Re: [PATCH bpf 1/9] bpf: Acquire map uref in .init_seq_private for array map iterator

[Yonghong Song <yhs@fb.com>]

On 8/6/22 12:40 AM, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
> 
> During bpf(BPF_LINK_CREATE) for BPF_TRACE_ITER, bpf_iter_attach_map()
> has already acquired a map uref, but the uref may be released by
> bpf_link_release() during th reading of map iterator.

some wording issue:
bpf_iter_attach_map() acquires a map uref, and the uref may be released
before or in the middle of iterating map elements. For example, the uref
could be released in bpf_iter_detach_map() as part of
bpf_link_release(), or could be released in bpf_map_put_with_uref()
as part of bpf_map_release().

> 
> Alternative fix is acquiring an extra bpf_link reference just like
> a pinned map iterator does, but it introduces unnecessary dependency
> on bpf_link instead of bpf_map.
> 
> So choose another fix: acquiring an extra map uref in .init_seq_private
> for array map iterator.
> 
> Fixes: d3cc2ab546ad ("bpf: Implement bpf iterator for array maps")
> Signed-off-by: Hou Tao <houtao1@huawei.com>

Acked-by: Yonghong Song <yhs@fb.com>

> ---
>   kernel/bpf/arraymap.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index d3e734bf8056..bf6898bb7cb8 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -649,6 +649,12 @@ static int bpf_iter_init_array_map(void *priv_data,
>   		seq_info->percpu_value_buf = value_buf;
>   	}
>   
> +	/*
> +	 * During bpf(BPF_LINK_CREATE), bpf_iter_attach_map() has already
> +	 * acquired a map uref, but the uref may be released by
> +	 * bpf_link_release(), so acquire an extra map uref for iterator.
> +	 */
> +	bpf_map_inc_with_uref(map);
>   	seq_info->map = map;
>   	return 0;
>   }
> @@ -657,6 +663,7 @@ static void bpf_iter_fini_array_map(void *priv_data)
>   {
>   	struct bpf_iter_seq_array_map_info *seq_info = priv_data;
>   
> +	bpf_map_put_with_uref(seq_info->map);
>   	kfree(seq_info->percpu_value_buf);
>   }
>