Re: what's next for the linux kernel?
From: Bodo Eggert (7eggert_at_gmx.de)
Date: 10/05/05
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Date: Wed, 5 Oct 2005 23:05:34 +0200 (CEST) To: Marc Perkel <marc@perkel.com>
On Wed, 5 Oct 2005, Marc Perkel wrote:
> What you don't get is that if you don't have rights to write to a file
> then you shouldn't have the right to delete the file.
In unix, nobody but the kernel has the right to *delete* a file. Therefore
nobody can delete a file without write permission.
Files are deleted if the last reference is gone. If you play a music file
and unlink it while it's playing, it won't be deleted untill the player
closes the file, since an open filehandle is a reference.
If you like, you can think of it as a kind of instant garbage collection:
Files are the objects referenced by these lists, and if you own
the object, you can change it. However, as long as there is a reference,
you can't destroy it, since this would invalidate all references.
Instead, you must remove all references.
Directories are lists of references, and these lists are independent from
the referenced file-objects. If you own the list, you can change it by
adding or removing files. You can even link files not owned or accessable
by you:
7eggert@be1:~/tmp > ls -l /tmp/foo/foo
---------- 1 7eggert_b users 0 2005-10-05 22:32 /tmp/foo/foo
7eggert@be1:~/tmp > ln /tmp/foo/foo .
7eggert@be1:~/tmp > ls -l
total 0
---------- 2 7eggert_b users 0 2005-10-05 22:32 foo
<snip>
Do you notice the link count in the second column?
Let's remove a link:
<snip>
*switch*
7eggert_b@be1:/tmp/foo> rm foo
rm: remove write-protected regular empty file `foo'? y
*switch*
7eggert@be1:~/tmp > ls -l
total 0
---------- 1 7eggert_b users 0 2005-10-05 22:32 foo
<snip>
As you can see, each directory-owner can independantly unlink the file.
BTW: The owner can change the permissions on the linked file anytime, so
even if you couldn't link non-accessable files, you could end up with
entries in your private directory you could neither access nor delete.
I can also open the file as 7eggert_b, delete it from another tty and
still access it's contents:
<snip>
7eggert_b@be1:/tmp/foo> cat > foo
as
*switch*
be1:/home/7eggert/tmp # rm /tmp/foo/foo
be1:/home/7eggert/tmp # ll /proc/4820/fd/1
l-wx------ 1 7eggert_b users 64 Oct 5 22:43 /proc/4820/fd/1
-> /tmp/foo/foo (deleted)
be1:/home/7eggert/tmp # cat /proc/4820/fd/1
as
*switch*
df
be1:/home/7eggert/tmp # cat /proc/4820/fd/1
as
df
<snip>
As you can see, the directory entry is deleted, but the file is still
there. However, making a hard link from a /proc/pid/fd entry is not (yet?)
possible: "ln: creating hard link `baz' to `/proc/4927/fd/1': Invalid
cross-device link".
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- Previous message: Dave Neuer: "Re: what's next for the linux kernel?"
- In reply to: Marc Perkel: "Re: what's next for the linux kernel?"
- Next in thread: Giuseppe Bilotta: "'Undeleting' an open file"
- Reply: Giuseppe Bilotta: "'Undeleting' an open file"
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