linux-kernel - [PATCH v2 2/2] hfsplus: return error when node already exists in hfs_bnode

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Message-Id: <20251224151347.1861896-3-shardul.b@mpiricsoftware.com>
Date: Wed, 24 Dec 2025 20:43:47 +0530
From: Shardul Bankar <shardul.b@...ricsoftware.com>
To: slava@...eyko.com,
	zippel@...ux-m68k.org,
	linux-fsdevel@...r.kernel.org,
	glaubitz@...sik.fu-berlin.de,
	frank.li@...o.com
Cc: akpm@...l.org,
	linux-kernel@...r.kernel.org,
	janak@...ricsoftware.com,
	shardulsb08@...il.com,
	stable@...r.kernel.org,
	syzbot+1c8ff72d0cd8a50dfeaa@...kaller.appspotmail.com,
	Shardul Bankar <shardul.b@...ricsoftware.com>
Subject: [PATCH v2 2/2] hfsplus: return error when node already exists in hfs_bnode_create

When hfs_bnode_create() finds that a node is already hashed (which should
not happen in normal operation), it currently returns the existing node
without incrementing its reference count. This causes a reference count
inconsistency that leads to a kernel panic when the node is later freed
in hfs_bnode_put():

    kernel BUG at fs/hfsplus/bnode.c:676!
    BUG_ON(!atomic_read(&node->refcnt))

This scenario can occur when hfs_bmap_alloc() attempts to allocate a node
that is already in use (e.g., when node 0's bitmap bit is incorrectly
unset), or due to filesystem corruption.

Returning an existing node from a create path is not normal operation.

Fix this by returning ERR_PTR(-EEXIST) instead of the node when it's
already hashed. This properly signals the error condition to callers,
which already check for IS_ERR() return values.

Reported-by: syzbot+1c8ff72d0cd8a50dfeaa@...kaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=1c8ff72d0cd8a50dfeaa
Fixes: 634725a92938 ("[PATCH] hfs: cleanup HFS+ prints")
Signed-off-by: Shardul Bankar <shardul.b@...ricsoftware.com>
---
 v2:
 - Return ERR_PTR(-EEXIST) for already-hashed nodes, per Slava's suggestion.
 fs/hfsplus/bnode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/hfsplus/bnode.c b/fs/hfsplus/bnode.c
index 191661af9677..250a226336ea 100644
--- a/fs/hfsplus/bnode.c
+++ b/fs/hfsplus/bnode.c
@@ -629,7 +629,7 @@ struct hfs_bnode *hfs_bnode_create(struct hfs_btree *tree, u32 num)
 	if (node) {
 		pr_crit("new node %u already hashed?\n", num);
 		WARN_ON(1);
-		return node;
+		return ERR_PTR(-EEXIST);
 	}
 	node = __hfs_bnode_create(tree, num);
 	if (!node)
-- 
2.34.1