[TUHS] Device special files

Dan Cross crossd at gmail.com
Thu Feb 8 02:34:21 AEST 2018


On Wed, Feb 7, 2018 at 11:24 AM, Arthur Krewat <krewat at kilonet.net> wrote:

> medusa# mount | egrep '^/dev'
> /devices on /devices read/write/setuid/devices/rstchown/dev=9640000 on
> Fri Jan 19 16:33:07 2018
> /dev on /dev read/write/setuid/devices/rstchown/dev=9680000 on Fri Jan 19
> 16:33:07 2018
> /dev/fd on fd read/write/setuid/devices/rstchown/dev=9940001 on Fri Jan
> 19 16:33:22 2018
> medusa# ls -l /dev/rdsk/c1t*d0
> lrwxrwxrwx   1 root     root          65 Jan  2  2015 /dev/rdsk/c1t0d0 ->
> ../../devices/pci at 0,0/pci8086,340b at 4/pci1028,1f10 at 0/sd at 0,0:wd,raw
> lrwxrwxrwx   1 root     root          65 Jan  2  2015 /dev/rdsk/c1t1d0 ->
> ../../devices/pci at 0,0/pci8086,340b at 4/pci1028,1f10 at 0/sd at 1,0:wd,raw
> medusa# uname -a
> SunOS medusa 5.11 11.3 i86pc i386 i86pc
>

Some more commentary would be useful here, but yeah: Solaris went pretty
crazy with the cheez-whiz on this stuff. /devices was the "this is what we
found from autoconfiguration at boot time" thingy, and /dev was the "and
here are a bunch of symlinks to those things." I don't really remember why
they chose to do it that way.... Linux and BSD's devfs (as in 4.4 and its
descendants) was rather simpler (pretty much what I described until udev
came around and elaborated on the theme).

        - Dan C.


On 2/6/2018 9:06 PM, Dan Cross wrote:
>
> On Tue, Feb 6, 2018 at 8:48 PM, Dave Horsfall <dave at horsfall.org> wrote:
>
>> On Wed, 7 Feb 2018, Greg 'groggy' Lehey wrote:
>>
>>> V3 and earlier still *called* them special files, but it seems they were
>>>> essentially just magic inode numbers (there was no physical file on disk,
>>>> just any directory entry with the given inode would be the special file).
>>>>
>>>
>>> Isn't that still the case?
>>>
>>
>> Wasn't that "devfs" (which Penguin/OS calls "udev")?  I've never grokked
>> that concept.
>>
>
> No. devfs was (is?) a pseudo-filesystem where only special files
> corresponding to the devices enumerated by the kernel during
> autoconfiguration are present. The contents are synthesized at boot time
> and the filesystem is mounted at some canonical location (like /dev), but
> is otherwise ephemeral. This is in contrast to the older /dev, which is
> usually just a directory on the root filesystem, wherein one created a
> number of device files that may (or may not) correspond to an actual
> hardware device in the system (remember the old dance of, "cd /dev &&
> ./MAKEDEV foo" when you added a "foo" onto your system?). The inodes and
> directory entries for those files actually exist in the disk-resident
> filesystem structures (though of course data blocks aren't allocated to
> those files and the inode doesn't refer to any data blocks).
>
> My understanding is that udev is an elaboration on devfs on Linux that
> includes a rules engine that supports things like assigning set names to
> specific devices, setting permissions, group/user ownership and the like.
> For example, one can configure a rule so that when USB UART device with
> serial number 0xdeadbeef gets attached to the system, it appears as
> /dev/console_for_foo, owned by group "fooadmin" and mod 660. Presumably
> whoever configured that knows that that serial device is physically
> connected to the serial console for "foo".
>
>         - Dan C.
>
>
>
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