RFC: /dev/stdin, symlinks & permissions

Michael Tokarev
2008-03-17T23:26:42Z

I just come across an.. interesting (to me anyway) issue.

There are files - /dev/stdin, /dev/stdout & /dev/stderr -
which are handy sometimes and are available on several
*nix variants (including at least Solaris).

On Linux they're usually "implemented" as symlinks pointed
to /proc/self/fd/{0,1,2}, respectively. Which, in turn,
are symlinks pointing to the actual files.

For example, in a root ssh session, /dev/stdin may look
like (omitting details):

# ls -l /dev/stdin
/dev/stdin -> /proc/self/fd/0
# ls -l /proc/self/fd/0
/proc/self/fd/0 -> /dev/pts/0
# ls -l /dev/pts/0
crw--w---- 1 root tty 136, 0 Mar 18 02:19 /dev/pts/0

So far so good. Now, I change uid to something else, --
doing su(8) to "mjt". /proc/self changed obviously,
but stdin &Co is still here, and points to the same
/dev/pts/0. But *its* permissions/ownership did not
change! So now I can't, for example,

$ echo x > /dev/stdout
bash: /dev/stdout: Permission denied

which is quite unexpected - I for one expect /dev/stdout
to work the way very similar to /dev/tty, to mean "current
standard output regardless of any permissions etc".

For example in Solaris the whole /proc/self/fd (equivalent)
is in /dev/, and all the files in there has permissions
similar to /dev/tty:

# ls -l /dev/stdout
/dev/stdin -> ./fd/0
#ls -l /dev/fd/0
crw-rw-rw- 1 root root 306, 0 Mar 17 18:03 /dev/fd/0

To summarize. I understand where the whole thing comes from.
I understand kernel does not provide /dev/stdin &Co, this
interface is provided by distributions.

But the current way is half-broken, and it can't be corrected
from userspace.

Should kernel support something similar to other systems, less
broken than current /dev/stdin&Co symlinks?

Thanks!

/mjt