(resending as plain text as the first got bounced) Hi, I'm the maintainer of the psmisc package that provides system tools for things like fuser and killall. I am trying to establish if something I have found with the proc filesystem is as intended (knowing why would be nice) or if it's a strange corner-case bug. Apologies to the non-procfs maintainers but these two lists are what MAINTAINER said to go to. If you could CC me on replies that would be great. The proc file descriptor for a block device mounted in a different namespace will show the device id of that different namespace and not the device id of the process stat()ing the file. The issue came up in fuser not finding certain processes that were directly accessing a block device, see https://gitlab.com/psmisc/psmisc/-/issues/39 Programs such as lsof are caught by this too. My question is: When I am in the bash mount namespace (4026531840 below) then shouldn't all the device IDs be from that namespace? In other words, the device id of the dereferenced symlink and what it points to are the same (device id 5) and not symlink has 44 and /dev/dm-8 has 5. I get that if I could look at the device IDs in qemu or use nsenter to switch to its namespace, then the device should be 44 for the symlink and device (which it is and seems correct to me). How to replicate ============= # uname -a Linux elmo 5.16.0-5-amd64 #1 SMP PREEMPT Debian 5.16.14-1 (2022-03-15) x86_64 GNU/Linux The easiest way to replicate this is to make a qemu virtual machine and have it mount a block device. I suspect there are other ways, but I don't have many things that mount a device and switch namespaces. The qemu process (here it is 136775) will have a different mount namespace. # ps -o pid,mntns,comm $$ 136775 PID MNTNS COMMAND 136775 4026532762 qemu-system-x86 142359 4026531840 bash File descriptor 23 is what qemu is using to mount the block device # ls -l /proc/136775/fd/23 lrwx------ 1 libvirt-qemu libvirt-qemu 64 Apr 12 16:34 /proc/136775/fd/23 -> /dev/dm-8 However, the dereferenced symlink and where the symlink points to show different data. # stat -L /proc/136775/fd/23 File: /proc/136775/fd/23 Size: 0 Blocks: 0 IO Block: 4096 block special file Device: 2ch/44d Inode: 9 Links: 1 Device type: fd,8 Access: (0660/brw-rw----) Uid: (64055/libvirt-qemu) Gid: (64055/libvirt-qemu) Access: 2022-04-12 16:34:25.687147886 +1000 Modify: 2022-04-12 16:34:25.519151533 +1000 Change: 2022-04-12 16:34:25.595149882 +1000 Birth: - # stat /dev/dm-8 File: /dev/dm-8 Size: 0 Blocks: 0 IO Block: 4096 block special file Device: 5h/5d Inode: 348 Links: 1 Device type: fd,8 Access: (0660/brw-rw----) Uid: ( 0/ root) Gid: ( 0/ root) Access: 2022-04-12 16:15:12.684434884 +1000 Modify: 2022-04-12 16:15:12.684434884 +1000 Change: 2022-04-12 16:15:12.684434884 +1000 Birth: - If we change to the qemu process' mount namespace then we do see that /dev/dm-8 has the same device/inode as the symlink. # nsenter -m -t 136775 stat /dev/dm-8 File: /dev/dm-8 Size: 0 Blocks: 0 IO Block: 4096 block special file Device: 2ch/44d Inode: 9 Links: 1 Device type: fd,8 Access: (0660/brw-rw----) Uid: (64055/libvirt-qemu) Gid: (64055/libvirt-qemu) Access: 2022-04-12 16:34:25.687147886 +1000 Modify: 2022-04-12 16:34:25.519151533 +1000 Change: 2022-04-12 16:34:25.595149882 +1000 Birth: - Thanks for your time. - Craig
