namespace: chase symlinks for mounts to set up in userspace

This adds logic to chase symlinks for all mount points that shall be created in
a namespace environment in userspace, instead of leaving this to the kernel.
This has the advantage that we can correctly handle absolute symlinks that
shall be taken relative to a specific root directory. Moreover, we can properly
handle mounts created on symlinked files or directories as we can merge their
mounts as necessary.

(This also drops the "done" flag in the namespace logic, which was never
actually working, but was supposed to permit a partial rollback of the
namespace logic, which however is only mildly useful as it wasn't clear in
which case it would or would not be able to roll back.)

Fixes: #3867

src/basic/fs-util.c

@@ -597,3 +597,190 @@ int inotify_add_watch_fd(int fd, int what, uint32_t mask) {
 
         return r;
 }
+
+int chase_symlinks(const char *path, const char *_root, char **ret) {
+        _cleanup_free_ char *buffer = NULL, *done = NULL, *root = NULL;
+        _cleanup_close_ int fd = -1;
+        unsigned max_follow = 32; /* how many symlinks to follow before giving up and returning ELOOP */
+        char *todo;
+        int r;
+
+        assert(path);
+
+        /* This is a lot like canonicalize_file_name(), but takes an additional "root" parameter, that allows following
+         * symlinks relative to a root directory, instead of the root of the host.
+         *
+         * Note that "root" matters only if we encounter an absolute symlink, it's unused otherwise. Most importantly
+         * this means the path parameter passed in is not prefixed by it.
+         *
+         * Algorithmically this operates on two path buffers: "done" are the components of the path we already
+         * processed and resolved symlinks, "." and ".." of. "todo" are the components of the path we still need to
+         * process. On each iteration, we move one component from "todo" to "done", processing it's special meaning
+         * each time. The "todo" path always starts with at least one slash, the "done" path always ends in no
+         * slash. We always keep an O_PATH fd to the component we are currently processing, thus keeping lookup races
+         * at a minimum. */
+
+        r = path_make_absolute_cwd(path, &buffer);
+        if (r < 0)
+                return r;
+
+        if (_root) {
+                r = path_make_absolute_cwd(_root, &root);
+                if (r < 0)
+                        return r;
+        }
+
+        fd = open("/", O_CLOEXEC|O_NOFOLLOW|O_PATH);
+        if (fd < 0)
+                return -errno;
+
+        todo = buffer;
+        for (;;) {
+                _cleanup_free_ char *first = NULL;
+                _cleanup_close_ int child = -1;
+                struct stat st;
+                size_t n, m;
+
+                /* Determine length of first component in the path */
+                n = strspn(todo, "/");                  /* The slashes */
+                m = n + strcspn(todo + n, "/");         /* The entire length of the component */
+
+                /* Extract the first component. */
+                first = strndup(todo, m);
+                if (!first)
+                        return -ENOMEM;
+
+                todo += m;
+
+                /* Just a single slash? Then we reached the end. */
+                if (isempty(first) || path_equal(first, "/"))
+                        break;
+
+                /* Just a dot? Then let's eat this up. */
+                if (path_equal(first, "/."))
+                        continue;
+
+                /* Two dots? Then chop off the last bit of what we already found out. */
+                if (path_equal(first, "/..")) {
+                        _cleanup_free_ char *parent = NULL;
+                        int fd_parent = -1;
+
+                        if (isempty(done) || path_equal(done, "/"))
+                                return -EINVAL;
+
+                        parent = dirname_malloc(done);
+                        if (!parent)
+                                return -ENOMEM;
+
+                        /* Don't allow this to leave the root dir */
+                        if (root &&
+                            path_startswith(done, root) &&
+                            !path_startswith(parent, root))
+                                return -EINVAL;
+
+                        free(done);
+                        done = parent;
+                        parent = NULL;
+
+                        fd_parent = openat(fd, "..", O_CLOEXEC|O_NOFOLLOW|O_PATH);
+                        if (fd_parent < 0)
+                                return -errno;
+
+                        safe_close(fd);
+                        fd = fd_parent;
+
+                        continue;
+                }
+
+                /* Otherwise let's see what this is. */
+                child = openat(fd, first + n, O_CLOEXEC|O_NOFOLLOW|O_PATH);
+                if (child < 0)
+                        return -errno;
+
+                if (fstat(child, &st) < 0)
+                        return -errno;
+
+                if (S_ISLNK(st.st_mode)) {
+                        _cleanup_free_ char *destination = NULL;
+
+                        /* This is a symlink, in this case read the destination. But let's make sure we don't follow
+                         * symlinks without bounds. */
+                        if (--max_follow <= 0)
+                                return -ELOOP;
+
+                        r = readlinkat_malloc(fd, first + n, &destination);
+                        if (r < 0)
+                                return r;
+                        if (isempty(destination))
+                                return -EINVAL;
+
+                        if (path_is_absolute(destination)) {
+
+                                /* An absolute destination. Start the loop from the beginning, but use the root
+                                 * directory as base. */
+
+                                safe_close(fd);
+                                fd = open(root ?: "/", O_CLOEXEC|O_NOFOLLOW|O_PATH);
+                                if (fd < 0)
+                                        return -errno;
+
+                                free(buffer);
+                                buffer = destination;
+                                destination = NULL;
+
+                                todo = buffer;
+                                free(done);
+
+                                /* Note that we do not revalidate the root, we take it as is. */
+                                if (isempty(root))
+                                        done = NULL;
+                                else {
+                                        done = strdup(root);
+                                        if (!done)
+                                                return -ENOMEM;
+                                }
+
+                        } else {
+                                char *joined;
+
+                                /* A relative destination. If so, this is what we'll prefix what's left to do with what
+                                 * we just read, and start the loop again, but remain in the current directory. */
+
+                                joined = strjoin("/", destination, todo, NULL);
+                                if (!joined)
+                                        return -ENOMEM;
+
+                                free(buffer);
+                                todo = buffer = joined;
+                        }
+
+                        continue;
+                }
+
+                /* If this is not a symlink, then let's just add the name we read to what we already verified. */
+                if (!done) {
+                        done = first;
+                        first = NULL;
+                } else {
+                        if (!strextend(&done, first, NULL))
+                                return -ENOMEM;
+                }
+
+                /* And iterate again, but go one directory further down. */
+                safe_close(fd);
+                fd = child;
+                child = -1;
+        }
+
+        if (!done) {
+                /* Special case, turn the empty string into "/", to indicate the root directory. */
+                done = strdup("/");
+                if (!done)
+                        return -ENOMEM;
+        }
+
+        *ret = done;
+        done = NULL;
+
+        return 0;
+}

src/basic/fs-util.h

@@ -77,3 +77,5 @@ union inotify_event_buffer {
 };
 
 int inotify_add_watch_fd(int fd, int what, uint32_t mask);
+
+int chase_symlinks(const char *path, const char *_root, char **ret);