[PATCH v2] audit: fix suffixed '/' filename matching in __audit_inode_child()

[Ricardo Robaina <rrobaina@redhat.com>]

When the user specifies a directory to delete with the suffix '/',
the audit record fails to collect the filename, resulting in the
following logs:

 type=PATH msg=audit(10/30/2024 14:11:17.796:6304) : item=2 name=(null)
 type=PATH msg=audit(10/30/2024 14:11:17.796:6304) : item=1 name=(null)

It happens because the value of the variables dname, and n->name->name
in __audit_inode_child() differ only by the suffix '/'. This commit
treats this corner case by handling pathname's trailing slashes in
audit_compare_dname_path().

Steps to reproduce the issue:

 # auditctl -w /tmp
 $ mkdir /tmp/foo
 $ rm -r /tmp/foo/
 # ausearch -i | grep PATH | tail -3

The first version of this patch was based on a GitHub patch/PR by
user @hqh2010 [1].

Link: https://github.com/linux-audit/audit-kernel/pull/148 [1]

Suggested-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Reviewed-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Ricardo Robaina <rrobaina@redhat.com>

---
v2: handling pathname's trailing slashes in audit_compare_dname_path()
v1: https://lore.kernel.org/audit/20241114040948.GK3387508@ZenIV/T/#t
---
 kernel/auditfilter.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/auditfilter.c b/kernel/auditfilter.c
index 470041c49a44..8ddccdb4a2a7 100644
--- a/kernel/auditfilter.c
+++ b/kernel/auditfilter.c
@@ -1319,13 +1319,20 @@ int audit_compare_dname_path(const struct qstr *dname, const char *path, int par
 	if (pathlen < dlen)
 		return 1;
 
-	parentlen = parentlen == AUDIT_NAME_FULL ? parent_len(path) : parentlen;
-	if (pathlen - parentlen != dlen)
-		return 1;
+	if (parentlen == AUDIT_NAME_FULL)
+		parentlen = parent_len(path);
 
 	p = path + parentlen;
 
-	return strncmp(p, dname->name, dlen);
+	/* handle trailing slashes */
+	pathlen -= parentlen;
+	while (p[pathlen - 1] == '/')
+		pathlen--;
+
+	if (pathlen != dlen)
+		return 1;
+
+	return memcmp(p, dname->name, dlen);
 }
 
 int audit_filter(int msgtype, unsigned int listtype)
-- 
2.47.0