From: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
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 read-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 read locked by the
caller (lookup_slow), or it is called with no flags (lookup_one*).
The read lock suffices to prevent ->d_name modifications, with the
exception of one case: __d_unalias, will call __d_move to fix a
directory accessible from multiple dentries, which effectively swaps
->d_name while holding only the shared read lock. This happens
through this flow:
lookup_slow() //LOOKUP_CREATE
d_lookup()
->d_lookup()
d_splice_alias()
__d_unalias()
__d_move()
Nevertheless, this case is not a problem because negative dentries
are not allowed to be moved with __d_move.
- 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 locked.
Reviewed-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
---
Changes since v3:
- Add comment regarding creation (Eric)
- Reorder checks to clarify !flags meaning (Eric)
- Add commit message explanaton of the inode read lock wrt.
__d_move. (Eric)
Changes since v2:
- Add comments to all rejection cases (Eric)
- safeguard against filesystem creating dentries without LOOKUP flags
---
fs/libfs.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 55 insertions(+)
diff --git a/fs/libfs.c b/fs/libfs.c
index 5b851315eeed..ed04c4dcc312 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1462,9 +1462,64 @@ 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)
+{
+ 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)) {
+ /*
+ * Negative dentries created prior to turning the
+ * directory case-insensitive cannot be trusted, since
+ * they don't ensure any possible case version of the
+ * filename doesn't exist.
+ */
+ if (!d_is_casefold_lookup(dentry))
+ return 0;
+
+ /*
+ * Filesystems will call into d_revalidate without
+ * setting LOOKUP_ flags even for file creation (see
+ * lookup_one* variants). Reject negative dentries in
+ * this case, since we can't know for sure it won't be
+ * used for creation.
+ */
+ if (!flags)
+ return 0;
+
+ /*
+ * If the lookup is for creation, then a negative dentry
+ * can only be reused if it's a case-sensitive match,
+ * not just a case-insensitive one. This is needed to
+ * make the new file be created with the name the user
+ * specified, preserving case.
+ */
+ if (flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET)) {
+ /*
+ * ->d_name won't change from under us in the
+ * creation path only, since d_revalidate during
+ * creation and renames is always called with
+ * the parent inode locked. It isn't the case
+ * for all lookup callpaths, so ->d_name must
+ * not be touched 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
--
2.41.0
