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Date:   Tue, 5 Jul 2022 00:15:12 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
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Subject: [PATCH 4/7] step_into(): lose inode argument
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Sender: Al Viro <viro@ftp.linux.org.uk>
Precedence: bulk

make handle_mounts() always fetch it.  This is just the first step -
the callers of step_into() will stop trying to calculate the sucker,
etc.

The passed value should be equal to dentry->d_inode in all cases;
in RCU mode - fetched after we'd sampled ->d_seq.  Might as well
fetch it here.  We do need to validate ->d_seq, which duplicates
the check currently done in lookup_fast(); that duplication will
go away shortly.

After that change handle_mounts() always ignores the initial value of
*inode and always sets it on success.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/namei.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index c7c9e88add85..dddbebf92b48 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1532,6 +1532,11 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
 	path->dentry = dentry;
 	if (nd->flags & LOOKUP_RCU) {
 		unsigned int seq = nd->next_seq;
+		*inode = dentry->d_inode;
+		if (read_seqcount_retry(&dentry->d_seq, seq))
+			return -ECHILD;
+		if (unlikely(!*inode))
+			return -ENOENT;
 		if (likely(__follow_mount_rcu(nd, path, inode)))
 			return 0;
 		// *path and nd->next_seq might've been clobbered
@@ -1842,9 +1847,10 @@ static const char *pick_link(struct nameidata *nd, struct path *link,
  * NOTE: dentry must be what nd->next_seq had been sampled from.
  */
 static const char *step_into(struct nameidata *nd, int flags,
-		     struct dentry *dentry, struct inode *inode)
+		     struct dentry *dentry)
 {
 	struct path path;
+	struct inode *inode;
 	int err = handle_mounts(nd, dentry, &path, &inode);
 
 	if (err < 0)
@@ -1970,7 +1976,7 @@ static const char *handle_dots(struct nameidata *nd, int type)
 			parent = follow_dotdot(nd, &inode);
 		if (IS_ERR(parent))
 			return ERR_CAST(parent);
-		error = step_into(nd, WALK_NOFOLLOW, parent, inode);
+		error = step_into(nd, WALK_NOFOLLOW, parent);
 		if (unlikely(error))
 			return error;
 
@@ -2015,7 +2021,7 @@ static const char *walk_component(struct nameidata *nd, int flags)
 	}
 	if (!(flags & WALK_MORE) && nd->depth)
 		put_link(nd);
-	return step_into(nd, flags, dentry, inode);
+	return step_into(nd, flags, dentry);
 }
 
 /*
@@ -2474,8 +2480,7 @@ static int handle_lookup_down(struct nameidata *nd)
 	if (!(nd->flags & LOOKUP_RCU))
 		dget(nd->path.dentry);
 	nd->next_seq = nd->seq;
-	return PTR_ERR(step_into(nd, WALK_NOFOLLOW,
-			nd->path.dentry, nd->inode));
+	return PTR_ERR(step_into(nd, WALK_NOFOLLOW, nd->path.dentry));
 }
 
 /* Returns 0 and nd will be valid on success; Retuns error, otherwise. */
@@ -3464,7 +3469,7 @@ static const char *open_last_lookups(struct nameidata *nd,
 finish_lookup:
 	if (nd->depth)
 		put_link(nd);
-	res = step_into(nd, WALK_TRAILING, dentry, inode);
+	res = step_into(nd, WALK_TRAILING, dentry);
 	if (unlikely(res))
 		nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL);
 	return res;
-- 
2.30.2