read_lock();
> +	bpf_spin_lock(&glock);
> +	bpf_rbtree_add(&groot, &n->node, less);
> +	bpf_rcu_read_unlock();
> +
> +	bpf_rcu_read_lock();
> +	bpf_spin_unlock(&glock);
> +	bpf_rcu_read_unlock();
> +
> +	return 0;
> +}
> +
>   char _license[] SEC("license") = "GPL";