[PATCH v2 17/25] fs: add vfs_get_fscaps()

["Seth Forshee (DigitalOcean)" <sforshee@kernel.org>]

Provide a type-safe interface for retrieving filesystem capabilities and
a generic implementation suitable for most filesystems. Also add an
internal interface, vfs_get_fscaps_nosec(), which skips security checks
for later use from the capability code.

Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
---
 fs/xattr.c         | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h |  4 ++++
 2 files changed, 68 insertions(+)

diff --git a/fs/xattr.c b/fs/xattr.c
index 06290e4ebc03..10d1b1f78fc2 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -181,6 +181,70 @@ xattr_supports_user_prefix(struct inode *inode)
 }
 EXPORT_SYMBOL(xattr_supports_user_prefix);
 
+static int generic_get_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
+			      struct vfs_caps *caps)
+{
+	struct inode *inode = d_inode(dentry);
+	struct vfs_ns_cap_data nscaps;
+	int ret;
+
+	ret = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, &nscaps, sizeof(nscaps));
+
+	if (ret >= 0)
+		ret = vfs_caps_from_xattr(idmap, i_user_ns(inode), caps, &nscaps, ret);
+
+	return ret;
+}
+
+/**
+ * vfs_get_fscaps_nosec - get filesystem capabilities without security checks
+ * @idmap: idmap of the mount the inode was found from
+ * @dentry: the dentry from which to get filesystem capabilities
+ * @caps: storage in which to return the filesystem capabilities
+ *
+ * This function gets the filesystem capabilities for the dentry and returns
+ * them in @caps. It does not perform security checks.
+ *
+ * Return: 0 on success, a negative errno on error.
+ */
+int vfs_get_fscaps_nosec(struct mnt_idmap *idmap, struct dentry *dentry,
+			 struct vfs_caps *caps)
+{
+	struct inode *inode = d_inode(dentry);
+
+	if (inode->i_op->get_fscaps)
+		return inode->i_op->get_fscaps(idmap, dentry, caps);
+	return generic_get_fscaps(idmap, dentry, caps);
+}
+
+/**
+ * vfs_get_fscaps - get filesystem capabilities
+ * @idmap: idmap of the mount the inode was found from
+ * @dentry: the dentry from which to get filesystem capabilities
+ * @caps: storage in which to return the filesystem capabilities
+ *
+ * This function gets the filesystem capabilities for the dentry and returns
+ * them in @caps.
+ *
+ * Return: 0 on success, a negative errno on error.
+ */
+int vfs_get_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
+		   struct vfs_caps *caps)
+{
+	int error;
+
+	/*
+	 * The VFS has no restrictions on reading security.* xattrs, so
+	 * xattr_permission() isn't needed. Only LSMs get a say.
+	 */
+	error = security_inode_get_fscaps(idmap, dentry);
+	if (error)
+		return error;
+
+	return vfs_get_fscaps_nosec(idmap, dentry, caps);
+}
+EXPORT_SYMBOL(vfs_get_fscaps);
+
 int
 __vfs_setxattr(struct mnt_idmap *idmap, struct dentry *dentry,
 	       struct inode *inode, const char *name, const void *value,
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 89163e0f7aad..d7cd2467e1ea 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2116,6 +2116,10 @@ extern int vfs_dedupe_file_range(struct file *file,
 extern loff_t vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
 					struct file *dst_file, loff_t dst_pos,
 					loff_t len, unsigned int remap_flags);
+extern int vfs_get_fscaps_nosec(struct mnt_idmap *idmap, struct dentry *dentry,
+				struct vfs_caps *caps);
+extern int vfs_get_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
+			  struct vfs_caps *caps);
 
 /**
  * enum freeze_holder - holder of the freeze

-- 
2.43.0