tmpfiles: add '=' action modifier.

Add the '=' action modifier that instructs tmpfiles.d to check the file
type of a path and remove objects that do not match before trying to
open or create the path.

BUG=chromium:1186405
TEST=./test/test-systemd-tmpfiles.py "$(which systemd-tmpfiles)"

Change-Id: If807dc0db427393e9e0047aba640d0d114897c26

src/tmpfiles/tmpfiles.c

@@ -142,6 +142,8 @@ typedef struct Item {
 
         bool allow_failure:1;
 
+        bool try_replace:1;
+
         OperationMask done;
 } Item;
 
@@ -2002,6 +2004,180 @@ static int glob_item_recursively(Item *i, fdaction_t action) {
         return r;
 }
 
+static int rm_if_wrong_type_safe(
+                mode_t mode,
+                int parent_fd,
+                const struct stat *parent_st /* Only used if follow is true. */,
+                const char *name,
+                int flags) {
+        _cleanup_free_ char *parent_name = NULL;
+        bool follow_links = !FLAGS_SET(flags, AT_SYMLINK_NOFOLLOW);
+        struct stat st;
+        int r;
+
+        assert(name);
+        assert((mode & ~S_IFMT) == 0);
+        assert(!follow_links || parent_st);
+        assert((flags & ~AT_SYMLINK_NOFOLLOW) == 0);
+
+        if (!filename_is_valid(name))
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "\"%s\" is not a valid filename.", name);
+
+        r = fstatat_harder(parent_fd, name, &st, flags, REMOVE_CHMOD | REMOVE_CHMOD_RESTORE);
+        if (r < 0) {
+                (void) fd_get_path(parent_fd, &parent_name);
+                return log_full_errno(r == -ENOENT? LOG_DEBUG : LOG_ERR, r,
+                              "Failed to stat \"%s\" at \"%s\": %m", name, strna(parent_name));
+        }
+
+        /* Fail before removing anything if this is an unsafe transition. */
+        if (follow_links && unsafe_transition(parent_st, &st)) {
+                (void) fd_get_path(parent_fd, &parent_name);
+                return log_error_errno(SYNTHETIC_ERRNO(ENOLINK),
+                                "Unsafe transition from \"%s\" to \"%s\".", parent_name, name);
+        }
+
+        if ((st.st_mode & S_IFMT) == mode)
+                return 0;
+
+        (void) fd_get_path(parent_fd, &parent_name);
+        log_notice("Wrong file type 0x%x; rm -rf \"%s/%s\"", st.st_mode & S_IFMT, strna(parent_name), name);
+
+        /* If the target of the symlink was the wrong type, the link needs to be removed instead of the
+         * target, so make sure it is identified as a link and not a directory. */
+        if (follow_links) {
+                r = fstatat_harder(parent_fd, name, &st, AT_SYMLINK_NOFOLLOW, REMOVE_CHMOD | REMOVE_CHMOD_RESTORE);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to stat \"%s\" at \"%s\": %m", name, strna(parent_name));
+        }
+
+        /* Do not remove mount points. */
+        r = fd_is_mount_point(parent_fd, name, follow_links ? AT_SYMLINK_FOLLOW : 0);
+        if (r < 0)
+                (void) log_warning_errno(r, "Failed to check if  \"%s/%s\" is a mount point: %m; Continuing",
+                                strna(parent_name), name);
+        else if (r > 0)
+                return log_error_errno(SYNTHETIC_ERRNO(EBUSY),
+                                "Not removing  \"%s/%s\" because it is a mount point.", strna(parent_name), name);
+
+        if ((st.st_mode & S_IFMT) == S_IFDIR) {
+                _cleanup_close_ int child_fd = -1;
+
+                child_fd = openat(parent_fd, name, O_NOCTTY | O_CLOEXEC | O_DIRECTORY);
+                if (child_fd < 0)
+                        return log_error_errno(errno, "Failed to open \"%s\" at \"%s\": %m", name, strna(parent_name));
+
+                r = rm_rf_children(TAKE_FD(child_fd), REMOVE_ROOT|REMOVE_SUBVOLUME|REMOVE_PHYSICAL, &st);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to remove contents of \"%s\" at \"%s\": %m", name, strna(parent_name));
+
+                r = unlinkat_harder(parent_fd, name, AT_REMOVEDIR, REMOVE_CHMOD | REMOVE_CHMOD_RESTORE);
+        } else
+                r = unlinkat_harder(parent_fd, name, 0, REMOVE_CHMOD | REMOVE_CHMOD_RESTORE);
+        if (r < 0)
+                return log_error_errno(r, "Failed to remove \"%s\" at \"%s\": %m", name, strna(parent_name));
+
+        /* This is covered by the log_notice "Wrong file type..." It is logged earlier because it gives
+         * context to other error messages that might follow. */
+        return -ENOENT;
+}
+
+/* If child_mode is non-zero, rm_if_wrong_type_safe will be executed for the last path component. */
+static int mkdir_parents_rm_if_wrong_type(mode_t child_mode, const char *path) {
+        _cleanup_close_ int parent_fd = -1, next_fd = -1;
+        _cleanup_free_ char *parent_name = NULL;
+        struct stat parent_st;
+        const char *s, *e;
+        int r;
+        size_t path_len;
+
+        assert(path);
+        assert((child_mode & ~S_IFMT) == 0);
+
+        path_len = strlen(path);
+
+        if (!is_path(path))
+                /* rm_if_wrong_type_safe already logs errors. */
+                return child_mode != 0 ? rm_if_wrong_type_safe(child_mode, AT_FDCWD, NULL, path, AT_SYMLINK_NOFOLLOW) : 0;
+
+        if (child_mode != 0 && endswith(path, "/"))
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                "Trailing path separators are only allowed if child_mode is not set; got \"%s\"", path);
+
+        /* Get the parent_fd and stat. */
+        parent_fd = AT_FDCWD;
+        if (path_is_absolute(path)) {
+                parent_fd = open("/", O_NOCTTY | O_CLOEXEC | O_DIRECTORY);
+                if (parent_fd < 0)
+                        return log_error_errno(errno, "Failed to open root: %m");
+        }
+        if (fstat(parent_fd, &parent_st) < 0)
+                return log_error_errno(errno, "Failed to stat root: %m");
+
+        /* Check every parent directory in the path, except the last component */
+        e = path;
+        for (;;) {
+                char t[path_len + 1];
+
+                /* Find the start of the next path component. */
+                s = e + strspn(e, "/");
+                /* Find the end of the next path component. */
+                e = s + strcspn(s, "/");
+
+                /* Copy the path component to t so it can be a null terminated string. */
+                *((char*) mempcpy(t, s, e - s)) = 0;
+
+                /* Is this the last component? If so, then check the type */
+                if (*e == 0)
+                        return child_mode != 0 ? rm_if_wrong_type_safe(child_mode, parent_fd, &parent_st, t, AT_SYMLINK_NOFOLLOW) : 0;
+                else {
+                        r = rm_if_wrong_type_safe(S_IFDIR, parent_fd, &parent_st, t, 0);
+                        /* Remove dangling symlinks. */
+                        if (r == -ENOENT)
+                                r = rm_if_wrong_type_safe(S_IFDIR, parent_fd, &parent_st, t, AT_SYMLINK_NOFOLLOW);
+                }
+
+                if (r == -ENOENT) {
+                        RUN_WITH_UMASK(0000)
+                                r = mkdirat_label(parent_fd, t, 0755);
+                        if (r < 0) {
+                                (void) fd_get_path(parent_fd, &parent_name);
+                                return log_error_errno(r, "Failed to mkdir \"%s\" at \"%s\": %m", t, parent_name);
+                        }
+                } else if (r < 0)
+                        /* rm_if_wrong_type_safe already logs errors. */
+                        return r;
+
+                next_fd = openat(parent_fd, t, O_NOCTTY | O_CLOEXEC | O_DIRECTORY);
+                if (next_fd < 0) {
+                        r = -errno;
+                        (void) fd_get_path(parent_fd, &parent_name);
+                        return log_error_errno(r, "Failed to open \"%s\" at \"%s\": %m", t, parent_name);
+                }
+                if (fstat(next_fd, &parent_st) < 0) {
+                        r = -errno;
+                        (void) fd_get_path(parent_fd, &parent_name);
+                        return log_error_errno(r, "Failed to stat \"%s\" at \"%s\": %m", t, parent_name);
+                }
+
+                safe_close(parent_fd);
+                parent_fd = TAKE_FD(next_fd);
+        }
+}
+
+static int mkdir_parents_item(Item *i, mode_t child_mode) {
+        int r;
+        if (i->try_replace) {
+                r = mkdir_parents_rm_if_wrong_type(child_mode, i->path);
+                if (r < 0 && r != -ENOENT)
+                        return r;
+        } else
+                RUN_WITH_UMASK(0000)
+                        (void) mkdir_parents_label(i->path, 0755);
+
+        return 0;
+}
+
 static int create_item(Item *i) {
         CreationMode creation;
         int r = 0;
@@ -2020,8 +2196,9 @@ static int create_item(Item *i) {
 
         case TRUNCATE_FILE:
         case CREATE_FILE:
-                RUN_WITH_UMASK(0000)
-                        (void) mkdir_parents_label(i->path, 0755);
+                r = mkdir_parents_item(i, S_IFREG);
+                if (r < 0)
+                        return r;
 
                 if ((i->type == CREATE_FILE && i->append_or_force) || i->type == TRUNCATE_FILE)
                         r = truncate_file(i, i->path);
@@ -2033,8 +2210,9 @@ static int create_item(Item *i) {
                 break;
 
         case COPY_FILES:
-                RUN_WITH_UMASK(0000)
-                        (void) mkdir_parents_label(i->path, 0755);
+                r = mkdir_parents_item(i, 0);
+                if (r < 0)
+                        return r;
 
                 r = copy_files(i);
                 if (r < 0)
@@ -2050,8 +2228,9 @@ static int create_item(Item *i) {
 
         case CREATE_DIRECTORY:
         case TRUNCATE_DIRECTORY:
-                RUN_WITH_UMASK(0000)
-                        (void) mkdir_parents_label(i->path, 0755);
+                r = mkdir_parents_item(i, S_IFDIR);
+                if (r < 0)
+                        return r;
 
                 r = create_directory(i, i->path);
                 if (r < 0)
@@ -2061,8 +2240,9 @@ static int create_item(Item *i) {
         case CREATE_SUBVOLUME:
         case CREATE_SUBVOLUME_INHERIT_QUOTA:
         case CREATE_SUBVOLUME_NEW_QUOTA:
-                RUN_WITH_UMASK(0000)
-                        (void) mkdir_parents_label(i->path, 0755);
+                r = mkdir_parents_item(i, S_IFDIR);
+                if (r < 0)
+                        return r;
 
                 r = create_subvolume(i, i->path);
                 if (r < 0)
@@ -2076,8 +2256,9 @@ static int create_item(Item *i) {
                 break;
 
         case CREATE_FIFO:
-                RUN_WITH_UMASK(0000)
-                        (void) mkdir_parents_label(i->path, 0755);
+                r = mkdir_parents_item(i, S_IFIFO);
+                if (r < 0)
+                        return r;
 
                 r = create_fifo(i, i->path);
                 if (r < 0)
@@ -2085,8 +2266,9 @@ static int create_item(Item *i) {
                 break;
 
         case CREATE_SYMLINK: {
-                RUN_WITH_UMASK(0000)
-                        (void) mkdir_parents_label(i->path, 0755);
+                r = mkdir_parents_item(i, S_IFLNK);
+                if (r < 0)
+                        return r;
 
                 mac_selinux_create_file_prepare(i->path, S_IFLNK);
                 r = symlink(i->argument, i->path);
@@ -2142,8 +2324,9 @@ static int create_item(Item *i) {
                         return 0;
                 }
 
-                RUN_WITH_UMASK(0000)
-                        (void) mkdir_parents_label(i->path, 0755);
+                r = mkdir_parents_item(i, i->type == CREATE_BLOCK_DEVICE ? S_IFBLK : S_IFCHR);
+                if (r < 0)
+                        return r;
 
                 r = create_device(i, i->type == CREATE_BLOCK_DEVICE ? S_IFBLK : S_IFCHR);
                 if (r < 0)
@@ -2659,7 +2842,7 @@ static int parse_line(
         ItemArray *existing;
         OrderedHashmap *h;
         int r, pos;
-        bool append_or_force = false, boot = false, allow_failure = false;
+        bool append_or_force = false, boot = false, allow_failure = false, try_replace = false;
 
         assert(fname);
         assert(line >= 1);
@@ -2704,6 +2887,8 @@ static int parse_line(
                         append_or_force = true;
                 else if (action[pos] == '-' && !allow_failure)
                         allow_failure = true;
+                else if (action[pos] == '=' && !try_replace)
+                        try_replace = true;
                 else {
                         *invalid_config = true;
                         return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EBADMSG), "Unknown modifiers in command '%s'", action);
@@ -2718,6 +2903,7 @@ static int parse_line(
         i.type = action[0];
         i.append_or_force = append_or_force;
         i.allow_failure = allow_failure;
+        i.try_replace = try_replace;
 
         r = specifier_printf(path, PATH_MAX-1, specifier_table, NULL, &i.path);
         if (r == -ENXIO)