Adding upstream version 255.4.upstream/255.4

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1156 insertions, 0 deletions

diff --git a/src/basic/chase.c b/src/basic/chase.c
new file mode 100644
index 0000000..26bc2d6
--- /dev/null
+++ b/src/basic/chase.c
@@ -0,0 +1,1156 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <linux/magic.h>
+
+#include "alloc-util.h"
+#include "chase.h"
+#include "fd-util.h"
+#include "fileio.h"
+#include "fs-util.h"
+#include "glyph-util.h"
+#include "log.h"
+#include "path-util.h"
+#include "string-util.h"
+#include "user-util.h"
+
+bool unsafe_transition(const struct stat *a, const struct stat *b) {
+        /* Returns true if the transition from a to b is safe, i.e. that we never transition from unprivileged to
+         * privileged files or directories. Why bother? So that unprivileged code can't symlink to privileged files
+         * making us believe we read something safe even though it isn't safe in the specific context we open it in. */
+
+        if (a->st_uid == 0) /* Transitioning from privileged to unprivileged is always fine */
+                return false;
+
+        return a->st_uid != b->st_uid; /* Otherwise we need to stay within the same UID */
+}
+
+static int log_unsafe_transition(int a, int b, const char *path, ChaseFlags flags) {
+        _cleanup_free_ char *n1 = NULL, *n2 = NULL, *user_a = NULL, *user_b = NULL;
+        struct stat st;
+
+        if (!FLAGS_SET(flags, CHASE_WARN))
+                return -ENOLINK;
+
+        (void) fd_get_path(a, &n1);
+        (void) fd_get_path(b, &n2);
+
+        if (fstat(a, &st) == 0)
+                user_a = uid_to_name(st.st_uid);
+        if (fstat(b, &st) == 0)
+                user_b = uid_to_name(st.st_uid);
+
+        return log_warning_errno(SYNTHETIC_ERRNO(ENOLINK),
+                                 "Detected unsafe path transition %s (owned by %s) %s %s (owned by %s) during canonicalization of %s.",
+                                 strna(n1), strna(user_a), special_glyph(SPECIAL_GLYPH_ARROW_RIGHT), strna(n2), strna(user_b), path);
+}
+
+static int log_autofs_mount_point(int fd, const char *path, ChaseFlags flags) {
+        _cleanup_free_ char *n1 = NULL;
+
+        if (!FLAGS_SET(flags, CHASE_WARN))
+                return -EREMOTE;
+
+        (void) fd_get_path(fd, &n1);
+
+        return log_warning_errno(SYNTHETIC_ERRNO(EREMOTE),
+                                 "Detected autofs mount point %s during canonicalization of %s.",
+                                 strna(n1), path);
+}
+
+static int log_prohibited_symlink(int fd, ChaseFlags flags) {
+        _cleanup_free_ char *n1 = NULL;
+
+        assert(fd >= 0);
+
+        if (!FLAGS_SET(flags, CHASE_WARN))
+                return -EREMCHG;
+
+        (void) fd_get_path(fd, &n1);
+
+        return log_warning_errno(SYNTHETIC_ERRNO(EREMCHG),
+                                 "Detected symlink where not symlink is allowed at %s, refusing.",
+                                 strna(n1));
+}
+
+static int chaseat_needs_absolute(int dir_fd, const char *path) {
+        if (dir_fd < 0)
+                return path_is_absolute(path);
+
+        return dir_fd_is_root(dir_fd);
+}
+
+int chaseat(int dir_fd, const char *path, ChaseFlags flags, char **ret_path, int *ret_fd) {
+        _cleanup_free_ char *buffer = NULL, *done = NULL;
+        _cleanup_close_ int fd = -EBADF, root_fd = -EBADF;
+        unsigned max_follow = CHASE_MAX; /* how many symlinks to follow before giving up and returning ELOOP */
+        bool exists = true, append_trail_slash = false;
+        struct stat st; /* stat obtained from fd */
+        const char *todo;
+        int r;
+
+        assert(!FLAGS_SET(flags, CHASE_PREFIX_ROOT));
+        assert(!FLAGS_SET(flags, CHASE_STEP|CHASE_EXTRACT_FILENAME));
+        assert(!FLAGS_SET(flags, CHASE_TRAIL_SLASH|CHASE_EXTRACT_FILENAME));
+        assert(!FLAGS_SET(flags, CHASE_MKDIR_0755) || (flags & (CHASE_NONEXISTENT | CHASE_PARENT)) != 0);
+        assert(dir_fd >= 0 || dir_fd == AT_FDCWD);
+
+        /* Either the file may be missing, or we return an fd to the final object, but both make no sense */
+        if (FLAGS_SET(flags, CHASE_NONEXISTENT))
+                assert(!ret_fd);
+
+        if (FLAGS_SET(flags, CHASE_STEP))
+                assert(!ret_fd);
+
+        if (isempty(path))
+                path = ".";
+
+        /* This function resolves symlinks of the path relative to the given directory file descriptor. If
+         * CHASE_AT_RESOLVE_IN_ROOT is specified and a directory file descriptor is provided, symlinks
+         * are resolved relative to the given directory file descriptor. Otherwise, they are resolved
+         * relative to the root directory of the host.
+         *
+         * Note that when a positive directory file descriptor is provided and CHASE_AT_RESOLVE_IN_ROOT is
+         * specified and we find an absolute symlink, it is resolved relative to given directory file
+         * descriptor and not the root of the host. Also, when following relative symlinks, this functions
+         * ensures they cannot be used to "escape" the given directory file descriptor. If a positive
+         * directory file descriptor is provided, the "path" parameter is always interpreted relative to the
+         * given directory file descriptor, even if it is absolute. If the given directory file descriptor is
+         * AT_FDCWD and "path" is absolute, it is interpreted relative to the root directory of the host.
+         *
+         * When "dir_fd" points to a non-root directory and CHASE_AT_RESOLVE_IN_ROOT is set, this function
+         * always returns a relative path in "ret_path", even if "path" is an absolute path, because openat()
+         * like functions generally ignore the directory fd if they are provided with an absolute path. When
+         * CHASE_AT_RESOLVE_IN_ROOT is not set, then this returns relative path to the specified file
+         * descriptor if all resolved symlinks are relative, otherwise absolute path will be returned. When
+         * "dir_fd" is AT_FDCWD and "path" is an absolute path, we return an absolute path in "ret_path"
+         * because otherwise, if the caller passes the returned relative path to another openat() like
+         * function, it would be resolved relative to the current working directory instead of to "/".
+         *
+         * Summary about the result path:
+         * - "dir_fd" points to the root directory
+         *    → result will be absolute
+         * - "dir_fd" points to a non-root directory, and CHASE_AT_RESOLVE_IN_ROOT is set
+         *    → relative
+         * - "dir_fd" points to a non-root directory, and CHASE_AT_RESOLVE_IN_ROOT is not set
+         *    → relative when all resolved symlinks are relative, otherwise absolute
+         * - "dir_fd" is AT_FDCWD, and "path" is absolute
+         *    → absolute
+         * - "dir_fd" is AT_FDCWD, and "path" is relative
+         *    → relative when all resolved symlinks are relative, otherwise absolute
+         *
+         * 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
+         * its special meaning each time. We always keep an O_PATH fd to the component we are currently
+         * processing, thus keeping lookup races to a minimum.
+         *
+         * Suggested usage: whenever you want to canonicalize a path, use this function. Pass the absolute
+         * path you got as-is: fully qualified and relative to your host's root. Optionally, specify the
+         * "dir_fd" parameter to tell this function what to do when encountering a symlink with an absolute
+         * path as directory: resolve it relative to the given directory file descriptor.
+         *
+         * There are five ways to invoke this function:
+         *
+         * 1. Without CHASE_STEP or ret_fd: in this case the path is resolved and the normalized path is
+         *    returned in `ret_path`. The return value is < 0 on error. If CHASE_NONEXISTENT is also set, 0
+         *    is returned if the file doesn't exist, > 0 otherwise. If CHASE_NONEXISTENT is not set, >= 0 is
+         *    returned if the destination was found, -ENOENT if it wasn't.
+         *
+         * 2. With ret_fd: in this case the destination is opened after chasing it as O_PATH and this file
+         *    descriptor is returned as return value. This is useful to open files relative to some root
+         *    directory. Note that the returned O_PATH file descriptors must be converted into a regular one
+         *    (using fd_reopen() or such) before it can be used for reading/writing. ret_fd may not be
+         *    combined with CHASE_NONEXISTENT.
+         *
+         * 3. With CHASE_STEP: in this case only a single step of the normalization is executed, i.e. only
+         *    the first symlink or ".." component of the path is resolved, and the resulting path is
+         *    returned. This is useful if a caller wants to trace the path through the file system verbosely.
+         *    Returns < 0 on error, > 0 if the path is fully normalized, and == 0 for each normalization
+         *    step. This may be combined with CHASE_NONEXISTENT, in which case 1 is returned when a component
+         *    is not found.
+         *
+         * 4. With CHASE_SAFE: in this case the path must not contain unsafe transitions, i.e. transitions
+         *    from unprivileged to privileged files or directories. In such cases the return value is
+         *    -ENOLINK. If CHASE_WARN is also set, a warning describing the unsafe transition is emitted.
+         *    CHASE_WARN cannot be used in PID 1.
+         *
+         * 5. With CHASE_NO_AUTOFS: in this case if an autofs mount point is encountered, path normalization
+         *    is aborted and -EREMOTE is returned. If CHASE_WARN is also set, a warning showing the path of
+         *    the mount point is emitted. CHASE_WARN cannot be used in PID 1.
+         */
+
+        if (FLAGS_SET(flags, CHASE_AT_RESOLVE_IN_ROOT)) {
+                /* If we get AT_FDCWD or dir_fd points to "/", then we always resolve symlinks relative to
+                 * the host's root. Hence, CHASE_AT_RESOLVE_IN_ROOT is meaningless. */
+
+                r = dir_fd_is_root_or_cwd(dir_fd);
+                if (r < 0)
+                        return r;
+                if (r > 0)
+                        flags &= ~CHASE_AT_RESOLVE_IN_ROOT;
+        }
+
+        if (!(flags &
+              (CHASE_AT_RESOLVE_IN_ROOT|CHASE_NONEXISTENT|CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_STEP|
+               CHASE_PROHIBIT_SYMLINKS|CHASE_MKDIR_0755)) &&
+            !ret_path && ret_fd) {
+
+                /* Shortcut the ret_fd case if the caller isn't interested in the actual path and has no root
+                 * set and doesn't care about any of the other special features we provide either. */
+                r = openat(dir_fd, path, O_PATH|O_CLOEXEC|(FLAGS_SET(flags, CHASE_NOFOLLOW) ? O_NOFOLLOW : 0));
+                if (r < 0)
+                        return -errno;
+
+                *ret_fd = r;
+                return 0;
+        }
+
+        buffer = strdup(path);
+        if (!buffer)
+                return -ENOMEM;
+
+        /* If we receive an absolute path together with AT_FDCWD, we need to return an absolute path, because
+         * a relative path would be interpreted relative to the current working directory. Also, let's make
+         * the result absolute when the file descriptor of the root directory is specified. */
+        r = chaseat_needs_absolute(dir_fd, path);
+        if (r < 0)
+                return r;
+
+        bool need_absolute = r;
+        if (need_absolute) {
+                done = strdup("/");
+                if (!done)
+                        return -ENOMEM;
+        }
+
+        /* If a positive directory file descriptor is provided, always resolve the given path relative to it,
+         * regardless of whether it is absolute or not. If we get AT_FDCWD, follow regular openat()
+         * semantics, if the path is relative, resolve against the current working directory. Otherwise,
+         * resolve against root. */
+        fd = openat(dir_fd, done ?: ".", O_CLOEXEC|O_DIRECTORY|O_PATH);
+        if (fd < 0)
+                return -errno;
+
+        if (fstat(fd, &st) < 0)
+                return -errno;
+
+        /* If we get AT_FDCWD, we always resolve symlinks relative to the host's root. Only if a positive
+         * directory file descriptor is provided we will look at CHASE_AT_RESOLVE_IN_ROOT to determine
+         * whether to resolve symlinks in it or not. */
+        if (dir_fd >= 0 && FLAGS_SET(flags, CHASE_AT_RESOLVE_IN_ROOT))
+                root_fd = openat(dir_fd, ".", O_CLOEXEC|O_DIRECTORY|O_PATH);
+        else
+                root_fd = open("/", O_CLOEXEC|O_DIRECTORY|O_PATH);
+        if (root_fd < 0)
+                return -errno;
+
+        if (FLAGS_SET(flags, CHASE_TRAIL_SLASH))
+                append_trail_slash = ENDSWITH_SET(buffer, "/", "/.");
+
+        for (todo = buffer;;) {
+                _cleanup_free_ char *first = NULL;
+                _cleanup_close_ int child = -EBADF;
+                struct stat st_child;
+                const char *e;
+
+                r = path_find_first_component(&todo, /* accept_dot_dot= */ true, &e);
+                if (r < 0)
+                        return r;
+                if (r == 0) { /* We reached the end. */
+                        if (append_trail_slash)
+                                if (!strextend(&done, "/"))
+                                        return -ENOMEM;
+                        break;
+                }
+
+                first = strndup(e, r);
+                if (!first)
+                        return -ENOMEM;
+
+                /* Two dots? Then chop off the last bit of what we already found out. */
+                if (path_equal(first, "..")) {
+                        _cleanup_free_ char *parent = NULL;
+                        _cleanup_close_ int fd_parent = -EBADF;
+                        struct stat st_parent;
+
+                        /* If we already are at the top, then going up will not change anything. This is
+                         * in-line with how the kernel handles this. */
+                        if (empty_or_root(done) && FLAGS_SET(flags, CHASE_AT_RESOLVE_IN_ROOT)) {
+                                if (FLAGS_SET(flags, CHASE_STEP))
+                                        goto chased_one;
+                                continue;
+                        }
+
+                        fd_parent = openat(fd, "..", O_CLOEXEC|O_NOFOLLOW|O_PATH|O_DIRECTORY);
+                        if (fd_parent < 0)
+                                return -errno;
+
+                        if (fstat(fd_parent, &st_parent) < 0)
+                                return -errno;
+
+                        /* If we opened the same directory, that _may_ indicate that we're at the host root
+                         * directory. Let's confirm that in more detail with dir_fd_is_root(). And if so,
+                         * going up won't change anything. */
+                        if (stat_inode_same(&st_parent, &st)) {
+                                r = dir_fd_is_root(fd);
+                                if (r < 0)
+                                        return r;
+                                if (r > 0) {
+                                        if (FLAGS_SET(flags, CHASE_STEP))
+                                                goto chased_one;
+                                        continue;
+                                }
+                        }
+
+                        r = path_extract_directory(done, &parent);
+                        if (r >= 0) {
+                                assert(!need_absolute || path_is_absolute(parent));
+                                free_and_replace(done, parent);
+                        } else if (r == -EDESTADDRREQ) {
+                                /* 'done' contains filename only (i.e. no slash). */
+                                assert(!need_absolute);
+                                done = mfree(done);
+                        } else if (r == -EADDRNOTAVAIL) {
+                                /* 'done' is "/". This branch should be already handled in the above. */
+                                assert(!FLAGS_SET(flags, CHASE_AT_RESOLVE_IN_ROOT));
+                                assert_not_reached();
+                        } else if (r == -EINVAL) {
+                                /* 'done' is an empty string, ends with '..', or an invalid path. */
+                                assert(!need_absolute);
+                                assert(!FLAGS_SET(flags, CHASE_AT_RESOLVE_IN_ROOT));
+
+                                if (!path_is_valid(done))
+                                        return -EINVAL;
+
+                                /* If we're at the top of "dir_fd", start appending ".." to "done". */
+                                if (!path_extend(&done, ".."))
+                                        return -ENOMEM;
+                        } else
+                                return r;
+
+                        if (FLAGS_SET(flags, CHASE_STEP))
+                                goto chased_one;
+
+                        if (FLAGS_SET(flags, CHASE_SAFE) &&
+                            unsafe_transition(&st, &st_parent))
+                                return log_unsafe_transition(fd, fd_parent, path, flags);
+
+                        /* If the path ends on a "..", and CHASE_PARENT is specified then our current 'fd' is
+                         * the child of the returned normalized path, not the parent as requested. To correct
+                         * this we have to go *two* levels up. */
+                        if (FLAGS_SET(flags, CHASE_PARENT) && isempty(todo)) {
+                                _cleanup_close_ int fd_grandparent = -EBADF;
+                                struct stat st_grandparent;
+
+                                fd_grandparent = openat(fd_parent, "..", O_CLOEXEC|O_NOFOLLOW|O_PATH|O_DIRECTORY);
+                                if (fd_grandparent < 0)
+                                        return -errno;
+
+                                if (fstat(fd_grandparent, &st_grandparent) < 0)
+                                        return -errno;
+
+                                if (FLAGS_SET(flags, CHASE_SAFE) &&
+                                    unsafe_transition(&st_parent, &st_grandparent))
+                                        return log_unsafe_transition(fd_parent, fd_grandparent, path, flags);
+
+                                st = st_grandparent;
+                                close_and_replace(fd, fd_grandparent);
+                                break;
+                        }
+
+                        /* update fd and stat */
+                        st = st_parent;
+                        close_and_replace(fd, fd_parent);
+                        continue;
+                }
+
+                /* Otherwise let's see what this is. */
+                child = r = RET_NERRNO(openat(fd, first, O_CLOEXEC|O_NOFOLLOW|O_PATH));
+                if (r < 0) {
+                        if (r != -ENOENT)
+                                return r;
+
+                        if (!isempty(todo) && !path_is_safe(todo))
+                                return r;
+
+                        if (FLAGS_SET(flags, CHASE_MKDIR_0755) && !isempty(todo)) {
+                                child = xopenat(fd,
+                                                first,
+                                                O_DIRECTORY|O_CREAT|O_EXCL|O_NOFOLLOW|O_CLOEXEC,
+                                                /* xopen_flags = */ 0,
+                                                0755);
+                                if (child < 0)
+                                        return child;
+                        } else if (FLAGS_SET(flags, CHASE_PARENT) && isempty(todo)) {
+                                if (!path_extend(&done, first))
+                                        return -ENOMEM;
+
+                                break;
+                        } else if (FLAGS_SET(flags, CHASE_NONEXISTENT)) {
+                                if (!path_extend(&done, first, todo))
+                                        return -ENOMEM;
+
+                                exists = false;
+                                break;
+                        } else
+                                return r;
+                }
+
+                if (fstat(child, &st_child) < 0)
+                        return -errno;
+
+                if (FLAGS_SET(flags, CHASE_SAFE) &&
+                    unsafe_transition(&st, &st_child))
+                        return log_unsafe_transition(fd, child, path, flags);
+
+                if (FLAGS_SET(flags, CHASE_NO_AUTOFS) &&
+                    fd_is_fs_type(child, AUTOFS_SUPER_MAGIC) > 0)
+                        return log_autofs_mount_point(child, path, flags);
+
+                if (S_ISLNK(st_child.st_mode) && !(FLAGS_SET(flags, CHASE_NOFOLLOW) && isempty(todo))) {
+                        _cleanup_free_ char *destination = NULL;
+
+                        if (FLAGS_SET(flags, CHASE_PROHIBIT_SYMLINKS))
+                                return log_prohibited_symlink(child, flags);
+
+                        /* 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, &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 file descriptor as base. */
+
+                                safe_close(fd);
+                                fd = fd_reopen(root_fd, O_CLOEXEC|O_PATH|O_DIRECTORY);
+                                if (fd < 0)
+                                        return fd;
+
+                                if (fstat(fd, &st) < 0)
+                                        return -errno;
+
+                                if (FLAGS_SET(flags, CHASE_SAFE) &&
+                                    unsafe_transition(&st_child, &st))
+                                        return log_unsafe_transition(child, fd, path, flags);
+
+                                /* When CHASE_AT_RESOLVE_IN_ROOT is not set, now the chased path may be
+                                 * outside of the specified dir_fd. Let's make the result absolute. */
+                                if (!FLAGS_SET(flags, CHASE_AT_RESOLVE_IN_ROOT))
+                                        need_absolute = true;
+
+                                r = free_and_strdup(&done, need_absolute ? "/" : NULL);
+                                if (r < 0)
+                                        return r;
+                        }
+
+                        /* Prefix what's left to do with what we just read, and start the loop again, but
+                         * remain in the current directory. */
+                        if (!path_extend(&destination, todo))
+                                return -ENOMEM;
+
+                        free_and_replace(buffer, destination);
+                        todo = buffer;
+
+                        if (FLAGS_SET(flags, CHASE_STEP))
+                                goto chased_one;
+
+                        continue;
+                }
+
+                /* If this is not a symlink, then let's just add the name we read to what we already verified. */
+                if (!path_extend(&done, first))
+                        return -ENOMEM;
+
+                if (FLAGS_SET(flags, CHASE_PARENT) && isempty(todo))
+                        break;
+
+                /* And iterate again, but go one directory further down. */
+                st = st_child;
+                close_and_replace(fd, child);
+        }
+
+        if (FLAGS_SET(flags, CHASE_PARENT)) {
+                r = stat_verify_directory(&st);
+                if (r < 0)
+                        return r;
+        }
+
+        if (ret_path) {
+                if (FLAGS_SET(flags, CHASE_EXTRACT_FILENAME) && done) {
+                        _cleanup_free_ char *f = NULL;
+
+                        r = path_extract_filename(done, &f);
+                        if (r < 0 && r != -EADDRNOTAVAIL)
+                                return r;
+
+                        /* If we get EADDRNOTAVAIL we clear done and it will get reinitialized by the next block. */
+                        free_and_replace(done, f);
+                }
+
+                if (!done) {
+                        assert(!need_absolute || FLAGS_SET(flags, CHASE_EXTRACT_FILENAME));
+                        done = strdup(append_trail_slash ? "./" : ".");
+                        if (!done)
+                                return -ENOMEM;
+                }
+
+                *ret_path = TAKE_PTR(done);
+        }
+
+        if (ret_fd) {
+                /* Return the O_PATH fd we currently are looking to the caller. It can translate it to a
+                 * proper fd by opening /proc/self/fd/xyz. */
+
+                assert(fd >= 0);
+                *ret_fd = TAKE_FD(fd);
+        }
+
+        if (FLAGS_SET(flags, CHASE_STEP))
+                return 1;
+
+        return exists;
+
+chased_one:
+        if (ret_path) {
+                const char *e;
+
+                if (!done) {
+                        assert(!need_absolute);
+                        done = strdup(append_trail_slash ? "./" : ".");
+                        if (!done)
+                                return -ENOMEM;
+                }
+
+                /* todo may contain slashes at the beginning. */
+                r = path_find_first_component(&todo, /* accept_dot_dot= */ true, &e);
+                if (r < 0)
+                        return r;
+                if (r == 0)
+                        *ret_path = TAKE_PTR(done);
+                else {
+                        char *c;
+
+                        c = path_join(done, e);
+                        if (!c)
+                                return -ENOMEM;
+
+                        *ret_path = c;
+                }
+        }
+
+        return 0;
+}
+
+static int empty_or_root_to_null(const char **path) {
+        int r;
+
+        assert(path);
+
+        /* This nullifies the input path when the path is empty or points to "/". */
+
+        if (empty_or_root(*path)) {
+                *path = NULL;
+                return 0;
+        }
+
+        r = path_is_root(*path);
+        if (r < 0)
+                return r;
+        if (r > 0)
+                *path = NULL;
+
+        return 0;
+}
+
+int chase(const char *path, const char *root, ChaseFlags flags, char **ret_path, int *ret_fd) {
+        _cleanup_free_ char *root_abs = NULL, *absolute = NULL, *p = NULL;
+        _cleanup_close_ int fd = -EBADF, pfd = -EBADF;
+        int r;
+
+        assert(path);
+
+        if (isempty(path))
+                return -EINVAL;
+
+        r = empty_or_root_to_null(&root);
+        if (r < 0)
+                return r;
+
+        /* A root directory of "/" or "" is identical to "/". */
+        if (empty_or_root(root)) {
+                root = "/";
+
+                /* When the root directory is "/", we will drop CHASE_AT_RESOLVE_IN_ROOT in chaseat(),
+                 * hence below is not necessary, but let's shortcut. */
+                flags &= ~CHASE_AT_RESOLVE_IN_ROOT;
+
+        } else {
+                r = path_make_absolute_cwd(root, &root_abs);
+                if (r < 0)
+                        return r;
+
+                /* Simplify the root directory, so that it has no duplicate slashes and nothing at the
+                 * end. While we won't resolve the root path we still simplify it. */
+                root = path_simplify(root_abs);
+
+                assert(path_is_absolute(root));
+                assert(!empty_or_root(root));
+
+                if (FLAGS_SET(flags, CHASE_PREFIX_ROOT)) {
+                        absolute = path_join(root, path);
+                        if (!absolute)
+                                return -ENOMEM;
+                }
+
+                flags |= CHASE_AT_RESOLVE_IN_ROOT;
+        }
+
+        if (!absolute) {
+                r = path_make_absolute_cwd(path, &absolute);
+                if (r < 0)
+                        return r;
+        }
+
+        path = path_startswith(absolute, root);
+        if (!path)
+                return log_full_errno(FLAGS_SET(flags, CHASE_WARN) ? LOG_WARNING : LOG_DEBUG,
+                                      SYNTHETIC_ERRNO(ECHRNG),
+                                      "Specified path '%s' is outside of specified root directory '%s', refusing to resolve.",
+                                      absolute, root);
+
+        fd = open(root, O_CLOEXEC|O_DIRECTORY|O_PATH);
+        if (fd < 0)
+                return -errno;
+
+        r = chaseat(fd, path, flags & ~CHASE_PREFIX_ROOT, ret_path ? &p : NULL, ret_fd ? &pfd : NULL);
+        if (r < 0)
+                return r;
+
+        if (ret_path) {
+                if (!FLAGS_SET(flags, CHASE_EXTRACT_FILENAME)) {
+
+                        /* When "root" points to the root directory, the result of chaseat() is always
+                         * absolute, hence it is not necessary to prefix with the root. When "root" points to
+                         * a non-root directory, the result path is always normalized and relative, hence
+                         * we can simply call path_join() and not necessary to call path_simplify().
+                         * Note that the result of chaseat() may start with "." (more specifically, it may be
+                         * "." or "./"), and we need to drop "." in that case. */
+
+                        if (empty_or_root(root))
+                                assert(path_is_absolute(p));
+                        else {
+                                char *q;
+
+                                assert(!path_is_absolute(p));
+
+                                q = path_join(root, p + (*p == '.'));
+                                if (!q)
+                                        return -ENOMEM;
+
+                                free_and_replace(p, q);
+                        }
+                }
+
+                *ret_path = TAKE_PTR(p);
+        }
+
+        if (ret_fd)
+                *ret_fd = TAKE_FD(pfd);
+
+        return r;
+}
+
+int chaseat_prefix_root(const char *path, const char *root, char **ret) {
+        char *q;
+        int r;
+
+        assert(path);
+        assert(ret);
+
+        /* This is mostly for prefixing the result of chaseat(). */
+
+        if (!path_is_absolute(path)) {
+                _cleanup_free_ char *root_abs = NULL;
+
+                r = empty_or_root_to_null(&root);
+                if (r < 0 && r != -ENOENT)
+                        return r;
+
+                /* If the dir_fd points to the root directory, chaseat() always returns an absolute path. */
+                if (empty_or_root(root))
+                        return -EINVAL;
+
+                r = path_make_absolute_cwd(root, &root_abs);
+                if (r < 0)
+                        return r;
+
+                root = path_simplify(root_abs);
+
+                q = path_join(root, path + (path[0] == '.' && IN_SET(path[1], '/', '\0')));
+        } else
+                q = strdup(path);
+        if (!q)
+                return -ENOMEM;
+
+        *ret = q;
+        return 0;
+}
+
+int chase_extract_filename(const char *path, const char *root, char **ret) {
+        int r;
+
+        /* This is similar to path_extract_filename(), but takes root directory.
+         * The result should be consistent with chase() with CHASE_EXTRACT_FILENAME. */
+
+        assert(path);
+        assert(ret);
+
+        if (isempty(path))
+                return -EINVAL;
+
+        if (!path_is_absolute(path))
+                return -EINVAL;
+
+        r = empty_or_root_to_null(&root);
+        if (r < 0 && r != -ENOENT)
+                return r;
+
+        if (!empty_or_root(root)) {
+                _cleanup_free_ char *root_abs = NULL;
+
+                r = path_make_absolute_cwd(root, &root_abs);
+                if (r < 0)
+                        return r;
+
+                path = path_startswith(path, root_abs);
+                if (!path)
+                        return -EINVAL;
+        }
+
+        if (!isempty(path)) {
+                r = path_extract_filename(path, ret);
+                if (r != -EADDRNOTAVAIL)
+                        return r;
+        }
+
+        char *fname = strdup(".");
+        if (!fname)
+                return -ENOMEM;
+
+        *ret = fname;
+        return 0;
+}
+
+int chase_and_open(const char *path, const char *root, ChaseFlags chase_flags, int open_flags, char **ret_path) {
+        _cleanup_close_ int path_fd = -EBADF;
+        _cleanup_free_ char *p = NULL, *fname = NULL;
+        mode_t mode = open_flags & O_DIRECTORY ? 0755 : 0644;
+        int r;
+
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP)));
+
+        if (empty_or_root(root) && !ret_path &&
+            (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_PROHIBIT_SYMLINKS|CHASE_PARENT|CHASE_MKDIR_0755)) == 0)
+                /* Shortcut this call if none of the special features of this call are requested */
+                return xopenat(AT_FDCWD, path,
+                               open_flags | (FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? O_NOFOLLOW : 0),
+                               /* xopen_flags = */ 0,
+                               mode);
+
+        r = chase(path, root, CHASE_PARENT|chase_flags, &p, &path_fd);
+        if (r < 0)
+                return r;
+        assert(path_fd >= 0);
+
+        if (!FLAGS_SET(chase_flags, CHASE_PARENT) &&
+            !FLAGS_SET(chase_flags, CHASE_EXTRACT_FILENAME)) {
+                r = chase_extract_filename(p, root, &fname);
+                if (r < 0)
+                        return r;
+        }
+
+        r = xopenat(path_fd, strempty(fname), open_flags|O_NOFOLLOW, /* xopen_flags = */ 0, mode);
+        if (r < 0)
+                return r;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(p);
+
+        return r;
+}
+
+int chase_and_opendir(const char *path, const char *root, ChaseFlags chase_flags, char **ret_path, DIR **ret_dir) {
+        _cleanup_close_ int path_fd = -EBADF;
+        _cleanup_free_ char *p = NULL;
+        DIR *d;
+        int r;
+
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP)));
+        assert(ret_dir);
+
+        if (empty_or_root(root) && !ret_path &&
+            (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_PROHIBIT_SYMLINKS|CHASE_PARENT|CHASE_MKDIR_0755)) == 0) {
+                /* Shortcut this call if none of the special features of this call are requested */
+                d = opendir(path);
+                if (!d)
+                        return -errno;
+
+                *ret_dir = d;
+                return 0;
+        }
+
+        r = chase(path, root, chase_flags, ret_path ? &p : NULL, &path_fd);
+        if (r < 0)
+                return r;
+        assert(path_fd >= 0);
+
+        d = xopendirat(path_fd, ".", O_NOFOLLOW);
+        if (!d)
+                return -errno;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(p);
+
+        *ret_dir = d;
+        return 0;
+}
+
+int chase_and_stat(const char *path, const char *root, ChaseFlags chase_flags, char **ret_path, struct stat *ret_stat) {
+        _cleanup_close_ int path_fd = -EBADF;
+        _cleanup_free_ char *p = NULL;
+        int r;
+
+        assert(path);
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP)));
+        assert(ret_stat);
+
+        if (empty_or_root(root) && !ret_path &&
+            (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_PROHIBIT_SYMLINKS|CHASE_PARENT|CHASE_MKDIR_0755)) == 0)
+                /* Shortcut this call if none of the special features of this call are requested */
+                return RET_NERRNO(fstatat(AT_FDCWD, path, ret_stat,
+                                          FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? AT_SYMLINK_NOFOLLOW : 0));
+
+        r = chase(path, root, chase_flags, ret_path ? &p : NULL, &path_fd);
+        if (r < 0)
+                return r;
+        assert(path_fd >= 0);
+
+        if (fstat(path_fd, ret_stat) < 0)
+                return -errno;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(p);
+
+        return 0;
+}
+
+int chase_and_access(const char *path, const char *root, ChaseFlags chase_flags, int access_mode, char **ret_path) {
+        _cleanup_close_ int path_fd = -EBADF;
+        _cleanup_free_ char *p = NULL;
+        int r;
+
+        assert(path);
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP)));
+
+        if (empty_or_root(root) && !ret_path &&
+            (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_PROHIBIT_SYMLINKS|CHASE_PARENT|CHASE_MKDIR_0755)) == 0)
+                /* Shortcut this call if none of the special features of this call are requested */
+                return RET_NERRNO(faccessat(AT_FDCWD, path, access_mode,
+                                            FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? AT_SYMLINK_NOFOLLOW : 0));
+
+        r = chase(path, root, chase_flags, ret_path ? &p : NULL, &path_fd);
+        if (r < 0)
+                return r;
+        assert(path_fd >= 0);
+
+        r = access_fd(path_fd, access_mode);
+        if (r < 0)
+                return r;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(p);
+
+        return 0;
+}
+
+int chase_and_fopen_unlocked(
+                const char *path,
+                const char *root,
+                ChaseFlags chase_flags,
+                const char *open_flags,
+                char **ret_path,
+                FILE **ret_file) {
+
+        _cleanup_free_ char *final_path = NULL;
+        _cleanup_close_ int fd = -EBADF;
+        int mode_flags, r;
+
+        assert(path);
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP|CHASE_PARENT)));
+        assert(open_flags);
+        assert(ret_file);
+
+        mode_flags = fopen_mode_to_flags(open_flags);
+        if (mode_flags < 0)
+                return mode_flags;
+
+        fd = chase_and_open(path, root, chase_flags, mode_flags, ret_path ? &final_path : NULL);
+        if (fd < 0)
+                return fd;
+
+        r = take_fdopen_unlocked(&fd, open_flags, ret_file);
+        if (r < 0)
+                return r;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(final_path);
+
+        return 0;
+}
+
+int chase_and_unlink(const char *path, const char *root, ChaseFlags chase_flags, int unlink_flags, char **ret_path) {
+        _cleanup_free_ char *p = NULL, *fname = NULL;
+        _cleanup_close_ int fd = -EBADF;
+        int r;
+
+        assert(path);
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP|CHASE_PARENT)));
+
+        fd = chase_and_open(path, root, chase_flags|CHASE_PARENT|CHASE_NOFOLLOW, O_PATH|O_DIRECTORY|O_CLOEXEC, &p);
+        if (fd < 0)
+                return fd;
+
+        r = path_extract_filename(p, &fname);
+        if (r < 0)
+                return r;
+
+        if (unlinkat(fd, fname, unlink_flags) < 0)
+                return -errno;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(p);
+
+        return 0;
+}
+
+int chase_and_open_parent(const char *path, const char *root, ChaseFlags chase_flags, char **ret_filename) {
+        int pfd, r;
+
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP)));
+
+        r = chase(path, root, CHASE_PARENT|CHASE_EXTRACT_FILENAME|chase_flags, ret_filename, &pfd);
+        if (r < 0)
+                return r;
+
+        return pfd;
+}
+
+int chase_and_openat(int dir_fd, const char *path, ChaseFlags chase_flags, int open_flags, char **ret_path) {
+        _cleanup_close_ int path_fd = -EBADF;
+        _cleanup_free_ char *p = NULL, *fname = NULL;
+        mode_t mode = open_flags & O_DIRECTORY ? 0755 : 0644;
+        int r;
+
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP)));
+
+        if (dir_fd == AT_FDCWD && !ret_path &&
+            (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_PROHIBIT_SYMLINKS|CHASE_PARENT|CHASE_MKDIR_0755)) == 0)
+                /* Shortcut this call if none of the special features of this call are requested */
+                return xopenat(dir_fd, path,
+                               open_flags | (FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? O_NOFOLLOW : 0),
+                               /* xopen_flags = */ 0,
+                               mode);
+
+        r = chaseat(dir_fd, path, chase_flags|CHASE_PARENT, &p, &path_fd);
+        if (r < 0)
+                return r;
+
+        if (!FLAGS_SET(chase_flags, CHASE_PARENT)) {
+                r = path_extract_filename(p, &fname);
+                if (r < 0 && r != -EADDRNOTAVAIL)
+                        return r;
+        }
+
+        r = xopenat(path_fd, strempty(fname), open_flags|O_NOFOLLOW, /* xopen_flags = */ 0, mode);
+        if (r < 0)
+                return r;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(p);
+
+        return r;
+}
+
+int chase_and_opendirat(int dir_fd, const char *path, ChaseFlags chase_flags, char **ret_path, DIR **ret_dir) {
+        _cleanup_close_ int path_fd = -EBADF;
+        _cleanup_free_ char *p = NULL;
+        DIR *d;
+        int r;
+
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP)));
+        assert(ret_dir);
+
+        if (dir_fd == AT_FDCWD && !ret_path &&
+            (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_PROHIBIT_SYMLINKS|CHASE_PARENT|CHASE_MKDIR_0755)) == 0) {
+                /* Shortcut this call if none of the special features of this call are requested */
+                d = opendir(path);
+                if (!d)
+                        return -errno;
+
+                *ret_dir = d;
+                return 0;
+        }
+
+        r = chaseat(dir_fd, path, chase_flags, ret_path ? &p : NULL, &path_fd);
+        if (r < 0)
+                return r;
+        assert(path_fd >= 0);
+
+        d = xopendirat(path_fd, ".", O_NOFOLLOW);
+        if (!d)
+                return -errno;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(p);
+
+        *ret_dir = d;
+        return 0;
+}
+
+int chase_and_statat(int dir_fd, const char *path, ChaseFlags chase_flags, char **ret_path, struct stat *ret_stat) {
+        _cleanup_close_ int path_fd = -EBADF;
+        _cleanup_free_ char *p = NULL;
+        int r;
+
+        assert(path);
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP)));
+        assert(ret_stat);
+
+        if (dir_fd == AT_FDCWD && !ret_path &&
+            (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_PROHIBIT_SYMLINKS|CHASE_PARENT|CHASE_MKDIR_0755)) == 0)
+                /* Shortcut this call if none of the special features of this call are requested */
+                return RET_NERRNO(fstatat(AT_FDCWD, path, ret_stat,
+                                          FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? AT_SYMLINK_NOFOLLOW : 0));
+
+        r = chaseat(dir_fd, path, chase_flags, ret_path ? &p : NULL, &path_fd);
+        if (r < 0)
+                return r;
+        assert(path_fd >= 0);
+
+        if (fstat(path_fd, ret_stat) < 0)
+                return -errno;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(p);
+
+        return 0;
+}
+
+int chase_and_accessat(int dir_fd, const char *path, ChaseFlags chase_flags, int access_mode, char **ret_path) {
+        _cleanup_close_ int path_fd = -EBADF;
+        _cleanup_free_ char *p = NULL;
+        int r;
+
+        assert(path);
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP)));
+
+        if (dir_fd == AT_FDCWD && !ret_path &&
+            (chase_flags & (CHASE_NO_AUTOFS|CHASE_SAFE|CHASE_PROHIBIT_SYMLINKS|CHASE_PARENT|CHASE_MKDIR_0755)) == 0)
+                /* Shortcut this call if none of the special features of this call are requested */
+                return RET_NERRNO(faccessat(AT_FDCWD, path, access_mode,
+                                            FLAGS_SET(chase_flags, CHASE_NOFOLLOW) ? AT_SYMLINK_NOFOLLOW : 0));
+
+        r = chaseat(dir_fd, path, chase_flags, ret_path ? &p : NULL, &path_fd);
+        if (r < 0)
+                return r;
+        assert(path_fd >= 0);
+
+        r = access_fd(path_fd, access_mode);
+        if (r < 0)
+                return r;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(p);
+
+        return 0;
+}
+
+int chase_and_fopenat_unlocked(
+                int dir_fd,
+                const char *path,
+                ChaseFlags chase_flags,
+                const char *open_flags,
+                char **ret_path,
+                FILE **ret_file) {
+
+        _cleanup_free_ char *final_path = NULL;
+        _cleanup_close_ int fd = -EBADF;
+        int mode_flags, r;
+
+        assert(path);
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP|CHASE_PARENT)));
+        assert(open_flags);
+        assert(ret_file);
+
+        mode_flags = fopen_mode_to_flags(open_flags);
+        if (mode_flags < 0)
+                return mode_flags;
+
+        fd = chase_and_openat(dir_fd, path, chase_flags, mode_flags, ret_path ? &final_path : NULL);
+        if (fd < 0)
+                return fd;
+
+        r = take_fdopen_unlocked(&fd, open_flags, ret_file);
+        if (r < 0)
+                return r;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(final_path);
+
+        return 0;
+}
+
+int chase_and_unlinkat(int dir_fd, const char *path, ChaseFlags chase_flags, int unlink_flags, char **ret_path) {
+        _cleanup_free_ char *p = NULL, *fname = NULL;
+        _cleanup_close_ int fd = -EBADF;
+        int r;
+
+        assert(path);
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP|CHASE_PARENT)));
+
+        fd = chase_and_openat(dir_fd, path, chase_flags|CHASE_PARENT|CHASE_NOFOLLOW, O_PATH|O_DIRECTORY|O_CLOEXEC, &p);
+        if (fd < 0)
+                return fd;
+
+        r = path_extract_filename(p, &fname);
+        if (r < 0)
+                return r;
+
+        if (unlinkat(fd, fname, unlink_flags) < 0)
+                return -errno;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(p);
+
+        return 0;
+}
+
+int chase_and_open_parent_at(int dir_fd, const char *path, ChaseFlags chase_flags, char **ret_filename) {
+        int pfd, r;
+
+        assert(!(chase_flags & (CHASE_NONEXISTENT|CHASE_STEP)));
+
+        r = chaseat(dir_fd, path, CHASE_PARENT|CHASE_EXTRACT_FILENAME|chase_flags, ret_filename, &pfd);
+        if (r < 0)
+                return r;
+
+        return pfd;
+}