linux-kernel - [PATCH] proc_sysctl: Fix up ->is

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Message-id: <174977507817.608730.3467596162021780258@noble.neil.brown.name>
Date: Fri, 13 Jun 2025 10:37:58 +1000
From: "NeilBrown" <neil@...wn.name>
To: Al Viro <viro@...iv.linux.org.uk>, Kees Cook <kees@...nel.org>,Joel
 Granados <joel.granados@...nel.org>
Cc: linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] proc_sysctl: Fix up ->is_seen() handling


Some sysctl tables can provide an is_seen() function which reports if
the sysctl should be visible to the current process.  This is currently
used to cause d_compare to fail for invisible sysctls.

This technique might have worked in 2.6.26 when it was implemented, but
it cannot work now.  In particular if ->d_compare always fails for a
particular name, then d_alloc_parallel() will always create a new dentry
and pass it to lookup() resulting in a new inode for every lookup.  I
tested this by changing sysctl_is_seen() to always return 0.  When
all sysctls were still visible and repeated lookups (ls -li) reported
different inode numbers.

This patch discards proc_sys_compare (the d_compare function) and
instead adds the is_seen functionality into proc_sys_revalidate (the
d_revalidate function).  If the sysctl table is unregistering, 0 is
returned.  Otherwise if is_seen() exists but fails, -ENOENT is returned.

The rcu_dereference() and RCU_INIT_POINTER() for ->sysctl are removed as
the field is not rcu-managed.  It is only changed when the inode is
created and when it is evicted, and in these cases there can be no
possible concurrent access.

Signed-off-by: NeilBrown <neil@...wn.name>
---
 fs/proc/inode.c       |  2 +-
 fs/proc/proc_sysctl.c | 35 ++++-------------------------------
 2 files changed, 5 insertions(+), 32 deletions(-)

diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index a3eb3b740f76..c3991dd314d9 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -42,7 +42,7 @@ static void proc_evict_inode(struct inode *inode)
 
 	head = ei->sysctl;
 	if (head) {
-		RCU_INIT_POINTER(ei->sysctl, NULL);
+		ei->sysctl = NULL;
 		proc_sys_evict_inode(inode, head);
 	}
 }
diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index cc9d74a06ff0..dbf652b50909 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -884,21 +884,15 @@ static const struct inode_operations proc_sys_dir_operations = {
 	.getattr	= proc_sys_getattr,
 };
 
-static int proc_sys_revalidate(struct inode *dir, const struct qstr *name,
-			       struct dentry *dentry, unsigned int flags)
-{
-	if (flags & LOOKUP_RCU)
-		return -ECHILD;
-	return !PROC_I(d_inode(dentry))->sysctl->unregistering;
-}
-
 static int proc_sys_delete(const struct dentry *dentry)
 {
 	return !!PROC_I(d_inode(dentry))->sysctl->unregistering;
 }
 
-static int sysctl_is_seen(struct ctl_table_header *p)
+static int proc_sys_revalidate(struct inode *dir, const struct qstr *name,
+			       struct dentry *dentry, unsigned int flags)
 {
+	struct ctl_table_header *p = PROC_I(d_inode(dentry))->sysctl;
 	struct ctl_table_set *set = p->set;
 	int res;
 	spin_lock(&sysctl_lock);
@@ -907,35 +901,14 @@ static int sysctl_is_seen(struct ctl_table_header *p)
 	else if (!set->is_seen)
 		res = 1;
 	else
-		res = set->is_seen(set);
+		res = set->is_seen(set) ? 1 : -ENOENT;
 	spin_unlock(&sysctl_lock);
 	return res;
 }
 
-static int proc_sys_compare(const struct dentry *dentry,
-		unsigned int len, const char *str, const struct qstr *name)
-{
-	struct ctl_table_header *head;
-	struct inode *inode;
-
-	/* Although proc doesn't have negative dentries, rcu-walk means
-	 * that inode here can be NULL */
-	/* AV: can it, indeed? */
-	inode = d_inode_rcu(dentry);
-	if (!inode)
-		return 1;
-	if (name->len != len)
-		return 1;
-	if (memcmp(name->name, str, len))
-		return 1;
-	head = rcu_dereference(PROC_I(inode)->sysctl);
-	return !head || !sysctl_is_seen(head);
-}
-
 static const struct dentry_operations proc_sys_dentry_operations = {
 	.d_revalidate	= proc_sys_revalidate,
 	.d_delete	= proc_sys_delete,
-	.d_compare	= proc_sys_compare,
 };
 
 static struct ctl_dir *find_subdir(struct ctl_dir *dir,
-- 
2.49.0