[PATCH v2 3/7] libfs: Validate negative dentries in case-insensitive directories

Gabriel Krisman Bertazi
2023-04-22T00:03:35Z

From: Gabriel Krisman Bertazi <krisman@collabora.com>

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.

dentry->d_name is only checked by the case-insensitive d_revalidate hook
in the LOOKUP_CREATE/LOOKUP_RENAME_TARGET case since, for these cases,
d_revalidate is always called with the parent inode locked, and
therefore the name cannot change from under us.

d_revalidate is only called in 4 places: lookup_dcache, __lookup_slow,
lookup_open and lookup_fast:

- lookup_dcache always calls it with zeroed flags, with the exception
of when coming from __lookup_hash, which needs the parent locked
already, for instance in the open/creation path, which is locked in
open_last_lookups.

- In __lookup_slow, either the parent inode is locked by the
caller (lookup_slow), or it is called with no
flags (lookup_one/lookup_one_len).

- lookup_open also requires the parent to be locked in the creation
case, which is done in open_last_lookups.

- lookup_fast will indeed be called with the parent unlocked, but it
shouldn't be called with LOOKUP_CREATE. Either it is called in the
link_path_walk, where nd->flags doesn't have LOOKUP_CREATE yet or in
open_last_lookups. But, in this case, it also never has LOOKUP_CREATE,
because it is only called on the !O_CREAT case, which means op->intent
doesn't have LOOKUP_CREAT (set in build_open_flags only if O_CREAT is
set).

Finally, for the LOOKUP_RENAME_TARGET, we are doing a rename, so the
parents inodes are also be locked.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
---
fs/libfs.c | 34 ++++++++++++++++++++++++++++++++++
1 file changed, 34 insertions(+)

diff --git a/fs/libfs.c b/fs/libfs.c
index 4eda519c3002..f8881e29c5d5 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1467,9 +1467,43 @@ 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 won't change from under us in
+ * the is_creation path only, since d_revalidate
+ * during creation and renames is always called
+ * with the parent inode locked. This isn't the
+ * case for all lookup callpaths, so it should
+ * not be accessed outside
+ * (LOOKUP_CREATE|LOOKUP_RENAME_TARGET) context.
+ */
+ if (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

--
2.40.0