[PATCH v4 15/30] acl: add vfs_set_acl()

[Christian Brauner]

In previous patches we implemented get and set inode operations for all
non-stacking filesystems that support posix acls but didn't yet
implement get and/or set acl inode operations. This specifically
affected cifs and 9p.

Now we can build a posix acl api based solely on get and set inode
operations. We add a new vfs_set_acl() api that can be used to set posix
acls. This finally removes all type unsafety and type conversion issues
explained in detail in [1] that we aim to get rid of.

After we finished building the vfs api we can switch stacking
filesystems to rely on the new posix api and then finally switch the
xattr system calls themselves to rely on the posix acl api.

Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org [1]
Signed-off-by: Christian Brauner (Microsoft) <brauner at kernel.org>
---

Notes:
    /* v2 */
    unchanged
    
    /* v3 */
    unchanged
    
    /* v4 */
    Christoph Hellwig <hch at lst.de>:
    - s/EXPORT_SYMBOL/EXPORT_SYMBOL_GPL/
    - return -EOPNOTSUPP from vfs_set_acl() if !CONFIG_FS_POSIX_ACL
    
    Christian Brauner (Microsoft) <brauner at kernel.org>:
    - use newly introduced may_write_xattr() helper

 fs/posix_acl.c            | 117 ++++++++++++++++++++++++++++++++++++++
 include/linux/posix_acl.h |  10 ++++
 2 files changed, 127 insertions(+)

diff --git a/fs/posix_acl.c b/fs/posix_acl.c
index 3749d07881cc..c920fb98e29b 100644
--- a/fs/posix_acl.c
+++ b/fs/posix_acl.c
@@ -24,6 +24,11 @@
 #include <linux/user_namespace.h>
 #include <linux/namei.h>
 #include <linux/mnt_idmapping.h>
+#include <linux/security.h>
+#include <linux/evm.h>
+#include <linux/fsnotify.h>
+
+#include "internal.h"
 
 static struct posix_acl **acl_by_type(struct inode *inode, int type)
 {
@@ -1254,3 +1259,115 @@ int simple_acl_create(struct inode *dir, struct inode *inode)
 		posix_acl_release(acl);
 	return 0;
 }
+
+static inline int posix_acl_type(const char *name)
+{
+	if (strcmp(name, XATTR_NAME_POSIX_ACL_ACCESS) == 0)
+		return ACL_TYPE_ACCESS;
+	else if (strcmp(name, XATTR_NAME_POSIX_ACL_DEFAULT) == 0)
+		return ACL_TYPE_DEFAULT;
+
+	return -1;
+}
+
+static int vfs_set_acl_idmapped_mnt(struct user_namespace *mnt_userns,
+				    struct user_namespace *fs_userns,
+				    struct posix_acl *acl)
+{
+	for (int n = 0; n < acl->a_count; n++) {
+		struct posix_acl_entry *acl_e = &acl->a_entries[n];
+
+		switch (acl_e->e_tag) {
+		case ACL_USER:
+			acl_e->e_uid = from_vfsuid(mnt_userns, fs_userns,
+						   VFSUIDT_INIT(acl_e->e_uid));
+			break;
+		case ACL_GROUP:
+			acl_e->e_gid = from_vfsgid(mnt_userns, fs_userns,
+						   VFSGIDT_INIT(acl_e->e_gid));
+			break;
+		}
+	}
+
+	return 0;
+}
+
+/**
+ * vfs_set_acl - set posix acls
+ * @mnt_userns: user namespace of the mount
+ * @dentry: the dentry based on which to set the posix acls
+ * @acl_name: the name of the posix acl
+ * @kacl: the posix acls in the appropriate VFS format
+ *
+ * This function sets @kacl. The caller must all posix_acl_release() on @kacl
+ * afterwards.
+ *
+ * Return: On success 0, on error negative errno.
+ */
+int vfs_set_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
+		const char *acl_name, struct posix_acl *kacl)
+{
+	int acl_type;
+	int error;
+	struct inode *inode = d_inode(dentry);
+	struct inode *delegated_inode = NULL;
+
+	acl_type = posix_acl_type(acl_name);
+	if (acl_type < 0)
+		return -EINVAL;
+
+	if (kacl) {
+		/*
+		 * If we're on an idmapped mount translate from mount specific
+		 * vfs{g,u}id_t into global filesystem k{g,u}id_t.
+		 * Afterwards we can cache the POSIX ACLs filesystem wide and -
+		 * if this is a filesystem with a backing store - ultimately
+		 * translate them to backing store values.
+		 */
+		error = vfs_set_acl_idmapped_mnt(mnt_userns, i_user_ns(inode), kacl);
+		if (error)
+			return error;
+	}
+
+retry_deleg:
+	inode_lock(inode);
+
+	/*
+	 * We only care about restrictions the inode struct itself places upon
+	 * us otherwise POSIX ACLs aren't subject to any VFS restrictions.
+	 */
+	error = may_write_xattr(mnt_userns, inode);
+	if (error)
+		goto out_inode_unlock;
+
+	error = security_inode_set_acl(mnt_userns, dentry, acl_name, kacl);
+	if (error)
+		goto out_inode_unlock;
+
+	error = try_break_deleg(inode, &delegated_inode);
+	if (error)
+		goto out_inode_unlock;
+
+	if (inode->i_opflags & IOP_XATTR)
+		error = set_posix_acl(mnt_userns, dentry, acl_type, kacl);
+	else if (unlikely(is_bad_inode(inode)))
+		error = -EIO;
+	else
+		error = -EOPNOTSUPP;
+	if (!error) {
+		fsnotify_xattr(dentry);
+		evm_inode_post_set_acl(dentry, acl_name, kacl);
+	}
+
+out_inode_unlock:
+	inode_unlock(inode);
+
+	if (delegated_inode) {
+		error = break_deleg_wait(&delegated_inode);
+		if (!error)
+			goto retry_deleg;
+	}
+
+	return error;
+}
+EXPORT_SYMBOL_GPL(vfs_set_acl);
diff --git a/include/linux/posix_acl.h b/include/linux/posix_acl.h
index 07e171b4428a..316b05c1dc97 100644
--- a/include/linux/posix_acl.h
+++ b/include/linux/posix_acl.h
@@ -99,6 +99,9 @@ static inline void cache_no_acl(struct inode *inode)
 	inode->i_acl = NULL;
 	inode->i_default_acl = NULL;
 }
+
+int vfs_set_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
+		const char *acl_name, struct posix_acl *kacl);
 #else
 static inline int posix_acl_chmod(struct user_namespace *mnt_userns,
 				  struct dentry *dentry, umode_t mode)
@@ -126,6 +129,13 @@ static inline int posix_acl_create(struct inode *inode, umode_t *mode,
 static inline void forget_all_cached_acls(struct inode *inode)
 {
 }
+
+static inline int vfs_set_acl(struct user_namespace *mnt_userns,
+			      struct dentry *dentry, const char *name,
+			      struct posix_acl *acl)
+{
+	return -EOPNOTSUPP;
+}
 #endif /* CONFIG_FS_POSIX_ACL */
 
 struct posix_acl *get_inode_acl(struct inode *inode, int type);
-- 
2.34.1