netdev - Misalignment, MIPS, and ip

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Message-ID: <CAHmME9o_eCNXpVztOZKW55kpRtE+1KSEQTQOjUBVn68Y2+or2g@mail.gmail.com>
Date:   Wed, 7 Dec 2016 19:35:51 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Netdev <netdev@...r.kernel.org>, linux-mips@...ux-mips.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        WireGuard mailing list <wireguard@...ts.zx2c4.com>
Subject: Misalignment, MIPS, and ip_hdr(skb)->version

Hey MIPS Networking People,

I receive encrypted packets with a 13 byte header. I decrypt the
ciphertext in place, and then discard the header. I then pass the
plaintext to the rest of the networking stack. The plaintext is an IP
packet. Due to the 13 byte header that was discarded, the plaintext
possibly begins at an unaligned location (depending on whether
dev->needed_headroom was respected).

Does this matter? Is this bad? Will there be a necessary performance hit?

In order to find out, I instrumented the MIPS unaligned access
exception handler to see where I was actually in trouble.
Surprisingly, the only part of the stack that seemed to be upset was
on calls to ip_hdr(skb)->version.

Two things disturb me about this. First, this seems too good to be
true. Does it seem reasonable to you that this is actually the only
place that would be problematic? Or was my testing methodology wrong
to arrive at such an optimistic conclusion?

Secondly, why should a call to ip_hdr(skb)->version cause an unaligned
access anyway? This struct member is simply the second half of a
single byte in a bit field. I'd expect for the compiler to generate a
single byte load, followed by a bitshift or a mask. Instead, the
compiler appears to generate a double byte load, hence the exception.
What's up with this? Stupid compiler that should be fixed? Some odd
optimization? What to do?

I'm considering just adding an extra byte of padding (see discussion
in [1]), but before I make any decision like that (and hopefully it
won't be necessary), I'd like to completely and entirely understand
the full effects and consequences of calling netif_rx(skb) when
skb->data is unaligned. Any insight you have to offer would be most
welcome.

Thanks,
Jason

[1] https://lists.zx2c4.com/pipermail/wireguard/2016-December/000709.html