From: David Gibson <david@gibson.dropbear.id.au>
To: Stefano Brivio <sbrivio@redhat.com>
Cc: passt-dev@passt.top, Laurent Vivier <lvivier@redhat.com>
Subject: Re: [PATCH 3/7] treewide: Standardise variable names for various packet lengths
Date: Wed, 1 May 2024 10:05:21 +1000 [thread overview]
Message-ID: <ZjGHQTSlH7zxp46t@zatzit> (raw)
In-Reply-To: <20240430204654.41bdb93e@elisabeth>
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On Tue, Apr 30, 2024 at 08:46:54PM +0200, Stefano Brivio wrote:
> On Mon, 29 Apr 2024 17:09:29 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
>
> > At various points we need to track the lengths of a packet including or
> > excluding various different sets of headers. We don't always use the same
> > variable names for doing so. Worse in some places we use the same name
> > for different things: e.g. tcp_fill_headers[46]() use ip_len for the
> > length including the IP headers, but then tcp_send_flag() which calls it
> > uses it to mean the IP payload length only.
> >
> > To improve clarity, standardise on these names:
> > plen: L4 protocol payload length
> > l4len: plen + length of L4 protocol header
> > l3len: l4len + length of IPv4/IPv6 header
>
> If we're dealing with IPv4, I guess tot_len is still a reasonable
> synonym for this, as you left for csum_ip4_header().
I'd prefer to stick with l3len, so we're consistent across both IPv4
and IPv6. csum_ip4_header() is a special case, becase..
>
> > l2len: l3len + length of L2 (ethernet) header
> >
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> > checksum.c | 42 +++++++++++++++++-----------------
> > checksum.h | 10 ++++-----
> > icmp.c | 8 +++----
> > passt.h | 4 ++--
> > tap.c | 48 +++++++++++++++++++--------------------
> > tcp.c | 66 +++++++++++++++++++++++++++---------------------------
> > udp.c | 24 ++++++++++----------
> > 7 files changed, 101 insertions(+), 101 deletions(-)
> >
> > diff --git a/checksum.c b/checksum.c
> > index 9cbe0b29..3602215a 100644
> > --- a/checksum.c
> > +++ b/checksum.c
> > @@ -116,7 +116,7 @@ uint16_t csum_fold(uint32_t sum)
> >
> > /**
> > * csum_ip4_header() - Calculate IPv4 header checksum
> > - * @tot_len: IPv4 payload length (data + IP header, network order)
> > + * @tot_len: IPv4 packet length (data + IP header, network order)
..the value here is network order. I'm sticking with "tot_len" to
emphasise that this is exactly the same bytes as the header field,
rather than a logical integer value. I don't love that - I never like
passing integers in non-native endianness - but I prefer it to using
the standard name for a value that doesn't have standard encoding.
I'm considering a separate cleanup for the endianness here, but that's
out of scope for this patch.
> > * @protocol: Protocol number (network order)
> > * @saddr: IPv4 source address (network order)
> > * @daddr: IPv4 destination address (network order)
> > @@ -140,13 +140,13 @@ uint16_t csum_ip4_header(uint16_t tot_len, uint8_t protocol,
> > /**
> > * proto_ipv4_header_psum() - Calculates the partial checksum of an
> > * IPv4 header for UDP or TCP
> > - * @tot_len: IPv4 Payload length (host order)
> > + * @l4len: IPv4 Payload length (host order)
>
> Ouch, how did this end up being called tot_len...
Yeah :/
[snip]
> > diff --git a/passt.h b/passt.h
> > index 76026f09..d1add470 100644
> > --- a/passt.h
> > +++ b/passt.h
> > @@ -22,11 +22,11 @@ struct tap_msg {
> > /**
> > * struct tap_l4_msg - Layer-4 message descriptor for protocol handlers
> > * @pkt_buf_offset: Offset of message from @pkt_buf
> > - * @l4_len: Length of Layer-4 payload, host order
> > + * @l4len: Length of Layer-4 payload, host order
>
> This is (and was) ambiguous, but in any case, we don't use the struct
> anymore since commit bb708111833e ("treewide: Packet abstraction with
> mandatory boundary checks"), so I'm fine either way, we don't necessarily
> need to drop it here, I think.
Huh, hadn't noticed that. I'll insert a patch removing the unused
structure before this one.
[snip]
> > @@ -1658,11 +1660,11 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
> > th->syn = !!(flags & SYN);
> > th->fin = !!(flags & FIN);
> >
> > - ip_len = tcp_l2_buf_fill_headers(c, conn, iov, optlen, NULL,
> > - conn->seq_to_tap);
> > - iov[TCP_IOV_PAYLOAD].iov_len = ip_len;
> > + l4len = tcp_l2_buf_fill_headers(c, conn, iov, optlen, NULL,
> > + conn->seq_to_tap);
> > + iov[TCP_IOV_PAYLOAD].iov_len = l4len;
> >
> > - *(uint32_t *)iov[TCP_IOV_VLEN].iov_base = htonl(vnet_len + ip_len);
> > + *(uint32_t *)iov[TCP_IOV_VLEN].iov_base = htonl(vnet_len + l4len);
>
> I was completely puzzled by this until I reached 7/7. Okay, this works
> because vnet_len includes everything that's not in l4len.
Yeah, 'vnet_len' is a bad name here , but fixing that is not within
this patch's scope.
> There's one bit of confusion left after this series, though. This series
> actually highlights that. I'm not sure if it's convenient to fix that
> here as well:
>
> l4len = tcp_l2_buf_fill_headers(c, conn, iov, plen,
> check, seq);
>
> This fills in Layer-2 buffers, and returns Layer-3 payload length (i.e.
> payload plus size of Layer-4 headers). I didn't really think about it
> when the variable was ip_len, but now it's apparent.
I'm not entirely clear what fix you're envisaging, and I wonder if
it's covered in my subsequent patches.
> Further on:
>
> iov[TCP_IOV_PAYLOAD].iov_len = l4len;
>
> ...so l4len is the length of the payload plus Layer-4 header, but we
> use that to set the buffer for TCP_IOV_PAYLOAD...
Yeah.. there's some ambiguity in the meaning of "payload". In
TCP_IOV_PAYLOAD it means the IP payload, but in 'plen' it means the L4
payload. I'm open to improved names for either "plen" or
"TCP_IOV_PAYLOAD".
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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next prev parent reply other threads:[~2024-05-01 0:10 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-29 7:09 [PATCH 0/7] Small improvements to IOV handling David Gibson
2024-04-29 7:09 ` [PATCH 1/7] checksum: Use proto_ipv6_header_psum() for ICMPv6 as well David Gibson
2024-04-29 7:09 ` [PATCH 2/7] tap: Split tap specific and L2 (ethernet) headers David Gibson
2024-04-30 18:46 ` Stefano Brivio
2024-04-30 23:53 ` David Gibson
2024-04-29 7:09 ` [PATCH 3/7] treewide: Standardise variable names for various packet lengths David Gibson
2024-04-30 18:46 ` Stefano Brivio
2024-05-01 0:05 ` David Gibson [this message]
2024-04-29 7:09 ` [PATCH 4/7] tcp: Simplify packet length calculation when preparing headers David Gibson
2024-04-29 7:09 ` [PATCH 5/7] tap, tcp: (Re-)abstract TAP specific header handling David Gibson
2024-04-30 18:47 ` Stefano Brivio
2024-05-01 0:06 ` David Gibson
2024-04-29 7:09 ` [PATCH 6/7] iov: Helper macro to construct iovs covering existing variables or fields David Gibson
2024-04-30 18:47 ` Stefano Brivio
2024-05-01 0:09 ` David Gibson
2024-04-29 7:09 ` [PATCH 7/7] tcp: Update tap specific header too in tcp_fill_headers[46]() David Gibson
2024-04-30 18:48 ` Stefano Brivio
2024-05-01 0:10 ` David Gibson
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