Updating my system this week broke network namespaces. I can create them but the command `ip netns exec ...` leads to the following error message:

`setting the network namespace "mynamespace" failed: Invalid argument`. Exit status 255.

I have attached the output of strace. Note how it gets a first failure right at the top at

```
execve("/usr/bin/ip", ["ip", "netns", "exec", "mynamespace", "fish"], 0x7ffff0408230 /* 31 vars */) = 0
brk(NULL) = 0x55b76ee3c000
arch_prctl(0x3001 /* ARCH_??? */, 0x7ffdbe08ccf0) = -1 EINVAL (Invalid argument)
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
```
On `arch_prctl`.

Later notable failures are:
```
prctl(PR_CAPBSET_READ, CAP_MAC_OVERRIDE) = 1
prctl(PR_CAPBSET_READ, 0x30 /* CAP_??? */) = -1 EINVAL (Invalid argument)
prctl(PR_CAPBSET_READ, CAP_CHECKPOINT_RESTORE) = 1
prctl(PR_CAPBSET_READ, 0x2c /* CAP_??? */) = -1 EINVAL (Invalid argument)
prctl(PR_CAPBSET_READ, 0x2a /* CAP_??? */) = -1 EINVAL (Invalid argument)
prctl(PR_CAPBSET_READ, 0x29 /* CAP_??? */) = -1 EINVAL (Invalid argument)
```
And at the end:
```
setns(5, CLONE_NEWNET) = -1 EINVAL (Invalid argument)

```

The network namespace exists. The `/etc/ld.so.preload` ENOENT is a red herring I think. I `touch`ed it and the error goes away, second attached file after touch.

I do not find any other bug reports anywhere about this. I have however updated a second PC and same issue, so it's not something very specific to my first PC. I tried downgrading a few versions of linux kernel and iproute2, but I hadn't upgraded my system since June 23rd, so there is quite a range. I'm going to try and downgrade to the versions at June 23 and if that fixes it I will bisect which version breaks it specifically.

Package changes in the upgrade which caused the issue (note I'm not exactly certain which package caused it):
-linux 6.3.9.arch1-1
+linux 6.4.8.arch1-1
-iproute2 6.3.0-2
+iproute2 6.4.0-1
-systemd 253.5-2
+systemd 254-1