libbtrfsutil: relax the privileges of subvolume_info()

Attempt to use the BTRFS_IOC_GET_SUBVOL_INFO ioctl (added in kernel
4.18) for subvolume_info() if not root. Also, rename
get_subvolume_info_root() -> get_subvolume_info_privileged() for
consistency with further changes.

This is based on a patch from Misono Tomohiro.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

libbtrfsutil/btrfsutil.h

@@ -63,6 +63,7 @@ enum btrfs_util_error {
 	BTRFS_UTIL_ERROR_SYNC_FAILED,
 	BTRFS_UTIL_ERROR_START_SYNC_FAILED,
 	BTRFS_UTIL_ERROR_WAIT_SYNC_FAILED,
+	BTRFS_UTIL_ERROR_GET_SUBVOL_INFO_FAILED,
 };
 
 /**
@@ -266,7 +267,8 @@ struct btrfs_util_subvolume_info {
  * to check whether the subvolume exists; %BTRFS_UTIL_ERROR_SUBVOLUME_NOT_FOUND
  * will be returned if it does not.
  *
- * This requires appropriate privilege (CAP_SYS_ADMIN).
+ * This requires appropriate privilege (CAP_SYS_ADMIN) unless @id is zero and
+ * the kernel supports BTRFS_IOC_GET_SUBVOL_INFO (kernel >= 4.18).
  *
  * Return: %BTRFS_UTIL_OK on success, non-zero error code on failure.
  */

libbtrfsutil/errors.c

@@ -45,6 +45,8 @@ static const char * const error_messages[] = {
 	[BTRFS_UTIL_ERROR_SYNC_FAILED] = "Could not sync filesystem",
 	[BTRFS_UTIL_ERROR_START_SYNC_FAILED] = "Could not start filesystem sync",
 	[BTRFS_UTIL_ERROR_WAIT_SYNC_FAILED] = "Could not wait for filesystem sync",
+	[BTRFS_UTIL_ERROR_GET_SUBVOL_INFO_FAILED] =
+		"Could not get subvolume information with BTRFS_IOC_GET_SUBVOL_INFO",
 };
 
 PUBLIC const char *btrfs_util_strerror(enum btrfs_util_error err)

libbtrfsutil/python/tests/test_subvolume.py

@@ -23,7 +23,12 @@ from pathlib import PurePath
 import traceback
 
 import btrfsutil
-from tests import BtrfsTestCase, HAVE_PATH_LIKE
+from tests import (
+    BtrfsTestCase,
+    drop_privs,
+    HAVE_PATH_LIKE,
+    skipUnlessHaveNobody,
+)
 
 
 class TestSubvolume(BtrfsTestCase):
@@ -87,7 +92,7 @@ class TestSubvolume(BtrfsTestCase):
         finally:
             os.chdir(pwd)
 
-    def test_subvolume_info(self):
+    def _test_subvolume_info(self, subvol, snapshot):
         for arg in self.path_or_fd(self.mountpoint):
             with self.subTest(type=type(arg)):
                 info = btrfsutil.subvolume_info(arg)
@@ -100,7 +105,7 @@ class TestSubvolume(BtrfsTestCase):
                 self.assertEqual(info.parent_uuid, bytes(16))
                 self.assertEqual(info.received_uuid, bytes(16))
                 self.assertNotEqual(info.generation, 0)
-                self.assertEqual(info.ctransid, 0)
+                self.assertGreaterEqual(info.ctransid, 0)
                 self.assertEqual(info.otransid, 0)
                 self.assertEqual(info.stransid, 0)
                 self.assertEqual(info.rtransid, 0)
@@ -109,9 +114,6 @@ class TestSubvolume(BtrfsTestCase):
                 self.assertEqual(info.stime, 0)
                 self.assertEqual(info.rtime, 0)
 
-        subvol = os.path.join(self.mountpoint, 'subvol')
-        btrfsutil.create_subvolume(subvol)
-
         info = btrfsutil.subvolume_info(subvol)
         self.assertEqual(info.id, 256)
         self.assertEqual(info.parent_id, 5)
@@ -132,19 +134,43 @@ class TestSubvolume(BtrfsTestCase):
         self.assertEqual(info.rtime, 0)
 
         subvol_uuid = info.uuid
-        snapshot = os.path.join(self.mountpoint, 'snapshot')
-        btrfsutil.create_snapshot(subvol, snapshot)
 
         info = btrfsutil.subvolume_info(snapshot)
         self.assertEqual(info.parent_uuid, subvol_uuid)
 
         # TODO: test received_uuid, stransid, rtransid, stime, and rtime
 
+    def test_subvolume_info(self):
+        subvol = os.path.join(self.mountpoint, 'subvol')
+        btrfsutil.create_subvolume(subvol)
+        snapshot = os.path.join(self.mountpoint, 'snapshot')
+        btrfsutil.create_snapshot(subvol, snapshot)
+
+        self._test_subvolume_info(subvol, snapshot)
+
         for arg in self.path_or_fd(self.mountpoint):
             with self.subTest(type=type(arg)):
                 with self.assertRaises(btrfsutil.BtrfsUtilError) as e:
                     # BTRFS_EXTENT_TREE_OBJECTID
                     btrfsutil.subvolume_info(arg, 2)
+                self.assertEqual(e.exception.btrfsutilerror,
+                                 btrfsutil.ERROR_SUBVOLUME_NOT_FOUND)
+
+    @skipUnlessHaveNobody
+    def test_subvolume_info_unprivileged(self):
+        subvol = os.path.join(self.mountpoint, 'subvol')
+        btrfsutil.create_subvolume(subvol)
+        snapshot = os.path.join(self.mountpoint, 'snapshot')
+        btrfsutil.create_snapshot(subvol, snapshot)
+
+        with drop_privs():
+            try:
+                self._test_subvolume_info(subvol, snapshot)
+            except OSError as e:
+                if e.errno == errno.ENOTTY:
+                    self.skipTest('BTRFS_IOC_GET_SUBVOL_INFO is not available')
+                else:
+                    raise
 
     def test_read_only(self):
         for arg in self.path_or_fd(self.mountpoint):

libbtrfsutil/subvolume.c

@@ -31,6 +31,11 @@
 
 #include "btrfsutil_internal.h"
 
+static bool is_root(void)
+{
+	return geteuid() == 0;
+}
+
 /*
  * This intentionally duplicates btrfs_util_is_subvolume_fd() instead of opening
  * a file descriptor and calling it, because fstat() and fstatfs() don't accept
@@ -295,8 +300,8 @@ PUBLIC enum btrfs_util_error btrfs_util_subvolume_info(const char *path,
 	return err;
 }
 
-static enum btrfs_util_error get_subvolume_info_root(int fd, uint64_t id,
+static enum btrfs_util_error get_subvolume_info_privileged(int fd, uint64_t id,
-						     struct btrfs_util_subvolume_info *subvol)
+							   struct btrfs_util_subvolume_info *subvol)
 {
 	struct btrfs_ioctl_search_args search = {
 		.key = {
@@ -383,6 +388,45 @@ static enum btrfs_util_error get_subvolume_info_root(int fd, uint64_t id,
 	return BTRFS_UTIL_OK;
 }
 
+static enum btrfs_util_error get_subvolume_info_unprivileged(int fd,
+							     struct btrfs_util_subvolume_info *subvol)
+{
+	struct btrfs_ioctl_get_subvol_info_args info;
+	int ret;
+
+	ret = ioctl(fd, BTRFS_IOC_GET_SUBVOL_INFO, &info);
+	if (ret == -1)
+		return BTRFS_UTIL_ERROR_GET_SUBVOL_INFO_FAILED;
+
+	subvol->id = info.treeid;
+	subvol->parent_id = info.parent_id;
+	subvol->dir_id = info.dirid;
+	subvol->flags = info.flags;
+	subvol->generation = info.generation;
+
+	memcpy(subvol->uuid, info.uuid, sizeof(subvol->uuid));
+	memcpy(subvol->parent_uuid, info.parent_uuid,
+	       sizeof(subvol->parent_uuid));
+	memcpy(subvol->received_uuid, info.received_uuid,
+	       sizeof(subvol->received_uuid));
+
+	subvol->ctransid = info.ctransid;
+	subvol->otransid = info.otransid;
+	subvol->stransid = info.stransid;
+	subvol->rtransid = info.rtransid;
+
+	subvol->ctime.tv_sec = info.ctime.sec;
+	subvol->ctime.tv_nsec = info.ctime.nsec;
+	subvol->otime.tv_sec = info.otime.sec;
+	subvol->otime.tv_nsec = info.otime.nsec;
+	subvol->stime.tv_sec = info.stime.sec;
+	subvol->stime.tv_nsec = info.stime.nsec;
+	subvol->rtime.tv_sec = info.rtime.sec;
+	subvol->rtime.tv_nsec = info.rtime.nsec;
+
+	return BTRFS_UTIL_OK;
+}
+
 PUBLIC enum btrfs_util_error btrfs_util_subvolume_info_fd(int fd, uint64_t id,
 							  struct btrfs_util_subvolume_info *subvol)
 {
@@ -393,6 +437,9 @@ PUBLIC enum btrfs_util_error btrfs_util_subvolume_info_fd(int fd, uint64_t id,
 		if (err)
 			return err;
 
+		if (!is_root())
+			return get_subvolume_info_unprivileged(fd, subvol);
+
 		err = btrfs_util_subvolume_id_fd(fd, &id);
 		if (err)
 			return err;
@@ -404,7 +451,7 @@ PUBLIC enum btrfs_util_error btrfs_util_subvolume_info_fd(int fd, uint64_t id,
 		return BTRFS_UTIL_ERROR_SUBVOLUME_NOT_FOUND;
 	}
 
-	return get_subvolume_info_root(fd, id, subvol);
+	return get_subvolume_info_privileged(fd, id, subvol);
 }
 
 PUBLIC enum btrfs_util_error btrfs_util_get_subvolume_read_only_fd(int fd,