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

[David Laight <David.Laight@ACULAB.COM>]

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;
> }
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)