Re: [PATCH] net: stmmac: guard FCS stripping against runt frames

[Aldo Ariel Panzardo]

Hi Andrew,

You're right that the relevant question is not merely whether the MAC can
forward frames shorter than 64 bytes, but whether a descriptor with a
reported length smaller than ETH_FCS_LEN can reach this path.

I found a closer NXP/Freescale reference: the i.MX RT1170 ENET_QOS
documentation (IMXRT1170RM, Chapter 61), which describes the Synopsys
DWC EQoS receive descriptors used by this driver:

  https://www.nxp.com/webapp/Download?colCode=IMXRT1170RM

The MTL receive configuration explicitly supports forwarding error packets
and undersized good packets (FEP/FUP), and RDES3.PL is documented as
the number of bytes transferred to system memory, including CRC. I don't
see a documented lower bound on PL.

However, I also noticed that dwmac4_wrback_get_rx_status() returns
discard_frame when RDES3_ERROR_SUMMARY is set, including CRC and
receive errors. So an ordinary CRC-error runt would be discarded before
reaching the FCS subtraction.

I therefore don't claim the documentation alone proves that a good-status
descriptor with PL < 4 can occur. I'll verify whether such a descriptor
can actually be produced by the hardware before claiming that as the
trigger.

The length check may still be useful as defensive validation of the
hardware-supplied descriptor length, but that is a different justification
from the runt-frame scenario in the current changelog. Happy to respin
with that framing if you prefer.

Thanks,
Aldo