Re: [PATCH] afs, bash: Fix open(O_CREAT) on an extant AFS file in a sticky dir

[David Howells <dhowells@redhat.com>]

Christian Brauner <brauner@kernel.org> wrote:

> > However, the bash work around is going to be removed:
> 
> Why is it removed? That's a very strange comment:

Because it makes bash output redirection work differently to other programs, I
would guess.  It's actually a simple security check to work around (just retry
the open() with O_CREAT dropped) - however, it does expose an... error, I
suppose, in the Linux kernel: namely that the VFS itself is treating foreign
files as if they had local system ownership.

We have the ->permission() inode op for this reason (I presume) - but that
only applies to certain checks.  The VFS must not assume that it can interpret
i_uid and i_gid on an inode and must not assume that it can compare them to
current->fsuid and current->fs_gid.

Now, in my patch, I added two inode ops because they VFS code involved makes
two distinct evaluations and so I made an op for each and, as such, those
evaluations may be applicable elsewhere, but I could make a combined op that
handles that specific situation instead.

David