On Fri, Oct 16, 2020 at 11:00:20AM +0200, Christian Eggers wrote: > On Friday, 16 October 2020, 09:45:42 CEST, Kurt Kanzenbach wrote: > > On Thu Oct 15 2020, Christian Eggers wrote: > > > On Wednesday, 14 October 2020, 19:31:03 CEST, Vladimir Oltean wrote: > > >> What problem are you actually trying to solve? > > > > > > After (hopefully) understanding the important bits, I would like to solve > > > the problem that after calling __skb_put_padto() there may be no tailroom > > > for the tail tag. > > > > > > The conditions where this can happen are quite special. You need a > > > skb->len < ETH_ZLEN and the skb must be marked as cloned. One condition > > > where this happens in practice is when the skb has been selected for TX > > > time stamping in dsa_skb_tx_timestamp() [cloned] and L2 is used as > > > transport for PTP [size < ETH_ZLEN]. But maybe cloned sk_buffs can also > > > happen for other reasons. > > Hmm. I've never observed any problems using DSA with L2 PTP time > > stamping with this tail tag code. What's the impact exactly? Memory > > corruption? > It looks like skb_put_padto() is only used by the tag_ksz driver. So it's > unlikely that other drivers are affected by the same problem. > > If I remember correctly, I got a skb_panic in skb_put() when adding the tail > tag. But with the current kernel I didn't manage to create packets where the > skb allocated by __skb_put_padto has not enough spare room for the tag tag. > Either I am trying with wrong packets, or something else has been changed in > between. > > I just sent a new patch which should solve the problem correctly here: > https://patchwork.ozlabs.org/project/netdev/list/?series=208269
Kurt is asking, and rightfully so, because his tag_hellcreek.c driver (for a 1588 switch with tail tags) is copied from tag_ksz.c. I have also attempted to replicate your issue at my end and failed to do so. In principle, it is indeed true that a cloned skb should not be modified without calling skb_unshare() first. The DSA core (dsa_slave_xmit) should do that. But that doesn't explain the symptoms you're seeing, which is why I asked for skb_dump.