3 ms·

True, device nodes and other special files exist in Unix, but they are usually contained to special directories.

cperciva 5y ago
You can create sockets and fifos anywhere. Device nodes are the only special ones which are generally confined to specific locations.
- deleted 5y ago
  [deleted]
- umanwizard 5y ago
  I think on BSDs at least you can theoretically put device nodes wherever you want (but there’s no good reason not to put them in /dev where everything expects them to be).
  - skissane 5y ago
    I'm pretty sure on Linux too there is nothing stopping you from putting a device node in any directory either. By convention they live in /dev (or its subdirectories), but there is nothing technically enforcing that convention. (udev assumes they live under /dev. I'm not sure if /dev is hardcoded in udev or configurable. But you don't have to use udev anyway, you can create device nodes manually.)
    - yjftsjthsd-h 5y ago
      Yeah, it's just a mknod away provided your filesystem isn't mounted nodev
    - tremon 5y ago
      Running a service inside a chroot might require some combination of /dev/{null,random,console,log} to be present inside the chroot, and bind-mounting /dev exposes the entire underlying system (raw disk nodes) inside the chroot. So there's actually a good reason for being able to create device nodes outside /dev.
      - cperciva 5y ago
        On FreeBSD you can mount devfs but tell it to only expose a subset of the device nodes, for exactly this reason.
      - kragen 5y ago
        Yeah, I used to do this to NFS-export a root filesystem for my diskless Sun-3. The NFS server was running Linux and the Sun was running IIRC NetBSD, so some of the device numbers in the Sun's /dev needed to be different. Pretty rickety setup but it worked okay. I don't recall ever accidentally overwriting my root filesystem or hanging up the modem line by accessing the bogus device nodes.
    - rwmj 5y ago
      We actually use this too (https://libguestfs.org/nbdkit-file-plugin.1.html#PARAMETERS https://libguestfs.org/nbdkit-file-plugin.1.html#PARAMETERS with "dir" parameter). It allows you to create a directory of devices to export. Symlinks are supported and generally better though.
  - cperciva 5y ago
    From FreeBSD 6.0 onwards, device nodes can only exist in devfs. (Ok, technically you can create them elsewhere, but you can't use them to access devices.)
    - umanwizard 5y ago
      I see. I’m more familiar with OpenBSD which I don’t think has such a restriction. (Though I could be wrong).
      - ben_bai 5y ago
        That's why most partitions have the nodev mount option. An (one of the rreasons) why there is pushback on a single big partition for everything.
anonymousiam 5y ago
Should have been more clear. Yes, (at least in Linux) you can mknod a special file anywhere, but the directory hierarchy is taken in context when you open the file. On MSDOS, you can open AUX or CON in any directory, even though no such named file exists anywhere in the filesystem. THAT is a poor implementation, and would probably be considered a bug by most developers if it were offered as a solution today.
- rbanffy 5y ago
  You know… it was called QDOS for a reason.
- hnlmorg 5y ago
  There wasn't directories in the first release of DOS. So I can forgive those special devices being location agnostic. However removing the device suffix (the colon) is really what causes this to be a bug. You either have persistent files with special attributes that identify it as a device, or you namespace your devices with characters that cannot be used in a file name. But you don't allow your devices to be valid file names and have them invisible on the file system.
  - mbreese 5y ago
    > However removing the device suffix (the colon) is really what causes this to be a bug. I never thought about it, but when you consider ‘:’ as the device suffix, this makes directory structures make more sense to me on Windows. C:\foo\bar makes more sense to me if it is the ‘C:’ device. Calling it a ‘C drive’ with a separate namespace never made sense to me. Somehow thinking of it the ‘C:’ device in an anonymous global hierarchy makes more sense to me.
- dredmorbius 5y ago
  Device files require that the mount point allow device files. That's a mount-time option, and "nodev" will prevent this. You'll still be able to create named pipes (mkfifo), but mknod will fail. This is handy for filesystems with a poorly developed sense of user ownership and permissions, say, msdos or vfat. I tend to mount my /tmp space nodev and nosuid, as occasionally you'll find poorly-written, or nefariously-intended programs attempting to do such things there.

You can create sockets and fifos anywhere. Device nodes are the only special ones which are generally confined to specific locations.

deleted 5y ago
[deleted]
umanwizard 5y ago
I think on BSDs at least you can theoretically put device nodes wherever you want (but there’s no good reason not to put them in /dev where everything expects them to be).
- skissane 5y ago
  I'm pretty sure on Linux too there is nothing stopping you from putting a device node in any directory either. By convention they live in /dev (or its subdirectories), but there is nothing technically enforcing that convention. (udev assumes they live under /dev. I'm not sure if /dev is hardcoded in udev or configurable. But you don't have to use udev anyway, you can create device nodes manually.)
  - yjftsjthsd-h 5y ago
    Yeah, it's just a mknod away provided your filesystem isn't mounted nodev
  - tremon 5y ago
    Running a service inside a chroot might require some combination of /dev/{null,random,console,log} to be present inside the chroot, and bind-mounting /dev exposes the entire underlying system (raw disk nodes) inside the chroot. So there's actually a good reason for being able to create device nodes outside /dev.
    - cperciva 5y ago
      On FreeBSD you can mount devfs but tell it to only expose a subset of the device nodes, for exactly this reason.
    - kragen 5y ago
      Yeah, I used to do this to NFS-export a root filesystem for my diskless Sun-3. The NFS server was running Linux and the Sun was running IIRC NetBSD, so some of the device numbers in the Sun's /dev needed to be different. Pretty rickety setup but it worked okay. I don't recall ever accidentally overwriting my root filesystem or hanging up the modem line by accessing the bogus device nodes.
  - rwmj 5y ago
    We actually use this too (https://libguestfs.org/nbdkit-file-plugin.1.html#PARAMETERS https://libguestfs.org/nbdkit-file-plugin.1.html#PARAMETERS with "dir" parameter). It allows you to create a directory of devices to export. Symlinks are supported and generally better though.
- cperciva 5y ago
  From FreeBSD 6.0 onwards, device nodes can only exist in devfs. (Ok, technically you can create them elsewhere, but you can't use them to access devices.)
  - umanwizard 5y ago
    I see. I’m more familiar with OpenBSD which I don’t think has such a restriction. (Though I could be wrong).
    - ben_bai 5y ago
      That's why most partitions have the nodev mount option. An (one of the rreasons) why there is pushback on a single big partition for everything.

anonymousiam 5y ago

Should have been more clear. Yes, (at least in Linux) you can mknod a special file anywhere, but the directory hierarchy is taken in context when you open the file. On MSDOS, you can open AUX or CON in any directory, even though no such named file exists anywhere in the filesystem. THAT is a poor implementation, and would probably be considered a bug by most developers if it were offered as a solution today.

rbanffy 5y ago
You know… it was called QDOS for a reason.
hnlmorg 5y ago
There wasn't directories in the first release of DOS. So I can forgive those special devices being location agnostic. However removing the device suffix (the colon) is really what causes this to be a bug. You either have persistent files with special attributes that identify it as a device, or you namespace your devices with characters that cannot be used in a file name. But you don't allow your devices to be valid file names and have them invisible on the file system.
- mbreese 5y ago
  > However removing the device suffix (the colon) is really what causes this to be a bug. I never thought about it, but when you consider ‘:’ as the device suffix, this makes directory structures make more sense to me on Windows. C:\foo\bar makes more sense to me if it is the ‘C:’ device. Calling it a ‘C drive’ with a separate namespace never made sense to me. Somehow thinking of it the ‘C:’ device in an anonymous global hierarchy makes more sense to me.
dredmorbius 5y ago
Device files require that the mount point allow device files. That's a mount-time option, and "nodev" will prevent this. You'll still be able to create named pipes (mkfifo), but mknod will fail. This is handy for filesystems with a poorly developed sense of user ownership and permissions, say, msdos or vfat. I tend to mount my /tmp space nodev and nosuid, as occasionally you'll find poorly-written, or nefariously-intended programs attempting to do such things there.