Re: [PATCH 2/2] af_packet: pass checksum validation status to the user

On 19.03.2015 21:50:03 +0300 Willem de Bruijn wrote:

On Thu, Mar 19, 2015 at 2:08 PM, Alexander Drozdov [off-list ref] wrote:

quoted

On Thu, 19 Mar 2015 11:29:32 -0400, Willem de Bruijn [off-list ref]
wrote:

quoted

On Thu, Mar 19, 2015 at 4:01 AM, Alexander Drozdov [off-list ref]
wrote:

quoted

Introduce TP_STATUS_CSUM_VALID tp_status flag to tell the
af_packet user that at least the transport header checksum
has been already validated.

This changes the interface slightly. Processes should be treating
this previously unused bit as reserved and other flags have
been added to the bitmap in this manner as well, so this should
then be safe here, too.

quoted

For now, the flag may be set for incoming packets only.

Why?

I can't figure out how af_packet could get that the outgoing
packet's checksum has been validated. "Checksumming on
output" from skbuff.h tells that skb->ip_summed should
equal to CHECKSUM_NONE in that case,

CHECKSUM_UNNECESSARY is a valid flag on the outgoing
path according to that documentation. And, indeed, I also see
no checksum scrubbing on forwarding paths in practice (but I
may be wrong there, only took a quick glance).

quoted

but that is not true for me (in my tests, skb->ip_summed ==
CHECKSUM_UNNECESSARY for forwarded packets in some
cases).

When you disable hardware checksum offload and generate
packets locally, you do see the expected CHECKSUM_NONE
value?

Yes, I do, but see below.

quoted

quoted

You cannot change the semantics of the flag afterwards.

I think the semantics of the flag won't be changed if one set the flag
for outgoing packets. If the flag is not set (for any directions)
then that is not mean that the packet checksum is invalid. The user just can
then
checksum the packet by itself. So, the user may check the flag for any
packet
right now.

I see. So it is a hint. Okay. It would be nice if it behaves as
expected in as many cases as possible from the outset. This
would include PACKET_OUTGOING and CHECKSUM_NONE.

I've just done some testing, and I've found that packets generated by
'nping --badsum' (socket(PF_INET, SOCK_RAW, IPPROTO_RAW)) have CHECKSUM_NONE
when they are viewed by af_packet. I've used rather old Linux kernel
for the tests, but isn't that a reason to not set TP_STATUS_CSUM_VALID for
outgoing packets right now?

Please also note the flag and semantics in
Documentation/networking/packet_mmap.txt

I'll do it and I'll resend the patches with the note.

quoted

quoted

Better to support both directions from the start.

quoted

Signed-off-by: Alexander Drozdov <redacted>
---
   include/uapi/linux/if_packet.h | 1 +
   net/packet/af_packet.c         | 9 +++++++++
   2 files changed, 10 insertions(+)

diff --git a/include/uapi/linux/if_packet.h

b/include/uapi/linux/if_packet.h
index da2d668..053bd10 100644

--- a/include/uapi/linux/if_packet.h
+++ b/include/uapi/linux/if_packet.h

@@ -99,6 +99,7 @@ struct tpacket_auxdata {
   #define TP_STATUS_VLAN_VALID           (1 << 4) /* auxdata has valid

tp_vlan_tci */
   #define TP_STATUS_BLK_TMO              (1 << 5)
   #define TP_STATUS_VLAN_TPID_VALID      (1 << 6) /* auxdata has valid
tp_vlan_tpid */
+#define TP_STATUS_CSUM_VALID           (1 << 7)

   /* Tx ring - header status */
   #define TP_STATUS_AVAILABLE          0

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 6ecf8dd..3f09dda 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c

@@ -1918,6 +1918,10 @@ static int tpacket_rcv(struct sk_buff *skb, struct

net_device *dev,

          if (skb->ip_summed == CHECKSUM_PARTIAL)
                  status |= TP_STATUS_CSUMNOTREADY;
+       else if (skb->pkt_type != PACKET_OUTGOING &&
+                (skb->ip_summed == CHECKSUM_COMPLETE ||
+                 skb_csum_unnecessary(skb)))
+               status |= TP_STATUS_CSUM_VALID;

          if (snaplen > res)
                  snaplen = res;

@@ -3015,6 +3019,11 @@ static int packet_recvmsg(struct kiocb *iocb,

struct socket *sock,
                  aux.tp_status = TP_STATUS_USER;
                  if (skb->ip_summed == CHECKSUM_PARTIAL)
                          aux.tp_status |= TP_STATUS_CSUMNOTREADY;
+               else if (skb->pkt_type != PACKET_OUTGOING &&
+                        (skb->ip_summed == CHECKSUM_COMPLETE ||
+                         skb_csum_unnecessary(skb)))
+                       aux.tp_status |= TP_STATUS_CSUM_VALID;
+

These two sections are near duplicates. I'd move the entire status
initialization, including existing TP_STATUS_USER and
TP_STATUS_CSUMNOTREADY fields to a helper function.

It's a bit unfortunately that we have to use an extra bit to add a signal
that is a near inverse of the existing TP_STATUS_CSUMNOTREADY
(bar CHECKSUM_NONE). I do not immediately see a better way,
either, though. And tp_status has plenty room at 32 bits.

quoted

                  aux.tp_len = PACKET_SKB_CB(skb)->origlen;
                  aux.tp_snaplen = skb->len;
                  aux.tp_mac = 0;
--
1.9.1