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

[Andrew Lunn]

On Wed, Sep 23, 2026 at 04:33:54PM -0300, Aldo Ariel Panzardo wrote:
> 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.

Will, a frame which is smaller than the FCS cannot pass the FCS
check. So you don't need to worry about undersized good packets
hitting this condition.

Given that an ethernet header is 14 octets the FCS is 4 octets, any
frame smaller than 18 octets should be dropped by the network
stack. So why not do one test at the beginning for ETH_HLEN +
ETH_FCS_LEN?

	Andrew