netdev - Re: [PATCH bpf] libbpf: Prevent overriding errno when logging errors

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Message-ID: <87d03u1fyj.fsf@toke.dk>
Date:   Thu, 13 Aug 2020 21:52:20 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf] libbpf: Prevent overriding errno when logging errors

Andrii Nakryiko <andrii.nakryiko@...il.com> writes:

> On Thu, Aug 13, 2020 at 7:29 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>
>> Turns out there were a few more instances where libbpf didn't save the
>> errno before writing an error message, causing errno to be overridden by
>> the printf() return and the error disappearing if logging is enabled.
>>
>> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
>> ---
>
> Acked-by: Andrii Nakryiko <andriin@...com>
>
>>  tools/lib/bpf/libbpf.c | 12 +++++++-----
>>  1 file changed, 7 insertions(+), 5 deletions(-)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 0a06124f7999..fd256440e233 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -3478,10 +3478,11 @@ bpf_object__probe_global_data(struct bpf_object *obj)
>>
>>         map = bpf_create_map_xattr(&map_attr);
>>         if (map < 0) {
>> -               cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
>> +               ret = -errno;
>> +               cp = libbpf_strerror_r(-ret, errmsg, sizeof(errmsg));
>
> fyi, libbpf_strerror_r() is smart enough to work with both negative
> and positive error numbers (it basically takes abs(err)), so no need
> to ensure it's positive here and below.

Noted. Although that also means it doesn't hurt either, I suppose; so
not going to bother respinning this unless someone insists :)

-Toke