Re: [PATCH bpf] bpftool: Use libbpf error code for flow dissector query

[Yonghong Song <yonghong.song@linux.dev>]

On 6/2/26 10:03 AM, Woojin Ji wrote:
> bpf_prog_query() returns a negative errno on failure. query_flow_dissector()
> currently closes the namespace fd and then reads errno to decide whether
> -EINVAL means that the running kernel does not support flow dissector queries.
>
> That errno check controls behavior, not just diagnostics: -EINVAL is handled
> as a non-fatal old-kernel case, while any other error makes bpftool net fail.
> Reading errno after close() is fragile, because close() can overwrite errno
> before the check. Use the libbpf-returned error code instead so the

Do you have evidence that close() is fragile?

> compatibility branch is based on the BPF_PROG_QUERY result itself.
>
> Keep the existing errno reset in the non-fatal path to preserve batch mode
> behavior. The success path is unchanged.
>
> Fixes: 7f0c57fec80f ("bpftool: show flow_dissector attachment status")
> Assisted-by: ChatGPT:gpt-5.5
> Signed-off-by: Woojin Ji <random6.xyz@gmail.com>
> ---
>   tools/bpf/bpftool/net.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/bpf/bpftool/net.c b/tools/bpf/bpftool/net.c
> index 974189da8a91..dba28755d284 100644
> --- a/tools/bpf/bpftool/net.c
> +++ b/tools/bpf/bpftool/net.c
> @@ -603,14 +603,14 @@ static int query_flow_dissector(struct bpf_attach_info *attach_info)
>   			     &attach_flags, prog_ids, &prog_cnt);
>   	close(fd);
>   	if (err) {
> -		if (errno == EINVAL) {
> +		if (err == -EINVAL) {
>   			/* Older kernel's don't support querying
>   			 * flow dissector programs.
>   			 */
>   			errno = 0;
>   			return 0;
>   		}
> -		p_err("can't query prog: %s", strerror(errno));
> +		p_err("can't query prog: %s", strerror(-err));
>   		return -1;
>   	}
>