linux-kernel - Re: [PATCH v11 08/14] net, arm64: untag user pointers in tcp_zerocopy

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Message-ID: <CAAeHK+w+koPnKP2=3yQsj5wCVY-c6+NLBnkKWQP8Q6M6pzO4dA@mail.gmail.com>
Date:   Mon, 18 Mar 2019 17:08:48 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
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        Kate Stewart <kstewart@...uxfoundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Szabolcs Nagy <Szabolcs.Nagy@....com>
Subject: Re: [PATCH v11 08/14] net, arm64: untag user pointers in tcp_zerocopy_receive

On Mon, Mar 18, 2019 at 3:45 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Mon, Mar 18, 2019 at 6:17 AM Andrey Konovalov <andreyknvl@...gle.com> wrote:
> >
>
> > Looking at the code, what's the point of this address != zc->address
> > check? Should I just remove it?
>
> No you must not remove it.
>
> The test detects if a u64 ->unsigned long  conversion might have truncated bits.
>
> Quite surprisingly some people still use 32bit kernels.
>
> The ABI is 64bit only, because we did not want to have yet another compat layer.
>
> struct tcp_zerocopy_receive {
>     __u64 address; /* in: address of mapping */
>     __u32 length; /* in/out: number of bytes to map/mapped */
>     __u32 recv_skip_hint; /* out: amount of bytes to skip */
> };

Ah, got it, thanks! I'll add a comment here then, otherwise this looks
confusing.