netdev - Re: [PATCH bpf] libbpf: reset errno after probing kernel features

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Message-ID: <CAEf4BzavUADiak9FboiThRC2W_agJXXh3dGm7zKqDNJ+dUFnHA@mail.gmail.com>
Date:   Mon, 30 Nov 2020 16:18:27 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...com>,
        Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] libbpf: reset errno after probing kernel features

On Mon, Nov 30, 2020 at 2:41 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>
> > On Mon, Nov 30, 2020 at 7:42 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >>
> >> The kernel feature probing results in 'errno' being set if the probing
> >> fails (as is often the case). This can stick around and leak to the caller,
> >> which can lead to confusion later. So let's make sure we always reset errno
> >> after calling a probe function.
> >
> > What specifically is the problem and what sort of confusion we are
> > talking about here? You are not supposed to check errno, unless the
> > function returned -1 or other error result.
> >
> > In some cases, you have to reset errno manually just to avoid
> > confusion (see how strtol() is used, as an example).
> >
> > I.e., I don't see the problem here, any printf() technically can set
> > errno to <0, we don't reset errno after each printf call though,
> > right?
>
> Well yeah, technically things work fine in the common case. But this

It works fine in all cases. Assuming "errno != 0 means last
libc/syscall failed" is just wrong.

> errno thing sent me on quite the wild goose chase when trying to find
> the root cause of the pinning issue I also sent a patch for...
>
> So since reseting errno doesn't hurt either I figured I'd save others
> ending up in similar trouble. If it's not to your taste feel free to
> just drop the patch :)

Yep, let's just drop it, no need to create a bad precedent.

>
> -Toke
>