...
read(3</proc/1709/mountinfo>, "", 1024) = 0
close(3</proc/1709/mountinfo>)          = 0
flock(4</run/snapd/lock/.lock>, LOCK_UN) = 0
close(4</run/snapd/lock/.lock>)         = 0
open("/proc/1/ns/mnt", O_RDONLY|O_LARGEFILE|O_NOFOLLOW|O_CLOEXEC|O_PATH) = 3</proc/1/ns/mnt>
open("/proc/self/ns/mnt", O_RDONLY|O_LARGEFILE|O_NOFOLLOW|O_CLOEXEC|O_PATH) = 4</proc/1709/ns/mnt>
readlinkat(3</proc/1/ns/mnt>, "", 0xbeaf4e4c, 128) = -1 ENOENT (No such file or directory)
write(2<pipe:[16042]>, "cannot perform readlinkat() on the mount namespace file descriptor of the init process", 86cannot perform readlinkat() on the mount namespace file descriptor of the init process) = 86
write(2<pipe:[16042]>, ": No such file or directory\n", 28: No such file or directory
) = 28
exit_group(1)                           = ?
+++ exited with 1 +++

$ ls -l /proc/1/ns/mnt
-r-------- 1 root root 0 May  1 00:04 /proc/1/ns/mnt