udev: don't force device ownership and mode on every event

This partially reverts 25de7aa7b90c23d33ea50ada1e50c5834a414237. I don't think the
change was intended there.

The problem I'm trying to solve: for /dev/kvm we get first an ADD uevent, and
then CHANGE whenever something connects or disconnects to the character device.
The rules in 50-default-udev.rules set UID, GID, and MODE on ADD, but not on
CHANGE. When the change event happens, we would reset the ownership and
permissions.

This happens because node_permissions_apply() would (after 25de7aa7b90c23d33)
set uid=gid=0 if they weren't set by the rules.

So let's only pass uid/gid/mode to node_permissions_apply() if appropriately
configured. Also let node_permissions_apply() do the skip of uid/gid/mode if
not set, and rename "always_apply" to more closely reflect its meaning.

2 files changed, 23 insertions, 27 deletions

diff --git a/src/udev/udev-event.c b/src/udev/udev-event.c
index 025a37bc26..dbdb9065dc 100644
--- a/src/udev/udev-event.c
+++ b/src/udev/udev-event.c
@@ -819,7 +819,6 @@ static int rename_netif(UdevEvent *event) {
 
 static int update_devnode(UdevEvent *event) {
         sd_device *dev = event->dev;
-        bool apply;
         int r;
 
         r = sd_device_get_devnum(dev, NULL);
@@ -834,17 +833,13 @@ static int update_devnode(UdevEvent *event) {
 
         if (!uid_is_valid(event->uid)) {
                 r = device_get_devnode_uid(dev, &event->uid);
-                if (r == -ENOENT)
-                        event->uid = 0;
-                else if (r < 0)
+                if (r < 0 && r != -ENOENT)
                         return log_device_error_errno(dev, r, "Failed to get devnode UID: %m");
         }
 
         if (!gid_is_valid(event->gid)) {
                 r = device_get_devnode_gid(dev, &event->gid);
-                if (r == -ENOENT)
-                        event->gid = 0;
-                else if (r < 0)
+                if (r < 0 && r != -ENOENT)
                         return log_device_error_errno(dev, r, "Failed to get devnode GID: %m");
         }
 
@@ -852,21 +847,14 @@ static int update_devnode(UdevEvent *event) {
                 r = device_get_devnode_mode(dev, &event->mode);
                 if (r < 0 && r != -ENOENT)
                         return log_device_error_errno(dev, r, "Failed to get devnode mode: %m");
-                if (r == -ENOENT) {
-                        if (event->gid > 0)
-                                /* default 0660 if a group is assigned */
-                                event->mode = 0660;
-                        else
-                                /* default 0600 */
-                                event->mode = 0600;
-                }
         }
+        if (event->mode == MODE_INVALID && gid_is_valid(event->gid) && event->gid > 0)
+                /* If group is set, but mode is not set, "upgrade" mode for the group. */
+                event->mode = 0660;
+
+        bool apply_mac = device_for_action(dev, DEVICE_ACTION_ADD);
 
-        apply = device_for_action(dev, DEVICE_ACTION_ADD) ||
-                uid_is_valid(event->uid) ||
-                gid_is_valid(event->gid) ||
-                event->mode != MODE_INVALID;
-        return udev_node_add(dev, apply, event->mode, event->uid, event->gid, event->seclabel_list);
+        return udev_node_add(dev, apply_mac, event->mode, event->uid, event->gid, event->seclabel_list);
 }
 
 static void event_execute_rules_on_remove(
diff --git a/src/udev/udev-node.c b/src/udev/udev-node.c
index 01cd2f408a..7e3447f7fa 100644
--- a/src/udev/udev-node.c
+++ b/src/udev/udev-node.c
@@ -26,6 +26,7 @@
 #include "string-util.h"
 #include "strxcpyx.h"
 #include "udev-node.h"
+#include "user-util.h"
 
 static int node_symlink(sd_device *dev, const char *node, const char *slink) {
         _cleanup_free_ char *slink_dirname = NULL, *target = NULL;
@@ -270,13 +271,14 @@ int udev_node_update_old_links(sd_device *dev, sd_device *dev_old) {
         return 0;
 }
 
-static int node_permissions_apply(sd_device *dev, bool apply,
+static int node_permissions_apply(sd_device *dev, bool apply_mac,
                                   mode_t mode, uid_t uid, gid_t gid,
                                   OrderedHashmap *seclabel_list) {
         const char *devnode, *subsystem, *id_filename = NULL;
         struct stat stats;
         dev_t devnum;
-        int r = 0;
+        bool apply_mode, apply_uid, apply_gid;
+        int r;
 
         assert(dev);
 
@@ -299,21 +301,27 @@ static int node_permissions_apply(sd_device *dev, bool apply,
         if (lstat(devnode, &stats) < 0)
                 return log_device_debug_errno(dev, errno, "cannot stat() node %s: %m", devnode);
 
-        if (((stats.st_mode & S_IFMT) != (mode & S_IFMT)) || (stats.st_rdev != devnum))
-                return log_device_debug_errno(dev, SYNTHETIC_ERRNO(EEXIST), "Found node '%s' with non-matching devnum %s, skip handling",
+        if ((mode != MODE_INVALID && (stats.st_mode & S_IFMT) != (mode & S_IFMT)) || stats.st_rdev != devnum)
+                return log_device_debug_errno(dev, SYNTHETIC_ERRNO(EEXIST),
+                                              "Found node '%s' with non-matching devnum %s, skip handling",
                                               devnode, id_filename);
 
-        if (apply) {
+        apply_mode = mode != MODE_INVALID && (stats.st_mode & 0777) != (mode & 0777);
+        apply_uid = uid_is_valid(uid) && stats.st_uid != uid;
+        apply_gid = gid_is_valid(gid) && stats.st_gid != gid;
+
+        if (apply_mode || apply_uid || apply_gid || apply_mac) {
                 bool selinux = false, smack = false;
                 const char *name, *label;
                 Iterator i;
 
-                if ((stats.st_mode & 0777) != (mode & 0777) || stats.st_uid != uid || stats.st_gid != gid) {
+                if (apply_mode || apply_uid || apply_gid) {
                         log_device_debug(dev, "Setting permissions %s, %#o, uid=%u, gid=%u", devnode, mode, uid, gid);
 
                         r = chmod_and_chown(devnode, mode, uid, gid);
                         if (r < 0)
-                                log_device_warning_errno(dev, r, "Failed to set owner/mode of %s to uid=" UID_FMT ", gid=" GID_FMT ", mode=%#o: %m", devnode, uid, gid, mode);
+                                log_device_warning_errno(dev, r, "Failed to set owner/mode of %s to uid=" UID_FMT ", gid=" GID_FMT ", mode=%#o: %m",
+                                                         devnode, uid, gid, mode);
                 } else
                         log_device_debug(dev, "Preserve permissions of %s, %#o, uid=%u, gid=%u", devnode, mode, uid, gid);