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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". -- Fun things to slip into your budget Not in a budget, but in an annual report: An employee stole 500,000+. They accounted for it on the annual report as 'involountary employee relations expense' - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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