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

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Date:   Mon, 8 Jun 2020 11:36:33 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.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>
Subject: Re: [PATCH bpf 1/3] bpf: syscall to start at file-descriptor 1

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?

>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---
>  fs/file.c            |    2 +-
>  include/linux/file.h |    1 +
>  kernel/bpf/syscall.c |   38 ++++++++++++++++++++++++++++++++------
>  3 files changed, 34 insertions(+), 7 deletions(-)
>

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