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

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Message-ID: <87a6yy1d6h.fsf@toke.dk>
Date:   Thu, 13 Aug 2020 22:52:22 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf] libbpf: Prevent overriding errno when logging errors

Daniel Borkmann <daniel@...earbox.net> writes:

> On 8/13/20 9:52 PM, Toke Høiland-Jørgensen wrote:
>> 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 :)
>
> Fixed up while applying, thanks!

Great, thanks!

-Toke