[freebsd-current]Who should reset M_PKTHDR flag in m_buf when IP packets are fragmented. m_unshare panic throw when IPSec is enabled

[Andrey V. Elsukov]

On 26.12.2017 13:22, Harsh Jain wrote:
>>> panic: m_unshare: m0 0xfffff80020f82600, m 0xfffff8005d054100 has M_PKTHDR
>>> cpuid = 15
>>> time = 1495578455
>>> KDB: stack backtrace:
>>> db_trace_self_wrapper() at db_trace_self_wrapper+0x2c/frame 0xfffffe044e9bb890
>>> kdb_backtrace() at kdb_backtrace+0x53/frame 0xfffffe044e9bb960
>>> vpanic() at vpanic+0x269/frame 0xfffffe044e9bba30
>>> kassert_panic() at kassert_panic+0xc7/frame 0xfffffe044e9bbac0
>>> m_unshare() at m_unshare+0x578/frame 0xfffffe044e9bbbc0
>>> esp_output() at esp_output+0x44c/frame 0xfffffe044e9bbe40
>>> ipsec4_perform_request() at ipsec4_perform_request+0x5df/frame 0xfffffe044e9bbff0
>> Hi,
>>
>> it seems unusual that IP reassembly happens on outbound path.
> It can be re-produced with single Ping packet on chelsio(cxgbe) NIC. I tried with Intel NIC. It seems they re-produce M_WRITEABLE() buffer(follows different path in m_unshare) which is not true for cxgbe.

In my view, IP fragmentation should occur in ip_output after IPsec
encryption. Something like:

1. rip_output() has mbuf chain where only first mbuf has M_PKTHDR flag
2. ip_output() -> IPSEC_OUTPUT() -> esp_output() -> m_unshare(). We
should still have only one mbuf with M_PKTHDR flag here.
3. esp_output_cb() -> ipsec_process_done() -> ip_output()
4. Now IP fragmentation should occur: ip_fragment() creates chain of
mbufs to send, where M_PKTHDR flag will be set for each fragment.

>> Do you have some packet normalization using firewall?
> Default FREEBSD current installation. No explicit firewall.
> What you think above patch makes sense.

It is not clear to me why it helps. The panic happens on outbound path,
where mbuf should be allocated by network stack and should be writeable.
ip_reass() usually used on inbound path. I think the patch just hides
the problem in another place.
Do you mean that cxgbe can produce !WRITEABLE mbuf for received packet
and then pass it to the network stack?

-- 
WBR, Andrey V. Elsukov

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