Messages in this thread | | | Date | Tue, 18 Mar 2008 10:24:03 +0300 | From | Michael Tokarev <> | Subject | Re: RFC: /dev/stdin, symlinks & permissions |
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Andreas Schwab wrote: > Michael Tokarev <mjt@tls.msk.ru> writes: > >> # 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". > > This has nothing to do with /dev/stdout. Your terminal simply does not > allow access by anyone except user root or group tty. You need to open > it up first, or mount /dev/pts with broader permissions (which is a bad > idea however).
No, you don't understand. It has nothing to do with permissions to the actual file /dev/stdin refers to, be it a tty or something else.
In the exact same situation, I don't have to do anything with /dev/tty (which refers to the same tty at the end) - it Just Works (tm). Ditto on Solaris for example, -- /dev/stdin etc Just Works too. But not on Linux.
Without digging into implementation details (in this case it's the fact that /dev/stdin in linux is implemented using a symlink to - f.e. - /dev/tty/0 etc), I expect /dev/stdin to always work as far as filedescriptor 0 is open, regardless of any permissions on the actual tty (if it's a tty in the first place, which is not necessary the case) -- exactly the same as /dev/tty works.
It's more: I can redirect stdout of a process to some file, and at that point /dev/stdout will return ENOENT on open.
The question is:
Are /dev/stdin &Co on linux are just convenience symlinks to know where your std* files are (if so, why they're in /dev and why they're so INconvenient - so many levels of symlinks), -- ie, the only thing I can do with them is ls/readlink them, or are they supposed to actually WORK, ie, I'm able to open them, as on other systems?
Thanks.
/mjt
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