Hi Grzegorz, On 2/12/22 20:44, Grzegorz Szpetkowski wrote: > Yes, this sounds right to me as well. > Attaching in git diff -u form: I applied the following patch <http://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/commit/?id=015e7241b392713f145f94cdfa9c2a68fbcb7f1d>. Cheers, Alex commit 015e7241b392713f145f94cdfa9c2a68fbcb7f1d (HEAD -> main) Author: Grzegorz Szpetkowski <gszpetkowski@xxxxxxxxx> Date: Tue Jan 25 16:22:17 2022 +0100 packet.7: add description of zero protocol for socket According to packet(7), whenever raw packet is created by socket(), it's immediately running, meaning that internal packet_rcv() handler will be triggered and socket buffer will begin allocation of sk_buff until sk_rcvbuf limit is reached. However, by examination of kernel's internal handler packet_create() it looks that kernel handles case of zero protocol in a special manner. When packet_create() is called with arg protocol = 0, __register_prot_hook is not executed, meaning running state is still 0 and most notably, packet handler is not added to kernel list (vide dev_add_pack). I found this behavior invaluable for solving a subtle issue. When process creates raw packet socket to listen for (let's say) all protocols, but limited to a single network interface, then while this interface can set by bind, it may to be too late due to preemption (e.g. if real-time scheduling is used) and/or high-rate of packets on other interfaces, meaning that undesired packets (any count) may be pulled into the socket buffer. The idea is that protocol zero means no packets on receive and an optional call to bind with nonzero sll_procol will act "as if" the originating socket API was called with this (nonzero) protocol. The call to bind(2) is optional. As an example, if user intends to treat the socket as Tx-only, then bind(2) may be skipped. Reported-by: Grzegorz Szpetkowski <gszpetkowski@xxxxxxxxx> Link: linux-man@ <https://lore.kernel.org/linux-man/CAMW=dumhWDu6LdhaQCJMskA4yNRBtOHs4iyrG6TP7xRv28AVWA@xxxxxxxxxxxxxx/> Cowritten-by: Alejandro Colomar <alx.manpages@xxxxxxxxx> Signed-off-by: Alejandro Colomar <alx.manpages@xxxxxxxxx> Signed-off-by Grzegorz Szpetkowski <gszpetkowski@xxxxxxxxx>: diff --git a/man7/packet.7 b/man7/packet.7 index effed18c7..182f628a4 100644 --- a/man7/packet.7 +++ b/man7/packet.7 @@ -47,6 +47,14 @@ is set to then all protocols are received. All incoming packets of that protocol type will be passed to the packet socket before they are passed to the protocols implemented in the kernel. +If +.I protocol +is set to zero, +no packets are received. +.BR bind (2) +can optionally be called with a nonzero +.IR sll_protocol +to start receiving packets for the protocols specified. .PP In order to create a packet socket, a process must have the .B CAP_NET_RAW -- Alejandro Colomar Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/ http://www.alejandro-colomar.es/
