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Linux xenfs vs privcmd
- To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 8 Oct 2025 14:33:51 +0100
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- Cc: Juergen Gross <jgross@xxxxxxxx>
- Delivery-date: Wed, 08 Oct 2025 13:34:07 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
Hello,
I'm doing a deployment of Xen on a remote system provisioned with Ubuntu
24.04, and I've found what I'm pretty sure is a bug.
In dom0, to start with:
user@host:~$ ls -la /dev/xen/
total 0
drwxr-xr-x 2 root root 140 Oct 8 20:04 .
drwxr-xr-x 18 root root 4620 Oct 8 20:04 ..
crw------- 1 root root 10, 120 Oct 8 20:04 evtchn
crw------- 1 root root 10, 118 Oct 8 20:04 gntalloc
crw------- 1 root root 10, 119 Oct 8 20:04 gntdev
crw------- 1 root root 10, 124 Oct 8 20:04 xenbus
crw------- 1 root root 10, 123 Oct 8 20:04 xenbus_backend
user@host:~$ ls -la /proc/xen/
total 0
dr-xr-xr-x 2 root root 0 Oct 8 20:04 .
dr-xr-xr-x 326 root root 0 Oct 8 20:04 ..
i.e. no /dev/xen/privcmd.
It turns out that mounting xenfs causes it to appear:
user@host:~$ sudo systemctl start proc-xen.mount
user@host:~$ ls -la /dev/xen/
total 0
drwxr-xr-x 2 root root 180 Oct 8 20:05 .
drwxr-xr-x 18 root root 4620 Oct 8 20:04 ..
crw------- 1 root root 10, 120 Oct 8 20:04 evtchn
crw------- 1 root root 10, 118 Oct 8 20:04 gntalloc
crw------- 1 root root 10, 119 Oct 8 20:04 gntdev
crw------- 1 root root 10, 115 Oct 8 20:05 hypercall
crw------- 1 root root 10, 116 Oct 8 20:05 privcmd
crw------- 1 root root 10, 124 Oct 8 20:04 xenbus
crw------- 1 root root 10, 123 Oct 8 20:04 xenbus_backend
user@host:~$ ls -la /proc/xen/
total 0
drwxr-xr-x 2 root root 0 Oct 8 20:05 .
dr-xr-xr-x 315 root root 0 Oct 8 20:04 ..
-r--r--r-- 1 root root 0 Oct 8 20:05 capabilities
-rw------- 1 root root 0 Oct 8 20:05 privcmd
-rw------- 1 root root 0 Oct 8 20:05 xenbus
-r-------- 1 root root 0 Oct 8 20:05 xensyms
-rw------- 1 root root 0 Oct 8 20:05 xsd_kva
-rw------- 1 root root 0 Oct 8 20:05 xsd_port
For good measure, I checked unmounting xenfs:
user@host:~$ sudo umount /proc/xen
user@host:~$ ls -la /dev/xen/
total 0
drwxr-xr-x 2 root root 180 Oct 8 20:05 .
drwxr-xr-x 18 root root 4620 Oct 8 20:04 ..
crw------- 1 root root 10, 120 Oct 8 20:04 evtchn
crw------- 1 root root 10, 118 Oct 8 20:04 gntalloc
crw------- 1 root root 10, 119 Oct 8 20:04 gntdev
crw------- 1 root root 10, 115 Oct 8 20:05 hypercall
crw------- 1 root root 10, 116 Oct 8 20:05 privcmd
crw------- 1 root root 10, 124 Oct 8 20:04 xenbus
crw------- 1 root root 10, 123 Oct 8 20:04 xenbus_backend
user@host:~$ ls -la /proc/xen/
total 0
dr-xr-xr-x 2 root root 0 Oct 8 20:04 .
dr-xr-xr-x 291 root root 0 Oct 8 20:04 ..
and /dev/xen/privcmd stayed.
Anyway - /dev/xen/privcmd (and /hypercall) shouldn't be tied to xenfs.
They should be SIF_PRIVILEGED alone, should they not?
~Andrew
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