Re: [PATCH bpf 1/2] Ignore RDONLY_PROG for devmaps in libbpf to allow re-loading of pinned devmaps

[Martin KaFai Lau <martin.lau@linux.dev>]

On 9/27/22 11:23 AM, Pramukh Naduthota wrote:
> Ignore BPF_F_RDONLY_PROG when checking for compatibility for devmaps. The
> kernel adds the flag to all devmap creates, and this breaks pinning
> behavior, as libbpf will then check the actual vs user supplied flags and
> see this difference. Work around this by adding RDONLY_PROG to the
> users's flags when testing against the pinned map
> 
> Fixes: 57a00f41644f ("libbpf: Add auto-pinning of maps when loading BPF objects")
> Signed-off-by: Pramukh Naduthota <pnaduthota@google.com>
> ---
>   tools/lib/bpf/libbpf.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 50d41815f4..a3dae26d82 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4818,6 +4818,7 @@ static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
>   	char msg[STRERR_BUFSIZE];
>   	__u32 map_info_len;
>   	int err;
> +	unsigned int effective_flags = map->def.map_flags;
>   
>   	map_info_len = sizeof(map_info);
>   
> @@ -4830,11 +4831,16 @@ static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
>   		return false;
>   	}
>   
> +	/* The kernel adds RDONLY_PROG to devmaps */
> +	if (map->def.type == BPF_MAP_TYPE_DEVMAP ||
> +	    map->def.type == BPF_MAP_TYPE_DEVMAP_HASH)
> +		effective_flags |= BPF_F_RDONLY_PROG;

May be set BPF_F_RDONLY_PROG in effective_flags only when map->def.map_flags 
does not have both BPF_F_RDONLY_PROG and BPF_F_WRONLY_PROG set?  Just in case 
the devmap may support setting them during map creation in the future.

> +
>   	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 == effective_flags &&
>   		map_info.map_extra == map->map_extra);
>   }
>