Hello,
I'm confused about the port_offset parameter to __inet_hash_connect.
When allocating the local port for an outgoing TCP connection the port search
looks something like this:
static u32 hint;
u32 offset = hint + port_offset;
inet_get_local_port_range(net, &low, &high);
remaining = (high - low) + 1;
for (i = 1; i <= remaining; i++) {
port = low + (i + offset) % remaining;
/* check port is free */
The port_offset is calculated for v4 and v6 based on a hash of src/dst
addresses, presumably in order to improve security.
I see a few issues with this:
- The port_offset is calculated even if the local port was already assigned
via bind. This wastes a few cycles.
- Keeping the last searched port as a static variable is a bad idea on
multicore cpus. Starting a lot of connections to the same target will result in
lock contention in the bind hash. This is probably only visible in highly
synthetic tests.
- When doing a port search at bind() time the search starts from
"prandom_32()". Is this "less secure" for port allocation? I bet most
applications are not aware of this difference.
Wouldn't it be better to use the same local port search mechanism at both bind
(inet_csk_get_port) and connect (__inet_hash_connect) time, based on starting
from a random point? It would also make connecting slightly faster.
Regards,
Leonard
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