Hello,

Bisected problems with setns() and /proc/net/sctp/snmp to this:

commit 1da4d377f943fe4194ffb9fb9c26cc58fad4dd24
Author: Alexey Dobriyan <adobriyan@gmail.com>
Date:   Fri Apr 13 15:35:42 2018 -0700

    proc: revalidate misc dentries

Reproduces for example with Fedora 5.9.10-100.fc32.x86_64, so 1fde6f21d90f
("proc: fix /proc/net/* after setns(2)") does not seem to cover
/proc/net/sctp/snmp


Reproducer attached, that does open+read+close of /proc/net/sctp/snmp before
and after setns() syscall. The second open+read+close of /proc/net/sctp/snmp
incorrectly produces results for the default namespace, not the target
namespace.


Example, create netns and do some sctp:

# ./iperf-netns
+ modprobe sctp
+ ip netns add test
+ ip netns exec test ip link set lo up
+ ip netns exec test iperf3 -s -1
-----------------------------------------------------------
Server listening on 5201
-----------------------------------------------------------
+ ip netns exec test iperf3 -c 127.0.0.1 --sctp --bitrate 50M --time 4
Connecting to host 127.0.0.1, port 5201
Accepted connection from 127.0.0.1, port 50696
[  5] local 127.0.0.1 port 54735 connected to 127.0.0.1 port 5201
[  5] local 127.0.0.1 port 5201 connected to 127.0.0.1 port 54735
[ ID] Interval           Transfer     Bitrate
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec  6.00 MBytes  50.3 Mbits/sec                  
[  5]   0.00-1.00   sec  6.00 MBytes  50.3 Mbits/sec                  
[  5]   1.00-2.00   sec  5.94 MBytes  49.8 Mbits/sec                  
[  5]   1.00-2.00   sec  5.94 MBytes  49.8 Mbits/sec                  
[  5]   2.00-3.00   sec  6.00 MBytes  50.3 Mbits/sec                  
[  5]   2.00-3.00   sec  6.00 MBytes  50.3 Mbits/sec                  
[  5]   3.00-4.00   sec  5.94 MBytes  49.8 Mbits/sec                  
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-4.00   sec  23.9 MBytes  50.1
Mbits/sec                  receiver
[  5]   3.00-4.00   sec  5.94 MBytes  49.8 Mbits/sec                  
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-4.00   sec  23.9 MBytes  50.1 Mbits/sec
[  5]   0.00-4.00   sec  23.9 MBytes  50.1

iperf Done.
+ cat /proc/net/sctp/snmp
SctpCurrEstab                           0
SctpActiveEstabs                        0
SctpPassiveEstabs                       0
SctpAborteds                            0
SctpShutdowns                           0
SctpOutOfBlues                          0
SctpChecksumErrors                      0
[...]
+ ip netns exec test cat /proc/net/sctp/snmp
SctpCurrEstab                           0
SctpActiveEstabs                        2
SctpPassiveEstabs                       2
SctpAborteds                            0
SctpShutdowns                           4
SctpOutOfBlues                          0
SctpChecksumErrors                      0
SctpOutCtrlChunks                       1544
SctpOutOrderChunks                      1530
[...]
+ wait


But now we see all zeroes in /proc/net/sctp/snmp with the reproducer:

$ gcc repro.c -o repro      
             
# ./repro
/proc/net/sctp/snmp [pid: 175998]
SctpCurrEstab                           0
SctpActiveEstabs                        0
SctpPassiveEstabs                       0
SctpAborteds                            0
SctpShutdowns                           0
[...]

setns(/run/netns/test) ...
/proc/net/sctp/snmp [pid: 175998]
SctpCurrEstab                           0
SctpActiveEstabs                        0
SctpPassiveEstabs                       0
SctpAborteds                            0
SctpShutdowns                           0
SctpOutOfBlues                          0
[...]


-Tommi