Messages in this thread | | | Date | Wed, 5 Oct 2005 23:05:34 +0200 (CEST) | From | Bodo Eggert <> | Subject | Re: what's next for the linux kernel? |
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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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