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

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Date:   Mon, 11 Jul 2022 14:51:59 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     "anquan.wu" <leiqi96@...mail.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, martin.lau@...ux.dev,
        john fastabend <john.fastabend@...il.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] libbpf: fix the name of a reused map

On Mon, Jul 11, 2022 at 3:25 AM anquan.wu <leiqi96@...mail.com> wrote:
>
> BPF map name was limited to BPF_OBJ_NAME_LEN.
> If 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>

please use your complete and capitalized name in Signed-off-by

Overall, looks good, I have a few stylistical nits, see below. But
also you forgot to cc bpf@...r.kernel.org, please send v2 and don't
forget to add mailing list. Without that our CI can't test your patch
properly.


> ---
>  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..5ad52a8accd1 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;

nit: __u32 len = sizeof(info), 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 ((BPF_OBJ_NAME_LEN - 1) == name_len && !strncmp(map->name, info.name, name_len))

nit, unnecessary () around BPF_OBJ_NAME_LEN, plus the order is a bit
weird. I also have general preference with strncmp/strcmp to compare
against zero explicitly, so can you rewrite this as:

if (name_len == BPF_OBJ_NAME_LEN - 1 && strncmp(map->name, info.name,
name_len) == 0)

?

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