Message-ID: <20240611041540.495840-1-mjguzik@gmail.com>
Date: Tue, 11 Jun 2024 06:15:40 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: viro@...iv.linux.org.uk,
	jack@...e.cz,
	dave@...morbit.com,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH] vfs: partially sanitize i_state zeroing on inode creation

new_inode used to have the following:
	spin_lock(&inode_lock);
	inodes_stat.nr_inodes++;
	list_add(&inode->i_list, &inode_in_use);
	list_add(&inode->i_sb_list, &sb->s_inodes);
	inode->i_ino = ++last_ino;
	inode->i_state = 0;
	spin_unlock(&inode_lock);

over time things disappeared, got moved around or got replaced (global
inode lock with a per-inode lock), eventually this got reduced to:
	spin_lock(&inode->i_lock);
	inode->i_state = 0;
	spin_unlock(&inode->i_lock);

But the lock acquire here does not synchronize against anyone.

Additionally iget5_locked performs i_state = 0 assignment without any
locks to begin with and the two combined look confusing at best.

It looks like the current state is a leftover which was not cleaned up.

Ideally it would be an invariant that i_state == 0 to begin with, but
achieving that would require dealing with all filesystem alloc handlers
one by one.

In the meantime drop the misleading locking and move i_state zeroing to
alloc_inode so that others don't need to deal with it by hand.

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---

I diffed this against fs-next + my inode hash patch as it adds one
i_state = 0 case. Should that patch not be accepted this bit can be
easily dropped from this one.

I brought the entire thing up quite some time ago [1] and Dave Chinner
noted that perhaps the lock has a side effect of providing memory
barriers which otherwise would not be there and which are needed by
someone.

For new_inode and alloc_inode consumers all fences are already there
anyway due to immediate lock usage.

Arguably new_inode_pseudo escape without it but I don't find the code at
hand to be affected in any meanignful way -- the only 2 consumers
(get_pipe_inode and sock_alloc) perform numerous other stores to the
inode immediately after. By the time it gets added to anything looking
at i_state, flushing that should be handled by whatever thing which adds
it. Mentioning this just in case.

[1] https://lore.kernel.org/all/CAGudoHF_Y0shcU+AMRRdN5RQgs9L_HHvBH8D4K=7_0X72kYy2g@mail.gmail.com/

 fs/inode.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 149adf8ab0ea..3967e68311a6 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -276,6 +276,10 @@ static struct inode *alloc_inode(struct super_block *sb)
 		return NULL;
 	}
 
+	/*
+	 * FIXME: the code should be able to assert i_state == 0 instead.
+	 */
+	inode->i_state = 0;
 	return inode;
 }
 
@@ -1023,14 +1027,7 @@ EXPORT_SYMBOL(get_next_ino);
  */
 struct inode *new_inode_pseudo(struct super_block *sb)
 {
-	struct inode *inode = alloc_inode(sb);
-
-	if (inode) {
-		spin_lock(&inode->i_lock);
-		inode->i_state = 0;
-		spin_unlock(&inode->i_lock);
-	}
-	return inode;
+	return alloc_inode(sb);
 }
 
 /**
@@ -1254,7 +1251,6 @@ struct inode *iget5_locked(struct super_block *sb, unsigned long hashval,
 		struct inode *new = alloc_inode(sb);
 
 		if (new) {
-			new->i_state = 0;
 			inode = inode_insert5(new, hashval, test, set, data);
 			if (unlikely(inode != new))
 				destroy_inode(new);
@@ -1297,7 +1293,6 @@ struct inode *iget5_locked_rcu(struct super_block *sb, unsigned long hashval,
 
 	new = alloc_inode(sb);
 	if (new) {
-		new->i_state = 0;
 		inode = inode_insert5(new, hashval, test, set, data);
 		if (unlikely(inode != new))
 			destroy_inode(new);
-- 
2.43.0

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