Re: [RFC] revoke(2) and generic handling of things likeremove_proc_entry()

From: Al Viro
Date: Thu Apr 11 2013 - 19:29:22 EST

On Thu, Apr 11, 2013 at 01:48:26PM -0700, Eric W. Biederman wrote:

> Last time I was looking at this I was noticing that there is a lock
> (mmap_sem?) that is held over every ->vm_op->foo() call. If that is
> true today it should be possible to just grab that lock and change
> vm_ops. That makes for a very cheap and easy implementation, except for
> the covolutions needed for taking the lock.

3-rd party down_write(&mm->mmap_sem) is a Bloody Bad Idea(tm). VM locking is
complicated enough as it is and making it cope with such things would make it
even more convoluted.

> If we can do add useful support at the fs and mm layers without
> affecting performance I am all for it. I remember that tends to make
> things easier. As an alternative let me suggest what I had intended to
> do if/when I ever got back to working on revoke.
> Make a library like libfs that can be used for files that want to
> implement revoke support.
> In that library implement what can be implemented reliably and correctly
> and error on the sophisticated cases we can't support.
> With the semantics and the basic users figured out move what bits we can
> into the vfs or the mm subsystem to make things easier.
> With a library at the very least we have one implementation that we can
> debug and work with instead of a different implementation of revoke for
> each different kind of file.

Yecchh... revoke() as a syscall or revoke as something that happens when
kernel decides that file has gone away? The latter includes
procfs/debugfs/sysfs at the very least. Do we want to require all of those
to use that library? I would rather try to avoid a need for wrappers, TBH...

You have a very good point re ->close() - the locking conditions for it are
such that making revoke do it is extremely inconvenient. IMO it means that
mmap should check for attempts to set ->vm_op on vma with non-NULL
->vm_file->f_revoke and fail if it runs into such.
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