On Mon, Aug 10, 2020 at 12:32 AM Willem de Bruijn <willemdebruijn.ker...@gmail.com> wrote: > > What happens when a tunnel device passes a packet to these devices? > That will also not have allocated the extra tailroom. Does that cause > a bug?
I looked at the code in net/ipv4/ip_tunnel.c. It indeed appeared to me that it didn't take needed_tailroom into consideration. However it does take needed_headroom into consideration through the macro LL_RESERVED_SPACE. I think it would be better for it to take needed_tailroom into consideration, too. However, looking at the comment of needed_tailroom in include/linux/netdevice.h, it says "Extra tailroom the hardware may need, but not in all cases can this be guaranteed". So if we take this comment as the spec, we can consider this to be not a bug. The reason the author of this comment said so, might be that he wanted to add needed_tailroom to solve some problems, but he was not able to change all code to take needed_tailroom into consideration, so he wrote in the comment saying that it was not necessary to always guarantee needed_tailroom. If we take this comment as the spec, to prevent bugs, any driver that sets needed_tailroom must always check (and re-allocate if necessary) before using the tailroom. However, I still think it would be better to always take into consideration needed_tailroom (and needed_headroom, too), so that eventually we can remove the words of "but not in all cases can this be guaranteed" from the comment. That would make the code more logical and consistent. Thank you for raising this important question about needed_tailroom!
