Merge pull request #26820 from DaanDeMeyer/dir-fd-is-root

fd-util: Add dir_fd_is_root()

src/basic/fd-util.c

@@ -21,6 +21,7 @@
 #include "missing_fcntl.h"
 #include "missing_fs.h"
 #include "missing_syscall.h"
+#include "mountpoint-util.h"
 #include "parse-util.h"
 #include "path-util.h"
 #include "process-util.h"
@@ -864,3 +865,54 @@ int fd_get_diskseq(int fd, uint64_t *ret) {
 
         return 0;
 }
+
+int dir_fd_is_root(int dir_fd) {
+        STRUCT_NEW_STATX_DEFINE(st);
+        STRUCT_NEW_STATX_DEFINE(pst);
+        int r;
+
+        assert(dir_fd >= 0);
+
+        r = statx_fallback(dir_fd, ".", 0, STATX_TYPE|STATX_INO|STATX_MNT_ID, &st.sx);
+        if (r == -ENOTDIR)
+                return false;
+        if (r < 0)
+                return r;
+
+        if (!FLAGS_SET(st.nsx.stx_mask, STATX_MNT_ID)) {
+                int mntid;
+
+                r = path_get_mnt_id_at(dir_fd, "", &mntid);
+                if (r < 0)
+                        return r;
+                assert(mntid >= 0);
+
+                st.nsx.stx_mnt_id = mntid;
+                st.nsx.stx_mask |= STATX_MNT_ID;
+        }
+
+        r = statx_fallback(dir_fd, "..", 0, STATX_TYPE|STATX_INO|STATX_MNT_ID, &pst.sx);
+        if (r < 0)
+                return r;
+
+        if (!FLAGS_SET(pst.nsx.stx_mask, STATX_MNT_ID)) {
+                int mntid;
+
+                r = path_get_mnt_id_at(dir_fd, "..", &mntid);
+                if (r < 0)
+                        return r;
+                assert(mntid >= 0);
+
+                pst.nsx.stx_mnt_id = mntid;
+                pst.nsx.stx_mask |= STATX_MNT_ID;
+        }
+
+        /* If the parent directory is the same inode, the fd points to the root directory "/". We also check
+         * that the mount ids are the same. Otherwise, a construct like the following could be used to trick
+         * us:
+         *
+         * $ mkdir /tmp/x /tmp/x/y
+         * $ mount --bind /tmp/x /tmp/x/y
+         */
+        return statx_inode_same(&st.sx, &pst.sx) && statx_mount_same(&st.nsx, &pst.nsx);
+}

src/basic/fd-util.h

@@ -100,6 +100,8 @@ int fd_is_opath(int fd);
 int read_nr_open(void);
 int fd_get_diskseq(int fd, uint64_t *ret);
 
+int dir_fd_is_root(int dir_fd);
+
 /* The maximum length a buffer for a /proc/self/fd/<fd> path needs */
 #define PROC_FD_PATH_MAX \
         (STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int))

src/basic/mountpoint-util.c

@@ -355,11 +355,19 @@ int path_is_mount_point(const char *t, const char *root, int flags) {
         return fd_is_mount_point(fd, last_path_component(t), flags);
 }
 
-int path_get_mnt_id(const char *path, int *ret) {
+int path_get_mnt_id_at(int dir_fd, const char *path, int *ret) {
         STRUCT_NEW_STATX_DEFINE(buf);
         int r;
 
-        if (statx(AT_FDCWD, path, AT_SYMLINK_NOFOLLOW|AT_NO_AUTOMOUNT, STATX_MNT_ID, &buf.sx) < 0) {
+        assert(dir_fd >= 0 || dir_fd == AT_FDCWD);
+        assert(path);
+        assert(ret);
+
+        if (statx(dir_fd,
+                  path,
+                  AT_NO_AUTOMOUNT|(isempty(path) ? AT_EMPTY_PATH : AT_SYMLINK_NOFOLLOW),
+                  STATX_MNT_ID,
+                  &buf.sx) < 0) {
                 if (!ERRNO_IS_NOT_SUPPORTED(errno) && !ERRNO_IS_PRIVILEGE(errno))
                         return -errno;
 
@@ -371,11 +379,11 @@ int path_get_mnt_id(const char *path, int *ret) {
                 return 0;
         }
 
-        r = name_to_handle_at_loop(AT_FDCWD, path, NULL, ret, 0);
+        r = name_to_handle_at_loop(dir_fd, path, NULL, ret, isempty(path) ? AT_EMPTY_PATH : 0);
         if (r == 0 || is_name_to_handle_at_fatal_error(r))
                 return r;
 
-        return fd_fdinfo_mnt_id(AT_FDCWD, path, 0, ret);
+        return fd_fdinfo_mnt_id(dir_fd, path, isempty(path) ? AT_EMPTY_PATH : 0, ret);
 }
 
 bool fstype_is_network(const char *fstype) {

src/basic/mountpoint-util.h

@@ -37,7 +37,10 @@
 
 int name_to_handle_at_loop(int fd, const char *path, struct file_handle **ret_handle, int *ret_mnt_id, int flags);
 
-int path_get_mnt_id(const char *path, int *ret);
+int path_get_mnt_id_at(int dir_fd, const char *path, int *ret);
+static inline int path_get_mnt_id(const char *path, int *ret) {
+        return path_get_mnt_id_at(AT_FDCWD, path, ret);
+}
 
 int fd_is_mount_point(int fd, const char *filename, int flags);
 int path_is_mount_point(const char *path, const char *root, int flags);

src/test/test-fd-util.c

@@ -569,4 +569,16 @@ TEST(take_fd) {
         assert_se(array[1] == -EBADF);
 }
 
+TEST(dir_fd_is_root) {
+        _cleanup_close_ int fd = -EBADF;
+
+        assert_se((fd = open("/", O_CLOEXEC|O_PATH|O_DIRECTORY|O_NOFOLLOW)) >= 0);
+        assert_se(dir_fd_is_root(fd) > 0);
+
+        fd = safe_close(fd);
+
+        assert_se((fd = open("/usr", O_CLOEXEC|O_PATH|O_DIRECTORY|O_NOFOLLOW)) >= 0);
+        assert_se(dir_fd_is_root(fd) == 0);
+}
+
 DEFINE_TEST_MAIN(LOG_DEBUG);