netdev - Re: [PATCH bpf 1/3] bpf: syscall to start at file-descriptor 1

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Date:   Mon, 8 Jun 2020 21:44:37 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     David Ahern <dsahern@...il.com>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Networking <netdev@...r.kernel.org>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Lorenzo Bianconi <lorenzo@...nel.org>, brouer@...hat.com
Subject: Re: [PATCH bpf 1/3] bpf: syscall to start at file-descriptor 1

On Mon, 8 Jun 2020 11:36:33 -0700
Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:

> On Mon, Jun 8, 2020 at 9:51 AM Jesper Dangaard Brouer <brouer@...hat.com> wrote:
> >
> > This patch change BPF syscall to avoid returning file descriptor value zero.
> >
> > As mentioned in cover letter, it is very impractical when extending kABI
> > that the file-descriptor value 'zero' is valid, as this requires new fields
> > must be initialised as minus-1. First step is to change the kernel such that
> > BPF-syscall simply doesn't return value zero as a FD number.
> >
> > This patch achieves this by similar code to anon_inode_getfd(), with the
> > exception of getting unused FD starting from 1. The kernel already supports
> > starting from a specific FD value, as this is used by f_dupfd(). It seems
> > simpler to replicate part of anon_inode_getfd() code and use this start from
> > offset feature, instead of using f_dupfd() handling afterwards.  
> 
> Wouldn't it be better to just handle that on libbpf side? That way it
> works on all kernels and doesn't require this duplication of logic
> inside kernel?

IMHO this is needed on the kernel side, to pair it with the API change.
I don't see how doing this in libbpf can cover all cases.

First of all, some users might not be using libbpf.

Second, a userspace application could be using an older version of
libbpf on a newer kernel. (Notice this is not only due to older
distros, but also because projects using git submodule libbpf will
freeze at some point in time that worked).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer