/proc/net/sctp/snmp, setns, proc: revalidate misc dentries

["Rantala, Tommi T. (Nokia - FI/Espoo)" <tommi.t.rantala@nokia.com>]

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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

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[-- Attachment #2: repro.c --]
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#define _GNU_SOURCE
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sched.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

void slurp(const char *fn)
{
	char buf[8192];
	ssize_t r;
	int fd;

	printf("%s [pid: %d]\n", fn, getpid()); fflush(stdout);

	fd = open(fn, O_RDONLY);
	if (fd < 0) { perror("open"); exit(1); }

	r = read(fd, buf, sizeof(buf)-1);
	if (r < 0) { perror("read"); exit(1); }
	buf[r] = 0;
	puts(buf); fflush(stdout);

	if (close(fd) < 0) { perror("close"); exit(1); }
}

void newnet(const char *ns)
{
	int fd;
	fd = open(ns, O_RDONLY);
	if (fd < 0) { perror("open"); exit(1); }
	if (setns(fd, CLONE_NEWNET) < 0) { perror("setns"); exit(1); }
	if (close(fd) < 0) { perror("close"); exit(1); }
}

int main(int argc, char **argv)
{
	const char *ns = "/run/netns/test";
	const char *fn = "/proc/net/sctp/snmp";
	int d = 1;

	// Optional args: /run/netns/... /proc/net/... n
	if (argc >= 2) ns = argv[1];
	if (argc >= 3) fn = argv[2];
	if (argc >= 4 && argv[3][0] == 'n') d = 0;

	if (d) slurp(fn);
	printf("setns(%s) ...\n", ns); fflush(stdout);
	newnet(ns);
	slurp(fn);
}

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