On Tue, Apr 09, 2019 at 09:04:15PM -0700, Eric Biggers wrote:
> On Wed, Apr 10, 2019 at 04:44:14AM +0100, Al Viro wrote:
> > On Tue, Apr 09, 2019 at 07:58:08PM -0700, Eric Biggers wrote:
> >
> > > It could check a flag IOP_GET_LINK in ->i_opflags instead, so it would be the
> > > same number of checks. See patch below.
> >
> > With that patch ->i_link is completely unused if ->get_link() is non-NULL,
> > so you get a method call on each traversal...
> >
>
> .get_link would be left NULL in all inode_operations that currently use
> simple_get_link, then simple_get_link() would be removed. My example patch just
> changed it in ext4 as an example.
>
> > > Benefits are that we get code that isn't actively misleading (via
> > > simple_get_link() existing but actually never being called), and filesystems can
> > > cache a symlink target in ->i_link if it becomes available later, i.e. if it's
> > > not immediately available at iget() time. Otherwise a filesystem-private field
> > > has to be used instead. (For fscrypt, I'd probably use fscrypt_info::ci_link.)
> >
> > What's to stop you from doing just that right now? You'd need to take
> > care with barriers, but you'd need that anyway... As soon as ->i_link is set
> > you'll get no more ->get_link() on that sucker, using the cached value
> > from that point on. IDGI...
>
> 1.) The VFS won't know to drop of RCU-walk mode, so waiting an RCU grace period
> before freeing the symlink target becomes mandatory. (Which I'd like to do
> for fscrypt anyway, but doing it sanely appears to require implementing
> .destroy_inode() for ext4, f2fs, and ubifs. I hoped I could do non-RCU mode
> as a simpler first step.)
>
> 2.) The VFS won't know to use a read memory barrier when loading i_link.
> The VFS could issue one unconditionally, but it would be unnecessary for
> regular fast symlinks.
>
> - Eric
Okay, actually all three filesystems have .destroy_inode() anyway. Not sure how
I missed that. So it should be possible to free the decrypted symlink target
from {ext4,f2fs,ubifs}_i_callback().
- Eric
