Re: [PATCH] fscrypt: cache decrypted symlink target in ->i_link

[Eric Biggers <ebiggers@kernel.org>]

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