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

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

On 6/2/26 12:43 PM, Yonghong Song wrote:
>
>
> 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?

The patch itself looks good to me. But it would be great in commit message
to answer why. Note that fd is readonly: fd = open("/proc/self/ns/net", O_RDONLY).

>
>> 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;
>>       }
>
>