netdev - getsockopt(XDP_MMAP_OFFSETS) syscall ABI breakage?

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Message-ID: <CAKxSbF2XaqwLAby0BBbhT_8vBviMvkA_7fiK-ivAs2DHWqARxw@mail.gmail.com>
Date:   Thu, 19 Dec 2019 15:46:16 -0600
From:   Alex Forster <aforster@...udflare.com>
To:     bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org
Subject: getsockopt(XDP_MMAP_OFFSETS) syscall ABI breakage?

The getsockopt(XDP_MMAP_OFFSETS) socket option returns a struct
xdp_mmap_offsets (from uapi/linux/if_xdp.h) which is defined as:

    struct xdp_mmap_offsets {
        struct xdp_ring_offset rx;
        struct xdp_ring_offset tx;
        struct xdp_ring_offset fr; /* Fill */
        struct xdp_ring_offset cr; /* Completion */
    };

Prior to kernel 5.4, struct xdp_ring_offset (from the same header) was
defined as:

    struct xdp_ring_offset {
        __u64 producer;
        __u64 consumer;
        __u64 desc;
    };

A few months ago, in 77cd0d7, it was changed to the following:

    struct xdp_ring_offset {
        __u64 producer;
        __u64 consumer;
        __u64 desc;
        __u64 flags;
    };

I believe this constitutes a syscall ABI breakage, which I did not
think was allowed. Have I misunderstood the current stability
guarantees for AF_XDP?

Alex Forster