PROBLEM: Old content of /proc/net after switching network namespace

[Mateusz StÄpieÅ]

Hello everyone,

After changing network namespace using setns, the content of /proc/net 
still represents the original namespace.
It looks like procfs dentries are not invalidated in dcache properly 
after the namespace switch.
It happens only, when you read content of /proc/net before changing 
namespace
The problem is reproducible in 4.19.13 but not in 4.14.X.
Bisecting the stable kernel tree shows that the commit
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1da4d377f943fe4194ffb9fb9c26cc58fad4dd24 
introduced the problem.
Reverting mentioned commit resolves it.

MCVE (slightly modified example from [man 2 setns]):

#define _GNU_SOURCE
#include <fcntl.h>
#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)

void print_dev()
{
    int fd2;
    fd2 = open("/proc/net/dev", O_RDONLY);
    char buf[2048] = {0};
    read(fd2, buf, 2048);
    printf("%s", buf);
    close(fd2);
}
int
main(int argc, char *argv[])
{
    int fd;

    printf("before namespace switch =========\n");
    print_dev();
    fd = open(argv[1], O_RDONLY); /* Get file descriptor for namespace */
    if (fd == -1)
        errExit("open");

    if (setns(fd, 0) == -1)       /* Join that namespace */
        errExit("setns");

    printf("after namespace switch ++++++++++\n");
    print_dev();
    return 0;
}


Steps to reproduce (assuming we have an interface named enp0s9):

ip netns add test
ip link set dev enp0s9 netns test
ip netns exec test sleep 30 &
gcc -o mcve mcve.c # mcve.c contains above C code
./mcve /proc/$(pidof sleep)/ns/net
# before namespace switch =========
# Inter-|   Receive                                                | 
Transmit
#  face |bytes    packets errs drop fifo frame compressed 
multicast|bytes    packets errs drop fifo colls carrier compressed
#     lo:       0       0    0    0    0     0          0         0 
  0       0    0    0    0     0       0          0
#    br0:       0       0    0    0    0     0          0         0 
  0       0    0    0    0     0       0          0
# enp0s3:  149625    1117    0    0    0     0          0         1 
61664     485    0    0    0     0       0          0
# docker0:       0       0    0    0    0     0          0         0 
   0       0    0    0    0     0       0          0
# enp0s8:   17086      60    0    0    0     0          0        29 
1006      13    0    0    0     0       0          0
# after namespace switch ++++++++++
# Inter-|   Receive                                                | 
Transmit
#  face |bytes    packets errs drop fifo frame compressed 
multicast|bytes    packets errs drop fifo colls carrier compressed
#     lo:       0       0    0    0    0     0          0         0 
  0       0    0    0    0     0       0          0
#    br0:       0       0    0    0    0     0          0         0 
  0       0    0    0    0     0       0          0
# enp0s3:  150813    1135    0    0    0     0          0         1 
64348     503    0    0    0     0       0          0
# docker0:       0       0    0    0    0     0          0         0 
   0       0    0    0    0     0       0          0
# enp0s8:   17086      60    0    0    0     0          0        29 
1006      13    0    0    0     0       0          0
ip netns exec test cat /proc/net/dev
## Should display
# Inter-|   Receive                                                | 
Transmit
# face |bytes    packets errs drop fifo frame compressed multicast|bytes 
   packets errs drop fifo colls carrier compressed
#    lo:       0       0    0    0    0     0          0         0 
 0       0    0    0    0     0       0          0
# enp0s9:   10438      33    0    0    0     0          0        17 
936      12    0    0    0     0       0          0

output from awk -f scripts/ver_linux

Linux test-agent 4.19.13 #1 SMP PREEMPT Thu Jan 3 12:03:20 UTC 2019 
x86_64 GNU/Linux

Util-linux          	2.29.2
Mount               	2.29.2
Module-init-tools   	23
E2fsprogs           	1.43.4
Linux C Library     	2.24
Dynamic linker (ldd)	2.24
Linux C++ Library   	6.0.22
Procps              	3.3.12
Net-tools           	2.10
Sh-utils            	8.26
Udev                	232
Modules Loaded      	ahci ata_generic ata_piix crc32c_intel e1000 
ehci_hcd ehci_pci i2c_core i2c_piix4 libahci serio_raw usb_common usbcore