linux-kernel - Re: [PATCH v2] libbpf: fix the name of a reused map

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Message-ID: <Ys0xcf2yRG4fjkBY@krava>
Date:   Tue, 12 Jul 2022 10:31:45 +0200
From:   Jiri Olsa <olsajiri@...il.com>
To:     Anquan Wu <leiqi96@...mail.com>
Cc:     andrii@...nel.org, daniel@...earbox.net, martin.lau@...ux.dev,
        song@...nel.org, yhs@...com, kpsingh@...nel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] libbpf: fix the name of a reused map

On Tue, Jul 12, 2022 at 11:15:40AM +0800, Anquan Wu wrote:
> BPF map name is limited to BPF_OBJ_NAME_LEN.
> A map name is defined as being longer than BPF_OBJ_NAME_LEN,
> it will be truncated to BPF_OBJ_NAME_LEN when a userspace program
> calls libbpf to create the map. A pinned map also generates a path
> in the /sys. If the previous program wanted to reuse the map，
> it can not get bpf_map by name, because the name of the map is only
> partially the same as the name which get from pinned path.
> 
> The syscall information below show that map name "process_pinned_map"
> is truncated to "process_pinned_".
> 
>     bpf(BPF_OBJ_GET, {pathname="/sys/fs/bpf/process_pinned_map",
>     bpf_fd=0, file_flags=0}, 144) = -1 ENOENT (No such file or directory)
> 
>     bpf(BPF_MAP_CREATE, {map_type=BPF_MAP_TYPE_HASH, key_size=4,
>     value_size=4,max_entries=1024, map_flags=0, inner_map_fd=0,
>     map_name="process_pinned_",map_ifindex=0, btf_fd=3, btf_key_type_id=6,
>     btf_value_type_id=10,btf_vmlinux_value_type_id=0}, 72) = 4
> 
> This patch check that if the name of pinned map are the same as the
> actual name for the first (BPF_OBJ_NAME_LEN - 1),
> bpf map still uses the name which is included in bpf object.
> 
> Signed-off-by: Anquan Wu <leiqi96@...mail.com>
> ---
> 
> v2: compare against zero explicitly
> 
> v1: https://lore.kernel.org/linux-kernel/OSZP286MB1725A2361FA2EE8432C4D5F4B8879@OSZP286MB1725.JPNP286.PROD.OUTLOOK.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 e89cc9c885b3..7b4d3604dfb4 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4328,6 +4328,7 @@ int bpf_map__reuse_fd(struct bpf_map *map, int fd)
>  {
>  	struct bpf_map_info info = {};
>  	__u32 len = sizeof(info);
> +	__u32 name_len;
>  	int new_fd, err;
>  	char *new_name;
>  
> @@ -4337,7 +4338,12 @@ int bpf_map__reuse_fd(struct bpf_map *map, int fd)
>  	if (err)
>  		return libbpf_err(err);
>  
> -	new_name = strdup(info.name);
> +	name_len = strlen(info.name);
> +	if (name_len == BPF_OBJ_NAME_LEN - 1 && strncmp(map->name, info.name, name_len) == 0)

so what if the map->name is different after 'name_len' ?

jirka

> +		new_name = strdup(map->name);
> +	else
> +		new_name = strdup(info.name);
> +
>  	if (!new_name)
>  		return libbpf_err(-errno);
>  
> -- 
> 2.32.0
>