man/man7/socket.7: Fix documentation for SO_ATTACH_REUSEPORT_EBPF

When using eBPF, there are two possible cases depending on the program
type.  Only the first case was described.  In the second case, the
program should not return an index, but a decision (SK_PASS/SK_DROP).
The socket should be selected using the sk_select_reuseport helper.

Signed-off-by: Vincent Bernat <vincent@bernat.ch>
Message-ID: <20251201195726.3669864-1-vincent@bernat.ch>
Acked-by: Craig Gallek <kraig@google.com>
Message-ID: <CAEfhGiwbaAuvZ=aVQoRw8+ZQPmMQnwp3WFO5ELCMPJOOa3FnWQ@mail.gmail.com>
[alx: reorganize text more schematically]
Signed-off-by: Alejandro Colomar <alx@kernel.org>

diff --git a/man/man7/socket.7 b/man/man7/socket.7
index 57ad0e108711e7819c5c3639b859b7e4ee64a770..4b133d85a1bcb687055ebb4ee6ef9c077a0c9c67 100644 (file)
--- a/man/man7/socket.7
+++ b/man/man7/socket.7
@@ -354,7 +354,7 @@ the sockets in the reuseport group (that is, all sockets which have
 .B SO_REUSEPORT
 set and are using the same local address to receive packets).
 .IP
-The BPF program must return an index between 0 and N\-1 representing
+The classic BPF program must return an index between 0 and N\-1 representing
 the socket which should receive the packet
 (where N is the number of sockets in the group).
 If the BPF program returns an invalid index,
@@ -368,6 +368,26 @@ Sockets are numbered in the order in which they are added to the group
 calls for UDP sockets or the order of
 .BR listen (2)
 calls for TCP sockets).
+.IP
+The extended BPF program can be of two types:
+.RS
+.TP
+.B BPF_PROG_TYPE_SOCKET_FILTER
+In this case,
+the extended BPF program must return an index between 0 and N\-1,
+like a classic BPF program.
+.TP
+.BR BPF_PROG_TYPE_SK_REUSEPORT " (since Linux 4.19)"
+In this case,
+it must return an action
+.RB ( SK_PASS
+or
+.BR SK_DROP )
+and the
+.B bpf_sk_select_reuseport
+helper can be used to select the socket which should receive the packet.
+.RE
+.IP
 New sockets added to a reuseport group will inherit the BPF program.
 When a socket is removed from a reuseport group (via
 .BR close (2)),