namespace: simplify, optimize and extend handling of mounts for namespace

This changes a couple of things in the namespace handling:

It merges the BindMount and TargetMount structures. They are mostly the same,
hence let's just use the same structue, and rely on C's implicit zero
initialization of partially initialized structures for the unneeded fields.

This reworks memory management of each entry a bit. It now contains one "const"
and one "malloc" path. We use the former whenever we can, but use the latter
when we have to, which is the case when we have to chase symlinks or prefix a
root directory. This means in the common case we don't actually need to
allocate any dynamic memory. To make this easy to use we add an accessor
function bind_mount_path() which retrieves the right path string from a
BindMount structure.

While we are at it, also permit "+" as prefix for dirs configured with
ReadOnlyPaths= and friends: if specified the root directory of the unit is
implicited prefixed.

This also drops set_bind_mount() and uses C99 structure initialization instead,
which I think is more readable and clarifies what is being done.

This drops append_protect_kernel_tunables() and
append_protect_kernel_modules() as append_static_mounts() is now simple enough
to be called directly.

Prefixing with the root dir is now done in an explicit step in
prefix_where_needed(). It will prepend the root directory on each entry that
doesn't have it prefixed yet. The latter is determined depending on an extra
bit in the BindMount structure.

1 files changed, 159 insertions, 173 deletions

diff --git a/src/core/namespace.c b/src/core/namespace.c
index 67b203ba76..e5e9fc425a 100644
--- a/src/core/namespace.c
+++ b/src/core/namespace.c
@@ -58,17 +58,13 @@ typedef enum MountMode {
 } MountMode;
 
 typedef struct BindMount {
-        char *path;
-        MountMode mode;
-        bool ignore; /* Ignore if path does not exist */
+        const char *path_const;   /* Memory allocated on stack or static */
+        MountMode mode:6;
+        bool ignore:1;            /* Ignore if path does not exist? */
+        bool has_prefix:1;        /* Already is prefixed by the root dir? */
+        char *path_malloc;        /* Use this instead of 'path' if we had to allocate memory */
 } BindMount;
 
-typedef struct TargetMount {
-        const char *path;
-        MountMode mode;
-        bool ignore; /* Ignore if path does not exist */
-} TargetMount;
-
 /*
  * The following Protect tables are to protect paths and mark some of them
  * READONLY, in case a path is covered by an option from another table, then
@@ -78,7 +74,7 @@ typedef struct TargetMount {
  */
 
 /* ProtectKernelTunables= option and the related filesystem APIs */
-static const TargetMount protect_kernel_tunables_table[] = {
+static const BindMount protect_kernel_tunables_table[] = {
         { "/proc/sys",                  READONLY,       false },
         { "/proc/sysrq-trigger",        READONLY,       true  },
         { "/proc/latency_stats",        READONLY,       true  },
@@ -97,7 +93,7 @@ static const TargetMount protect_kernel_tunables_table[] = {
 };
 
 /* ProtectKernelModules= option */
-static const TargetMount protect_kernel_modules_table[] = {
+static const BindMount protect_kernel_modules_table[] = {
 #ifdef HAVE_SPLIT_USR
         { "/lib/modules",               INACCESSIBLE,   true  },
 #endif
@@ -108,28 +104,28 @@ static const TargetMount protect_kernel_modules_table[] = {
  * ProtectHome=read-only table, protect $HOME and $XDG_RUNTIME_DIR and rest of
  * system should be protected by ProtectSystem=
  */
-static const TargetMount protect_home_read_only_table[] = {
+static const BindMount protect_home_read_only_table[] = {
         { "/home",      READONLY,       true },
         { "/run/user",  READONLY,       true },
         { "/root",      READONLY,       true },
 };
 
 /* ProtectHome=yes table */
-static const TargetMount protect_home_yes_table[] = {
+static const BindMount protect_home_yes_table[] = {
         { "/home",      INACCESSIBLE,   true },
         { "/run/user",  INACCESSIBLE,   true },
         { "/root",      INACCESSIBLE,   true },
 };
 
 /* ProtectSystem=yes table */
-static const TargetMount protect_system_yes_table[] = {
+static const BindMount protect_system_yes_table[] = {
         { "/usr",       READONLY,       false },
         { "/boot",      READONLY,       true  },
         { "/efi",       READONLY,       true  },
 };
 
 /* ProtectSystem=full includes ProtectSystem=yes */
-static const TargetMount protect_system_full_table[] = {
+static const BindMount protect_system_full_table[] = {
         { "/usr",       READONLY,       false },
         { "/boot",      READONLY,       true  },
         { "/efi",       READONLY,       true  },
@@ -144,7 +140,7 @@ static const TargetMount protect_system_full_table[] = {
  * (And of course /home and friends are also left writable, as ProtectHome=
  * shall manage those, orthogonally).
  */
-static const TargetMount protect_system_strict_table[] = {
+static const BindMount protect_system_strict_table[] = {
         { "/",          READONLY,       false },
         { "/proc",      READWRITE,      false },      /* ProtectKernelTunables= */
         { "/sys",       READWRITE,      false },      /* ProtectKernelTunables= */
@@ -154,154 +150,107 @@ static const TargetMount protect_system_strict_table[] = {
         { "/root",      READWRITE,      true  },      /* ProtectHome= */
 };
 
-static void set_bind_mount(BindMount *p, char *path, MountMode mode, bool ignore) {
-        p->path = path;
-        p->mode = mode;
-        p->ignore = ignore;
-}
-
-static int append_one_mount(BindMount **p, const char *root_directory,
-                            const char *path, MountMode mode, bool ignore) {
-        char *lpath;
+static const char *bind_mount_path(const BindMount *p) {
         assert(p);
 
-        lpath = prefix_root(root_directory, path);
-        if (!lpath)
-                return -ENOMEM;
+        /* Returns the path of this bind mount. If the malloc()-allocated ->path_buffer field is set we return that,
+         * otherwise the stack/static ->path field is returned. */
 
-        set_bind_mount((*p)++, lpath, mode, ignore);
-        return 0;
+        return p->path_malloc ?: p->path_const;
 }
 
-static int append_mounts(BindMount **p, char **strv, MountMode mode) {
+static int append_access_mounts(BindMount **p, char **strv, MountMode mode) {
         char **i;
 
         assert(p);
 
+        /* Adds a list of user-supplied READWRITE/READONLY/INACCESSIBLE entries */
+
         STRV_FOREACH(i, strv) {
-                bool ignore = false;
-                char *path;
+                bool ignore = false, needs_prefix = false;
+                const char *e = *i;
 
-                if (IN_SET(mode, INACCESSIBLE, READONLY, READWRITE) && startswith(*i, "-")) {
-                        (*i)++;
+                /* Look for any prefixes */
+                if (startswith(e, "-")) {
+                        e++;
                         ignore = true;
                 }
+                if (startswith(e, "+")) {
+                        e++;
+                        needs_prefix = true;
+                }
 
-                if (!path_is_absolute(*i))
+                if (!path_is_absolute(e))
                         return -EINVAL;
 
-                path = strdup(*i);
-                if (!path)
-                        return -ENOMEM;
-
-                set_bind_mount((*p)++, path, mode, ignore);
+                *((*p)++) = (BindMount) {
+                        .path_const = e,
+                        .mode = mode,
+                        .ignore = ignore,
+                        .has_prefix = !needs_prefix,
+                };
         }
 
         return 0;
 }
 
-static int append_target_mounts(BindMount **p, const char *root_directory,
-                                const TargetMount *mounts, const size_t size, bool ignore_protect) {
+static int append_static_mounts(BindMount **p, const BindMount *mounts, unsigned n, bool ignore_protect) {
         unsigned i;
 
         assert(p);
         assert(mounts);
 
-        for (i = 0; i < size; i++) {
-                bool ignore;
-                /*
-                 * Here we assume that the ignore field is set during
-                 * declaration we do not support "-" at the beginning.
-                 */
-                const TargetMount *m = &mounts[i];
-                char *path;
-
-                path = prefix_root(root_directory, m->path);
-                if (!path)
-                        return -ENOMEM;
-
-                if (!path_is_absolute(path))
-                        return -EINVAL;
+        /* Adds a list of static pre-defined entries */
 
-                /*
-                 * Ignore paths if they are not present. First we use our
-                 * static tables otherwise fallback to Unit context.
-                 */
-                ignore = m->ignore || ignore_protect;
-
-                set_bind_mount((*p)++, path, m->mode, ignore);
-        }
+        for (i = 0; i < n; i++)
+                *((*p)++) = (BindMount) {
+                        .path_const = bind_mount_path(mounts+i),
+                        .mode = mounts[i].mode,
+                        .ignore = mounts[i].ignore || ignore_protect,
+                };
 
         return 0;
 }
 
-static int append_protect_kernel_tunables(BindMount **p, const char *root_directory, bool ignore_protect) {
-        assert(p);
-
-        return append_target_mounts(p, root_directory, protect_kernel_tunables_table,
-                                    ELEMENTSOF(protect_kernel_tunables_table), ignore_protect);
-}
-
-static int append_protect_kernel_modules(BindMount **p, const char *root_directory, bool ignore_protect) {
+static int append_protect_home(BindMount **p, ProtectHome protect_home, bool ignore_protect) {
         assert(p);
 
-        return append_target_mounts(p, root_directory, protect_kernel_modules_table,
-                                    ELEMENTSOF(protect_kernel_modules_table), ignore_protect);
-}
-
-static int append_protect_home(BindMount **p, const char *root_directory, ProtectHome protect_home, bool ignore_protect) {
-        int r = 0;
-
-        assert(p);
+        switch (protect_home) {
 
-        if (protect_home == PROTECT_HOME_NO)
+        case PROTECT_HOME_NO:
                 return 0;
 
-        switch (protect_home) {
         case PROTECT_HOME_READ_ONLY:
-                r = append_target_mounts(p, root_directory, protect_home_read_only_table,
-                                         ELEMENTSOF(protect_home_read_only_table),
-                                         ignore_protect);
-                break;
+                return append_static_mounts(p, protect_home_read_only_table, ELEMENTSOF(protect_home_read_only_table), ignore_protect);
+
         case PROTECT_HOME_YES:
-                r = append_target_mounts(p, root_directory, protect_home_yes_table,
-                                         ELEMENTSOF(protect_home_yes_table), ignore_protect);
-                break;
+                return append_static_mounts(p, protect_home_yes_table, ELEMENTSOF(protect_home_yes_table), ignore_protect);
+
         default:
-                r = -EINVAL;
-                break;
+                assert_not_reached("Unexpected ProtectHome= value");
         }
-
-        return r;
 }
 
-static int append_protect_system(BindMount **p, const char *root_directory, ProtectSystem protect_system, bool ignore_protect) {
-        int r = 0;
-
+static int append_protect_system(BindMount **p, ProtectSystem protect_system, bool ignore_protect) {
         assert(p);
 
-        if (protect_system == PROTECT_SYSTEM_NO)
+        switch (protect_system) {
+
+        case PROTECT_SYSTEM_NO:
                 return 0;
 
-        switch (protect_system) {
         case PROTECT_SYSTEM_STRICT:
-                r = append_target_mounts(p, root_directory, protect_system_strict_table,
-                                         ELEMENTSOF(protect_system_strict_table), ignore_protect);
-                break;
+                return append_static_mounts(p, protect_system_strict_table, ELEMENTSOF(protect_system_strict_table), ignore_protect);
+
         case PROTECT_SYSTEM_YES:
-                r = append_target_mounts(p, root_directory, protect_system_yes_table,
-                                         ELEMENTSOF(protect_system_yes_table), ignore_protect);
-                break;
+                return append_static_mounts(p, protect_system_yes_table, ELEMENTSOF(protect_system_yes_table), ignore_protect);
+
         case PROTECT_SYSTEM_FULL:
-                r = append_target_mounts(p, root_directory, protect_system_full_table,
-                                         ELEMENTSOF(protect_system_full_table), ignore_protect);
-                break;
+                return append_static_mounts(p, protect_system_full_table, ELEMENTSOF(protect_system_full_table), ignore_protect);
+
         default:
-                r = -EINVAL;
-                break;
+                assert_not_reached("Unexpected ProtectSystem= value");
         }
-
-        return r;
 }
 
 static int mount_path_compare(const void *a, const void *b) {
@@ -309,7 +258,7 @@ static int mount_path_compare(const void *a, const void *b) {
         int d;
 
         /* If the paths are not equal, then order prefixes first */
-        d = path_compare(p->path, q->path);
+        d = path_compare(bind_mount_path(p), bind_mount_path(q));
         if (d != 0)
                 return d;
 
@@ -323,6 +272,34 @@ static int mount_path_compare(const void *a, const void *b) {
         return 0;
 }
 
+static int prefix_where_needed(BindMount *m, unsigned n, const char *root_directory) {
+        unsigned i;
+
+        /* Prefixes all paths in the bind mount table with the root directory if it is specified and the entry needs
+         * that. */
+
+        if (!root_directory)
+                return 0;
+
+        for (i = 0; i < n; i++) {
+                char *s;
+
+                if (m[i].has_prefix)
+                        continue;
+
+                s = prefix_root(root_directory, bind_mount_path(m+i));
+                if (!s)
+                        return -ENOMEM;
+
+                free(m[i].path_malloc);
+                m[i].path_malloc = s;
+
+                m[i].has_prefix = true;
+        }
+
+        return 0;
+}
+
 static void drop_duplicates(BindMount *m, unsigned *n) {
         BindMount *f, *t, *previous;
 
@@ -335,9 +312,9 @@ static void drop_duplicates(BindMount *m, unsigned *n) {
 
                 /* The first one wins (which is the one with the more restrictive mode), see mount_path_compare()
                  * above. */
-                if (previous && path_equal(f->path, previous->path)) {
-                        log_debug("%s is duplicate.", f->path);
-                        f->path = mfree(f->path);
+                if (previous && path_equal(bind_mount_path(f), bind_mount_path(previous))) {
+                        log_debug("%s is duplicate.", bind_mount_path(f));
+                        f->path_malloc = mfree(f->path_malloc);
                         continue;
                 }
 
@@ -363,13 +340,13 @@ static void drop_inaccessible(BindMount *m, unsigned *n) {
 
                 /* If we found a path set for INACCESSIBLE earlier, and this entry has it as prefix we should drop
                  * it, as inaccessible paths really should drop the entire subtree. */
-                if (clear && path_startswith(f->path, clear)) {
-                        log_debug("%s is masked by %s.", f->path, clear);
-                        f->path = mfree(f->path);
+                if (clear && path_startswith(bind_mount_path(f), clear)) {
+                        log_debug("%s is masked by %s.", bind_mount_path(f), clear);
+                        f->path_malloc = mfree(f->path_malloc);
                         continue;
                 }
 
-                clear = f->mode == INACCESSIBLE ? f->path : NULL;
+                clear = f->mode == INACCESSIBLE ? bind_mount_path(f) : NULL;
 
                 *t = *f;
                 t++;
@@ -396,7 +373,7 @@ static void drop_nop(BindMount *m, unsigned *n) {
 
                         /* Now let's find the first parent of the entry we are looking at. */
                         for (p = t-1; p >= m; p--) {
-                                if (path_startswith(f->path, p->path)) {
+                                if (path_startswith(bind_mount_path(f), bind_mount_path(p))) {
                                         found = true;
                                         break;
                                 }
@@ -404,8 +381,8 @@ static void drop_nop(BindMount *m, unsigned *n) {
 
                         /* We found it, let's see if it's the same mode, if so, we can drop this entry */
                         if (found && p->mode == f->mode) {
-                                log_debug("%s is redundant by %s", f->path, p->path);
-                                f->path = mfree(f->path);
+                                log_debug("%s is redundant by %s", bind_mount_path(f), bind_mount_path(p));
+                                f->path_malloc = mfree(f->path_malloc);
                                 continue;
                         }
                 }
@@ -431,9 +408,9 @@ static void drop_outside_root(const char *root_directory, BindMount *m, unsigned
 
         for (f = m, t = m; f < m + *n; f++) {
 
-                if (!path_startswith(f->path, root_directory)) {
-                        log_debug("%s is outside of root directory.", f->path);
-                        f->path = mfree(f->path);
+                if (!path_startswith(bind_mount_path(f), root_directory)) {
+                        log_debug("%s is outside of root directory.", bind_mount_path(f));
+                        f->path_malloc = mfree(f->path_malloc);
                         continue;
                 }
 
@@ -549,11 +526,11 @@ static int mount_dev(BindMount *m) {
          * missing when the service is started with RootDirectory. This is
          * consistent with mount units creating the mount points when missing.
          */
-        (void) mkdir_p_label(m->path, 0755);
+        (void) mkdir_p_label(bind_mount_path(m), 0755);
 
         /* Unmount everything in old /dev */
-        umount_recursive(m->path, 0);
-        if (mount(dev, m->path, NULL, MS_MOVE, NULL) < 0) {
+        umount_recursive(bind_mount_path(m), 0);
+        if (mount(dev, bind_mount_path(m), NULL, MS_MOVE, NULL) < 0) {
                 r = -errno;
                 goto fail;
         }
@@ -593,7 +570,7 @@ static int apply_mount(
 
         assert(m);
 
-        log_debug("Applying namespace mount on %s", m->path);
+        log_debug("Applying namespace mount on %s", bind_mount_path(m));
 
         switch (m->mode) {
 
@@ -603,10 +580,10 @@ static int apply_mount(
                 /* First, get rid of everything that is below if there
                  * is anything... Then, overmount it with an
                  * inaccessible path. */
-                (void) umount_recursive(m->path, 0);
+                (void) umount_recursive(bind_mount_path(m), 0);
 
-                if (lstat(m->path, &target) < 0)
-                        return log_debug_errno(errno, "Failed to lstat() %s to determine what to mount over it: %m", m->path);
+                if (lstat(bind_mount_path(m), &target) < 0)
+                        return log_debug_errno(errno, "Failed to lstat() %s to determine what to mount over it: %m", bind_mount_path(m));
 
                 what = mode_to_inaccessible_node(target.st_mode);
                 if (!what) {
@@ -619,13 +596,13 @@ static int apply_mount(
         case READONLY:
         case READWRITE:
 
-                r = path_is_mount_point(m->path, 0);
+                r = path_is_mount_point(bind_mount_path(m), 0);
                 if (r < 0)
-                        return log_debug_errno(r, "Failed to determine whether %s is already a mount point: %m", m->path);
+                        return log_debug_errno(r, "Failed to determine whether %s is already a mount point: %m", bind_mount_path(m));
                 if (r > 0) /* Nothing to do here, it is already a mount. We just later toggle the MS_RDONLY bit for the mount point if needed. */
                         return 0;
                 /* This isn't a mount point yet, let's make it one. */
-                what = m->path;
+                what = bind_mount_path(m);
                 break;
 
         case PRIVATE_TMP:
@@ -645,10 +622,10 @@ static int apply_mount(
 
         assert(what);
 
-        if (mount(what, m->path, NULL, MS_BIND|MS_REC, NULL) < 0)
-                return log_debug_errno(errno, "Failed to mount %s to %s: %m", what, m->path);
+        if (mount(what, bind_mount_path(m), NULL, MS_BIND|MS_REC, NULL) < 0)
+                return log_debug_errno(errno, "Failed to mount %s to %s: %m", what, bind_mount_path(m));
 
-        log_debug("Successfully mounted %s to %s", what, m->path);
+        log_debug("Successfully mounted %s to %s", what, bind_mount_path(m));
         return 0;
 }
 
@@ -658,9 +635,9 @@ static int make_read_only(BindMount *m, char **blacklist) {
         assert(m);
 
         if (IN_SET(m->mode, INACCESSIBLE, READONLY))
-                r = bind_remount_recursive(m->path, true, blacklist);
+                r = bind_remount_recursive(bind_mount_path(m), true, blacklist);
         else if (m->mode == PRIVATE_DEV) { /* Can be readonly but the submounts can't*/
-                if (mount(NULL, m->path, NULL, MS_REMOUNT|DEV_MOUNT_OPTIONS|MS_RDONLY, NULL) < 0)
+                if (mount(NULL, bind_mount_path(m), NULL, MS_REMOUNT|DEV_MOUNT_OPTIONS|MS_RDONLY, NULL) < 0)
                         r = -errno;
         } else
                 return 0;
@@ -685,24 +662,27 @@ static int chase_all_symlinks(const char *root_directory, BindMount *m, unsigned
          * can't resolve the path, and which have been marked for such removal. */
 
         for (f = m, t = m; f < m + *n; f++) {
-                int k;
                 _cleanup_free_ char *chased = NULL;
+                int k;
 
-                k = chase_symlinks(f->path, root_directory, &chased);
+                k = chase_symlinks(bind_mount_path(f), root_directory, &chased);
                 if (k < 0) {
                         /* Get only real errors */
                         if (r >= 0 && (k != -ENOENT || !f->ignore))
                                 r = k;
 
-                        log_debug_errno(r, "Failed to chase symlinks for %s: %m", f->path);
                         /* Doesn't exist or failed? Then remove it and continue! */
-                        f->path = mfree(f->path);
+                        log_debug_errno(k, "Failed to chase symlinks for %s: %m", bind_mount_path(f));
+                        f->path_malloc = mfree(f->path_malloc);
                         continue;
                 }
 
-                if (!path_equal(f->path, chased)) {
-                        log_debug("Chased %s → %s", f->path, chased);
-                        free_and_replace(f->path, chased);
+                if (!path_equal(bind_mount_path(f), chased)) {
+                        log_debug("Chased %s → %s", bind_mount_path(f), chased);
+
+                        free(f->path_malloc);
+                        f->path_malloc = chased;
+                        chased = NULL;
                 }
 
                 *t = *f;
@@ -782,67 +762,73 @@ int setup_namespace(
 
         if (n_mounts > 0) {
                 m = mounts = (BindMount *) alloca0(n_mounts * sizeof(BindMount));
-                r = append_mounts(&m, read_write_paths, READWRITE);
+                r = append_access_mounts(&m, read_write_paths, READWRITE);
                 if (r < 0)
                         goto finish;
 
-                r = append_mounts(&m, read_only_paths, READONLY);
+                r = append_access_mounts(&m, read_only_paths, READONLY);
                 if (r < 0)
                         goto finish;
 
-                r = append_mounts(&m, inaccessible_paths, INACCESSIBLE);
+                r = append_access_mounts(&m, inaccessible_paths, INACCESSIBLE);
                 if (r < 0)
                         goto finish;
 
                 if (tmp_dir) {
-                        r = append_one_mount(&m, root_directory, "/tmp", PRIVATE_TMP, false);
-                        if (r < 0)
-                                goto finish;
+                        *(m++) = (BindMount) {
+                                .path_const = "/tmp",
+                                .mode = PRIVATE_TMP,
+                        };
                 }
 
                 if (var_tmp_dir) {
-                        r = append_one_mount(&m, root_directory, "/var/tmp", PRIVATE_VAR_TMP, false);
-                        if (r < 0)
-                                goto finish;
+                        *(m++) = (BindMount) {
+                                .path_const = "/var/tmp",
+                                .mode = PRIVATE_VAR_TMP,
+                        };
                 }
 
                 if (ns_info->private_dev) {
-                        r = append_one_mount(&m, root_directory, "/dev", PRIVATE_DEV, false);
-                        if (r < 0)
-                                goto finish;
+                        *(m++) = (BindMount) {
+                                .path_const = "/dev",
+                                .mode = PRIVATE_DEV,
+                        };
                 }
 
                 if (ns_info->protect_kernel_tunables) {
-                        r = append_protect_kernel_tunables(&m, root_directory,
-                                                           ns_info->ignore_protect_paths);
+                        r = append_static_mounts(&m, protect_kernel_tunables_table, ELEMENTSOF(protect_kernel_tunables_table), ns_info->ignore_protect_paths);
                         if (r < 0)
                                 goto finish;
                 }
 
                 if (ns_info->protect_kernel_modules) {
-                        r = append_protect_kernel_modules(&m, root_directory,
-                                                          ns_info->ignore_protect_paths);
+                        r = append_static_mounts(&m, protect_kernel_modules_table, ELEMENTSOF(protect_kernel_modules_table), ns_info->ignore_protect_paths);
                         if (r < 0)
                                 goto finish;
                 }
 
                 if (ns_info->protect_control_groups) {
-                        r = append_one_mount(&m, root_directory, "/sys/fs/cgroup", READONLY, false);
-                        if (r < 0)
-                                goto finish;
+                        *(m++) = (BindMount) {
+                                .path_const = "/sys/fs/cgroup",
+                                .mode = READONLY,
+                        };
                 }
 
-                r = append_protect_home(&m, root_directory, protect_home,
-                                        ns_info->ignore_protect_paths);
+                r = append_protect_home(&m, protect_home, ns_info->ignore_protect_paths);
                 if (r < 0)
                         goto finish;
 
-                r = append_protect_system(&m, root_directory, protect_system, false);
+                r = append_protect_system(&m, protect_system, false);
                 if (r < 0)
                         goto finish;
 
                 assert(mounts + n_mounts == m);
 
+                /* Prepend the root directory where that's necessary */
+                r = prefix_where_needed(mounts, n_mounts, root_directory);
+                if (r < 0)
+                        goto finish;
+
                 /* Resolve symlinks manually first, as mount() will always follow them relative to the host's
                  * root. Moreover we want to suppress duplicates based on the resolved paths. This of course is a bit
                  * racy. */
@@ -899,7 +885,7 @@ int setup_namespace(
                 /* Create a blacklist we can pass to bind_mount_recursive() */
                 blacklist = newa(char*, n_mounts+1);
                 for (j = 0; j < n_mounts; j++)
-                        blacklist[j] = (char*) mounts[j].path;
+                        blacklist[j] = (char*) bind_mount_path(mounts+j);
                 blacklist[j] = NULL;
 
                 /* Second round, flip the ro bits if necessary. */
@@ -929,7 +915,7 @@ int setup_namespace(
 
 finish:
         for (m = mounts; m < mounts + n_mounts; m++)
-                free(m->path);
+                free(m->path_malloc);
 
         return r;
 }