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Thu, 27 Jul 2023 17:28:52 +0000 From: Gabriel Krisman Bertazi To: viro@zeniv.linux.org.uk, brauner@kernel.org, tytso@mit.edu, ebiggers@kernel.org, jaegeuk@kernel.org Date: Thu, 27 Jul 2023 13:28:39 -0400 Message-ID: <20230727172843.20542-4-krisman@suse.de> X-Mailer: git-send-email 2.41.0 In-Reply-To: <20230727172843.20542-1-krisman@suse.de> References: <20230727172843.20542-1-krisman@suse.de> MIME-Version: 1.0 X-Headers-End: 1qP4nN-004oJq-7m Subject: [f2fs-dev] [PATCH v4 3/7] libfs: Validate negative dentries in case-insensitive directories X-BeenThere: linux-f2fs-devel@lists.sourceforge.net X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-fsdevel@vger.kernel.org, Gabriel Krisman Bertazi , linux-ext4@vger.kernel.org, Gabriel Krisman Bertazi , linux-f2fs-devel@lists.sourceforge.net Errors-To: linux-f2fs-devel-bounces@lists.sourceforge.net From: Gabriel Krisman Bertazi 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 Signed-off-by: Gabriel Krisman Bertazi --- 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