Re: [PATCH net-next v4 0/4] net: selftest: improve test string formatting and checksum handling

[Jakub Kicinski <kuba@kernel.org>]

On Mon, 23 Jun 2025 13:45:41 +0200 Oleksij Rempel wrote:
> On Sat, Jun 21, 2025 at 06:46:00AM -0700, Jakub Kicinski wrote:
> > On Fri, 20 Jun 2025 12:53:23 +0200 Oleksij Rempel wrote:
> > > Let me first describe the setup where this issue was observed and my findings.
> > > The problem occurs on a system utilizing a Microchip DSA driver with an STMMAC
> > > Ethernet controller attached to the CPU port.
> > > 
> > > In the current selftest implementation, the TCP checksum validation fails,
> > > while the UDP test passes. The existing code prepares the skb for hardware
> > > checksum offload by setting skb->ip_summed = CHECKSUM_PARTIAL. For TCP, it sets
> > > the thdr->check field to the complement of the pseudo-header checksum, and for
> > > UDP, it uses udp4_hwcsum. If I understand it correct, this configuration tells
> > > the kernel that the hardware should perform the checksum calculation.
> > > 
> > > However, during testing, I noticed that "rx-checksumming" is enabled by default
> > > on the CPU port, and this leads to the TCP test failure.  Only after disabling
> > > "rx-checksumming" on the CPU port did the selftest pass. This suggests that the
> > > issue is specifically related to the hardware checksum offload mechanism in
> > > this particular setup. The behavior indicates that something on the path
> > > recalculated the checksum incorrectly.  
> > 
> > Interesting, that sounds like the smoking gun. When rx-checksumming 
> > is enabled the packet still reaches the stack right?  
> 
> No. It looks like this packets are just silently dropped, before they was
> seen by the stack. The only counter which confirms presence of this
> frames is HW specific mmc_rx_tcp_err. But it will be increasing even if
> rx-checksumming is disabled and packets are forwarded to the stack.

If you happen to have the docs for the STMMAC instantiation in the SoC
it'd be good to check if discarding frames with bad csum can be
disabled. Various monitoring systems will expect the L4 checksum errors
to appear in nstat, not some obscure ethtool -S counter.

> > If so does the frame enter the stack with CHECKSUM_COMPLETE or
> > UNNECESSARY?  
> 
> If rx-checksumming is enabled and packet has supported ethertype,
> then CHECKSUM_UNNECESSARY will be set. Otherwise CHECKSUM_NONE.
> 
> > > When examining the loopbacked frames, I observed that the TCP checksum was
> > > incorrect. Upon further investigation, the xmit helper in net/dsa/tag_ksz.c
> > > includes the following:
> > > 
> > > if (skb->ip_summed == CHECKSUM_PARTIAL && skb_checksum_help(skb))
> > >     return NULL;
> > > 
> > > I assume skb_checksum_help() is intended to calculate the proper checksum when
> > > CHECKSUM_PARTIAL is set, indicating that the software should complete the
> > > checksum before handing it to the hardware. My understanding is that the STMMAC
> > > hardware then calculates the checksum for egress frames if CHECKSUM_PARTIAL is
> > > used.  
> > 
> > stmmac shouldn't touch the frame, note that skb_checksum_help() sets
> > skb->ip_summed = CHECKSUM_NONE; so the skb should no longer be considered
> > for csum offload.  
> 
> It looks like skb_checksum_help(), which is used in tag_ksz.c, generates
> a TCP checksum without accounting for the IP pseudo-header. The
> resulting checksum is then incorrect and is filtered out by the STMMAC
> HW on ingress

The pseudo-header csum is filled in net_test_get_skb(), where it calls
tcp_v4_check(). But I think you're right, it's incorrect. Could you try:

diff --git a/net/core/selftests.c b/net/core/selftests.c
index 35f807ea9952..1166dd1ddb07 100644
--- a/net/core/selftests.c
+++ b/net/core/selftests.c
@@ -160,8 +160,10 @@ static struct sk_buff *net_test_get_skb(struct net_device *ndev,
        skb->csum = 0;
        skb->ip_summed = CHECKSUM_PARTIAL;
        if (attr->tcp) {
-               thdr->check = ~tcp_v4_check(skb->len, ihdr->saddr,
-                                           ihdr->daddr, 0);
+               int l4len;
+
+               l4len = skb->tail - skb_transport_header(skb);
+               thdr->check = ~tcp_v4_check(l4len, ihdr->saddr, ihdr->daddr, 0);
                skb->csum_start = skb_transport_header(skb) - skb->head;
                skb->csum_offset = offsetof(struct tcphdr, check);
        } else {

Or some such?

> If I generate the checksum manually by combining the result of
> skb_checksum() with the csum_tcpudp_magic() function - I get a different
> checksum from the skb_checksum_help() result, which is then not dropped
> by STMMAC on ingress.
> 
> Should tag_ksz.c use a different helper function instead of
> skb_checksum_help()?