RE: setns() affecting other threads in 5.10.132 and 6.0

7055197705709c59b8ab77e6a5c7d46d61edd96e Author: Alexey Dobriyan adobriyan@gmail.com Cc: Al Viro viro@zeniv.linux.org.uk Signed-off-by: Andrew Morton akpm@linux-foundation.org c6c75deda813 1fde6f21d90f

-----Original Message----- From: David Laight David.Laight@ACULAB.COM Sent: 04 September 2022 15:05

Sometime after 5.10.105 (5.10.132 and 6.0) there is a change that makes setns(open("/proc/1/ns/net")) in the main process change the behaviour of other process threads.

I don't know how much is broken, but the following fails.

Create a network namespace (eg "test"). Create a 'bond' interface (eg "test0") in the namespace.

Then /proc/net/bonding/test0 only exists inside the namespace.

However if you run a program in the "test" namespace that does:

• create a thread.
• change the main thread to in "init" namespace.
• try to open /proc/net/bonding/test0 in the thread.

then the open fails.

I don't know how much else is affected and haven't tried to bisect (I can't create bonds on my normal test kernel).

I've now bisected it. Prior to change 7055197705709c59b8ab77e6a5c7d46d61edd96e proc: fix dentry/inode overinstantiating under /proc/${pid}/net the setns() had no effect of either thread. Afterwards both threads see the entries in the init namespace.

However I think that in 5.10.105 the setns() did affect the thread it was run in. That might be the behaviour before c6c75deda813. proc: fix lookup in /proc/net subdirectories after setns(2)

There is also the earlier 1fde6f21d90f proc: fix /proc/net/* after setns(2)

From the commit messages it does look as though setns() should change what is seen, but just for the current thread. So it is currently broken - and has been since 5.18.0-rc4 and whichever stable branches the change was backported to.

David

The test program below shows the problem. Compile and run as: # ip netns exec test strace -f test_prog /proc/net/bonding/test0

The second open by the child should succeed, but fails.

I can't see any changes to the bonding code, so I suspect it is something much more fundamental. It might only affect /proc/net, but it might also affect which namespace sockets get created in. IIRC ls -l /proc/n/task/*/ns gives the correct namespaces.

David

#define _GNU_SOURCE

#include <fcntl.h> #include <unistd.h> #include <poll.h> #include <pthread.h> #include <sched.h>

#define delay(secs) poll(0,0, (secs) * 1000)

static void *thread_fn(void *file) { delay(2); open(file, O_RDONLY);

    delay(5);
    open(file, O_RDONLY);

    return NULL;

}

int main(int argc, char **argv) { pthread_t id;

    pthread_create(&id, NULL, thread_fn, argv[1]);

    delay(1);
    open(argv[1], O_RDONLY);

    delay(2);
    setns(open("/proc/1/ns/net", O_RDONLY), 0);

    delay(1);
    open(argv[1], O_RDONLY);

    delay(4);

    return 0;

}

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