* Re: [PATCH] bpf: fix reuse of DEVMAP
2025-08-11 9:39 ` [PATCH] bpf: fix reuse of DEVMAP Yureka Lilian
@ 2025-08-11 17:29 ` Eduard Zingerman
2025-08-11 19:38 ` Martin KaFai Lau
1 sibling, 0 replies; 3+ messages in thread
From: Eduard Zingerman @ 2025-08-11 17:29 UTC (permalink / raw)
To: Yureka Lilian, Andrii Nakryiko, Alexei Starovoitov,
Daniel Borkmann, Martin KaFai Lau, Song Liu, Yonghong Song,
John Fastabend, KP Singh, Stanislav Fomichev, Hao Luo, Jiri Olsa
Cc: bpf, linux-kernel
On Mon, 2025-08-11 at 11:39 +0200, Yureka Lilian 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,
> it BPF_F_RDONLY_PROG being set on a map being created is invalid.
>
> Thus, match the BPF_F_RDONLY_PROG flag being set on the new map when
> checking for compatibility with an existing DEVMAP
>
> The same problem is handled in third-party ebpf library:
> - https://github.com/cilium/ebpf/issues/925
> - https://github.com/cilium/ebpf/pull/930
>
> Signed-off-by: Yureka Lilian <yuka@yuka.dev>
> ---
The change makes sense to me, could you please add a selftest?
[...]
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] bpf: fix reuse of DEVMAP
2025-08-11 9:39 ` [PATCH] bpf: fix reuse of DEVMAP Yureka Lilian
2025-08-11 17:29 ` Eduard Zingerman
@ 2025-08-11 19:38 ` Martin KaFai Lau
1 sibling, 0 replies; 3+ messages in thread
From: Martin KaFai Lau @ 2025-08-11 19:38 UTC (permalink / raw)
To: Yureka Lilian
Cc: Andrii Nakryiko, Eduard Zingerman, Alexei Starovoitov,
Daniel Borkmann, Song Liu, Yonghong Song, John Fastabend,
KP Singh, Stanislav Fomichev, Hao Luo, Jiri Olsa,
Toke Høiland-Jørgensen, bpf, linux-kernel
On 8/11/25 2:39 AM, Yureka Lilian 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,
> it BPF_F_RDONLY_PROG being set on a map being created is invalid.
>
> Thus, match the BPF_F_RDONLY_PROG flag being set on the new map when
> checking for compatibility with an existing DEVMAP
>
> The same problem is handled in third-party ebpf library:
> - https://github.com/cilium/ebpf/issues/925
> - https://github.com/cilium/ebpf/pull/930
>
> Signed-off-by: Yureka Lilian <yuka@yuka.dev>
This should be a fix for this commit?
Fixes: 0cdbb4b09a06 ("devmap: Allow map lookups from eBPF")
> ---
> tools/lib/bpf/libbpf.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index fb4d92c5c..a554d7fff 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -5081,6 +5081,7 @@ 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);
> @@ -5093,11 +5094,20 @@ 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, match the BPF_F_RDONLY_PROG flag being set on the new
> + * map 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_flags_for_check |= BPF_F_RDONLY_PROG;
Does it break older kernels that do not set the RDONLY_PROG bit implicitly by
itself?
> +
> 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);
> }
>
^ permalink raw reply [flat|nested] 3+ messages in thread