linux-kernel - Re: [PATCH v2 1/2] bpf: fix reuse of DEVMAP

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Message-ID: <CAEf4Bzbw_02vrhUa+JEmuqo3snuf5xjNVL1wOu0BT1Z_MZ5S0g@mail.gmail.com>
Date: Wed, 13 Aug 2025 15:49:39 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Yureka Lilian <yuka@...a.dev>
Cc: Andrii Nakryiko <andrii@...nel.org>, Eduard Zingerman <eddyz87@...il.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] bpf: fix reuse of DEVMAP

On Wed, Aug 13, 2025 at 1:09 PM Yureka Lilian <yuka@...a.dev> wrote:
>
> Previously, re-using pinned DEVMAP maps would always fail, because
> get_map_info on a DEVMAP always returns flags with BPF_F_RDONLY_PROG set,
> but BPF_F_RDONLY_PROG being set on a map during creation is invalid.
>
> Thus, ignore the BPF_F_RDONLY_PROG flag on both sides when checking for
> compatibility with an existing DEVMAP.
>
> Ignoring it on both sides ensures that it continues to work on older
> kernels which don't set BPF_F_RDONLY_PROG on get_map_info.
>
> The same problem is handled in a third-party ebpf library:
> - https://github.com/cilium/ebpf/issues/925
> - https://github.com/cilium/ebpf/pull/930
>
> Fixes: 0cdbb4b09a06 ("devmap: Allow map lookups from eBPF")
> Signed-off-by: Yureka Lilian <yuka@...a.dev>
> ---
>  tools/lib/bpf/libbpf.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index d41ee26b9..049b0c400 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -5076,6 +5076,7 @@ static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
>  {
>         struct bpf_map_info map_info;
>         __u32 map_info_len = sizeof(map_info);
> +       __u32 map_flags_for_check = map->def.map_flags;
>         int err;
>
>         memset(&map_info, 0, map_info_len);
> @@ -5088,11 +5089,22 @@ static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
>                 return false;
>         }
>
> +       /* get_map_info on a DEVMAP will always return flags with
> +        * BPF_F_RDONLY_PROG set, but it will never be set on a map
> +        * being created.
> +        * Thus, ignore the BPF_F_RDONLY_PROG flag on both sides when
> +        * checking for compatibility with an existing DEVMAP.
> +        */
> +       if (map->def.type == BPF_MAP_TYPE_DEVMAP || map->def.type == BPF_MAP_TYPE_DEVMAP_HASH) {
> +               map_info.map_flags |= BPF_F_RDONLY_PROG;
> +               map_flags_for_check |= BPF_F_RDONLY_PROG;
> +       }

can we instead clear BPF_F_RDONLY_PROG in map_info.map_flags? and then
keep using map->def.map_flags directly

pw-bot: cr


> +
>         return (map_info.type == map->def.type &&
>                 map_info.key_size == map->def.key_size &&
>                 map_info.value_size == map->def.value_size &&
>                 map_info.max_entries == map->def.max_entries &&
> -               map_info.map_flags == map->def.map_flags &&
> +               map_info.map_flags == map_flags_for_check &&
>                 map_info.map_extra == map->map_extra);
>  }
>
> --
> 2.50.1
>