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

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Apr 10, 2019 at 02:04:25AM +0100, Al Viro wrote:
> On Tue, Apr 09, 2019 at 05:45:54PM -0700, Eric Biggers wrote:
> > On Wed, Apr 10, 2019 at 01:33:46AM +0100, Al Viro wrote:
> > > On Tue, Apr 09, 2019 at 04:35:44PM -0700, Eric Biggers wrote:
> > > > From: Eric Biggers <ebiggers@google.com>
> > > > 
> > > > Path lookups that traverse encrypted symlink(s) are very slow because
> > > > each encrypted symlink needs to be decrypted each time it's followed.
> > > > 
> > > > Make encrypted symlinks faster by caching the decrypted symlink target
> > > > in ->i_link.  The first call to ->get_link() sets it; later calls simply
> > > > return it.  ->symlink() also sets it when the symlink is created.
> > > > 
> > > > When the inode's ->i_crypt_info is freed, ->i_link is freed too.
> > > > 
> > > > Note: RCU-delayed freeing of ->i_link is not yet implemented.
> > > > Therefore, for now even when ->i_link is set, path lookups must continue
> > > > to drop out of RCU-walk mode when following an encrypted symlink.
> > > 
> > > And how the devil would they continue to do that, if I might ask?
> > > ->get_link() is *NOT* called if ->i_link is non-NULL, period.
> > 
> > You're right, I didn't notice that ->get_link() isn't called when
> > ->i_link is non-NULL.
> > 
> > But that being the case, what's the point of simple_get_link()?
> 
> Non-NULL ->get_link() => DCACHE_SYMLINK_TYPE in ->d_flags =>
> d_is_symlink() true => step_into() progresses to pick_link().
> 
> IOW, non-NULL ->get_link() is what tells you that we have
> a symlink there.

I think that's pretty unintuitive.  The fact that multiple filesystems including
ext4 set ->i_link on fast symlinks, then set ->get_link() to a function that
returns ->i_link, made me assume that's the mechanism by which such symlink
targets are returned to the VFS.  When in fact fs/namei.c just uses ->i_link,
and never calls ->get_link().

Is there any reason why d_flags_for_inode() doesn't check S_ISLNK() instead, and
then fs/namei.c would call ->get_link() if non-NULL, otherwise use ->i_link?

- Eric