linux-ext4 - [PATCH RFC 3/7] libfs: Validate negative dentries in case-insensitive directories

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Message-Id: <20220601204437.676872-4-krisman@collabora.com>
Date:   Wed,  1 Jun 2022 16:44:33 -0400
From:   Gabriel Krisman Bertazi <krisman@...labora.com>
To:     viro@...iv.linux.org.uk, tytso@....edu, jaegeuk@...nel.org
Cc:     ebiggers@...nel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        kernel@...labora.com
Subject: [PATCH RFC 3/7] libfs: Validate negative dentries in case-insensitive directories

Introduce a dentry revalidation helper to be used by case-insensitive
filesystems to check if it is safe to reuse a negative dentry.

A negative dentry is safe to be reused on a case-insensitive lookup if
it was created during a case-insensitive lookup and this is not a lookup
that will instantiate a dentry. If this is a creation lookup, we also
need to make sure the name matches sensitively the name under lookup in
order to assure the name preserving semantics.

Signed-off-by: Gabriel Krisman Bertazi <krisman@...labora.com>
---
 fs/libfs.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/fs/libfs.c b/fs/libfs.c
index e64bdedef168..618a85c08aa7 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1450,9 +1450,33 @@ static int generic_ci_d_hash(const struct dentry *dentry, struct qstr *str)
 	return 0;
 }
 
+static inline int generic_ci_d_revalidate(struct dentry *dentry,
+					  const struct qstr *name,
+					  unsigned int flags)
+{
+	int is_creation = flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET);
+
+	if (d_is_negative(dentry)) {
+		const struct dentry *parent = READ_ONCE(dentry->d_parent);
+		const struct inode *dir = READ_ONCE(parent->d_inode);
+
+		if (dir && needs_casefold(dir)) {
+			if (!d_is_casefold_lookup(dentry))
+				return 0;
+
+			if (is_creation &&
+			    (dentry->d_name.len != name->len ||
+			     memcmp(dentry->d_name.name, name->name, name->len)))
+				return 0;
+		}
+	}
+	return 1;
+}
+
 static const struct dentry_operations generic_ci_dentry_ops = {
 	.d_hash = generic_ci_d_hash,
 	.d_compare = generic_ci_d_compare,
+	.d_revalidate_name = generic_ci_d_revalidate,
 };
 #endif
 
-- 
2.36.1