[PATCH v2] proc: only bump parent nlink when registering directories

["Krzysztof Wilczyński" <kwilczynski@kernel.org>]

proc_register() increments the parent directory's link count for every
entry it registers, while remove_proc_entry() and remove_proc_subtree()
decrement it only when the removed entry is a directory.  Regular files
thus inflate the parent's count while they exist, and leak one link
permanently on every create and remove cycle.

For example, /proc/bus/pci/00 with twenty-two device files and no
subdirectories reports nlink 24 instead of 2, and SR-IOV VF enable
and disable cycles, each creating and removing the VF config space
entries under /proc/bus/pci/<bus>, inflate the link count of that
directory without bound.

Before commit e06689bf5701 ("proc: change ->nlink under
proc_subdir_lock"), the increment lived in proc_mkdir_data() and
proc_create_mount_point(), and was therefore applied only to
directories.  Moving it into proc_register() to bring it under
proc_subdir_lock dropped the S_ISDIR check.

Thus, move the nlink accounting into pde_subdir_insert() and
pde_erase(), only updating it for directories in both, so the link
count is always changed together with the directory entry itself.

Fixes: e06689bf5701 ("proc: change ->nlink under proc_subdir_lock")
Cc: stable@vger.kernel.org # v5.5+
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
---
Changes in v2:
  https://lore.kernel.org/linux-fsdevel/20260612153031.536525-1-kwilczynski@kernel.org/

  - Moved the nlink accounting into pde_subdir_insert() and
    pde_erase() instead of adding a check in proc_register(), as
    suggested by Alexey Dobriyan.

 fs/proc/generic.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index 8bb81e58c9d8..32b6b0f97967 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -112,6 +112,8 @@ static bool pde_subdir_insert(struct proc_dir_entry *dir,
 	/* Add new node and rebalance tree. */
 	rb_link_node(&de->subdir_node, parent, new);
 	rb_insert_color(&de->subdir_node, root);
+	if (S_ISDIR(de->mode))
+		dir->nlink++;
 	return true;
 }
 
@@ -404,7 +406,6 @@ struct proc_dir_entry *proc_register(struct proc_dir_entry *dir,
 		write_unlock(&proc_subdir_lock);
 		goto out_free_inum;
 	}
-	dir->nlink++;
 	write_unlock(&proc_subdir_lock);
 
 	return dp;
@@ -702,6 +703,8 @@ static void pde_erase(struct proc_dir_entry *pde, struct proc_dir_entry *parent)
 {
 	rb_erase(&pde->subdir_node, &parent->subdir);
 	RB_CLEAR_NODE(&pde->subdir_node);
+	if (S_ISDIR(pde->mode))
+		parent->nlink--;
 }
 
 /*
@@ -727,8 +730,6 @@ void remove_proc_entry(const char *name, struct proc_dir_entry *parent)
 			de = NULL;
 		} else {
 			pde_erase(de, parent);
-			if (S_ISDIR(de->mode))
-				parent->nlink--;
 		}
 	}
 	write_unlock(&proc_subdir_lock);
@@ -787,8 +788,6 @@ int remove_proc_subtree(const char *name, struct proc_dir_entry *parent)
 			continue;
 		}
 		next = de->parent;
-		if (S_ISDIR(de->mode))
-			next->nlink--;
 		write_unlock(&proc_subdir_lock);
 
 		proc_entry_rundown(de);
-- 
2.54.0