[PATCH v2 1/4] ovl: Set d->last properly during lookup

[Vivek Goyal <vgoyal@redhat.com>]

d->last signifies that this is the last layer we are looking into and there
is no more. And that means this allows for some optimzation opportunities
during lookup. For example, in ovl_lookup_single() we don't have to check
for opaque xattr of a directory is this is the last layer we are looking
into (d->last = true).

But knowing for sure whether we are looking into last layer can be very
tricky. If redirects are not enabled, then we can look at poe->numlower
and figure out if the lookup we are about to is last layer or not. But
if redircts are enabled then it is possible poe->numlower suggests that
we are looking in last layer, but there is an absolute redirect present
in found element and that redirects us to a layer in root and that means
lookup will continue in lower layers further.

For example, consider following.

/upperdir/pure (opaque=y)
/upperdir/pure/foo (opaque=y,redirect=/bar)
/lowerdir/bar

In this case pure is "pure upper". When we look for "foo", that time
poe->numlower=0. But that alone does not mean that we will not search
for a merge candidate in /lowerdir. Absolute redirect changes that.

IOW, d->last should not be set just based on poe->numlower if redirects
are enabled. That can lead to setting d->last while it should not have
and that means we will not check for opaque xattr while we should have.

So do this.

- If redirects are not enabled, then continue to rely on poe->numlower
  information to determine if it is last layer or not.

- If redirects are enabled, then set d->last = true only if this is the
  last layer in root ovl_entry (roe).

Suggested-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 fs/overlayfs/namei.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index de3e6da1d5a5..78798367c4f0 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -815,7 +815,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 		.is_dir = false,
 		.opaque = false,
 		.stop = false,
-		.last = !poe->numlower,
+		.last = ofs->config.redirect_follow ? false : !poe->numlower,
 		.redirect = NULL,
 	};
 
@@ -873,7 +873,11 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 	for (i = 0; !d.stop && i < poe->numlower; i++) {
 		struct ovl_path lower = poe->lowerstack[i];
 
-		d.last = i == poe->numlower - 1;
+		if (!ofs->config.redirect_follow)
+			d.last = i == poe->numlower - 1;
+		else
+			d.last = lower.layer->idx == roe->numlower;
+
 		err = ovl_lookup_layer(lower.dentry, &d, &this);
 		if (err)
 			goto out_put;
-- 
2.13.6