btrbk: add "compat" config option (busybox: add test -d command)

Add compat, compat_local, compat_remote configuration options.

Used for busybox: instead of running `readlink -e` (which is not
available on busybox), run `readlink -f` followed by `test -d`.

btrbk

@@ -132,6 +132,10 @@ my %config_options = (
   backend_local               => { default => undef,         accept => [ "no", "btrfs-progs", "btrfs-progs-btrbk", "btrfs-progs-sudo" ] },
   backend_remote              => { default => undef,         accept => [ "no", "btrfs-progs", "btrfs-progs-btrbk", "btrfs-progs-sudo" ] },
 
+  compat                      => { default => undef,     accept => [ "no", "busybox" ] },
+  compat_local                => { default => undef,     accept => [ "no", "busybox" ] },
+  compat_remote               => { default => undef,     accept => [ "no", "busybox" ] },
+
   snapshot_qgroup_destroy     => { default => undef,     accept => [ "yes", "no" ], context => [ "global", "volume", "subvolume" ] },
   target_qgroup_destroy       => { default => undef,     accept => [ "yes", "no" ] },
   archive_qgroup_destroy      => { default => undef,     accept => [ "yes", "no" ], context => [ "global" ] },
@@ -1883,23 +1887,43 @@ sub system_list_mountinfo($)
 }
 
 
+sub system_testdir($)
+{
+  my $vol = shift // die;
+  my $path = $vol->{PATH} // die;
+  my $ret = run_cmd(cmd => vinfo_cmd($vol, "test", '-d', { unsafe => $path } ),
+                    rsh => vinfo_rsh($vol),
+                    non_destructive => 1,
+                   );
+  return undef unless(defined($ret));
+  DEBUG "Directory exists: $vol->{PRINT}";
+  return 1;
+}
+
+
 sub system_realpath($)
 {
   my $vol = shift // die;
-
-  my $path = $vol->{PATH} // die;;
-  my $ret = run_cmd(cmd => vinfo_cmd($vol, "readlink", '-v', '-e', { unsafe => $path } ),
+  my $path = $vol->{PATH} // die;
+  my $compat = (($vol->{HOST} && config_key($vol, "compat_remote")) //
+                config_key($vol, "compat_local") //
+                config_key($vol, "compat")) // "" eq "busybox";
+  my @options = ("-v");                # report error messages
+  push @options, "-e" unless($compat); # all components must exist (not available in busybox!)
+  push @options, "-f" if($compat);     # all but the last component must exist.
+  my $ret = run_cmd(cmd => vinfo_cmd($vol, "readlink", @options, { unsafe => $path } ),
                     rsh => vinfo_rsh($vol),
                     non_destructive => 1,
                    );
   return undef unless(defined($ret));
 
-  unless(scalar(@$ret) && ($ret->[0] =~ /^($file_match)$/)) {
+  my $realpath = scalar(@$ret) ? (check_file($ret->[0], { absolute => 1 }) // "") : "";
+  unless($realpath) {
     ERROR "Failed to parse output of `realpath` for \"$vol->{PRINT}\": \"$ret->[0]\"";
     return undef;
   }
-  my $realpath = $1; # untaint argument
   DEBUG "Real path for \"$vol->{PRINT}\" is: $realpath";
+  return undef if($compat && !system_testdir($vol));
   return $realpath;
 }
 

doc/btrbk.conf.5.asciidoc

@@ -364,6 +364,9 @@ constraints.
 *backend* btrfs-progs|btrfs-progs-btrbk|btrfs-progs-sudo::
     Backend filesystem utilities to be used for btrfs specific
     operations. Defaults to ``btrfs-progs''.
+    If you want to set this option for local or remote hosts only, you
+    can set *backend_local* or *backend_remote*
+    (e.g. "backend_remote btrfs-progs-btrbk").
 +
 --
 btrfs-progs::
@@ -381,13 +384,17 @@ btrfs-progs-sudo::
     btrfs commands are prefixed with "sudo -n" (e.g. "sudo -n btrfs
     subvolume show" instead of "btrfs subvolume show"). Make sure to
     have appropriate (root) permissions for the "btrfs" command groups
-    and the "readlink" command in /etc/sudoers.
+    as well as the "readlink" and "test" commands in /etc/sudoers.
 --
 +
-For convenience, it is also possible to set *backend_local* or
-*backend_remote* options, which will override the backend only for
-local or remote sources/targets (e.g. "backend_remote
-btrfs-progs-btrbk").
+
+
+*compat* busybox|no::
+    If set to ``busybox'', use busybox compatible commands. Defaults
+    to ``no''.
+    If you want to set this option for local or remote hosts only, you
+    can set *compat_local* or *compat_remote*
+    (e.g. "compat_remote busybox").
 
 
 === Btrfs Specific Options

doc/ssh_filter_btrbk.1.asciidoc

@@ -37,6 +37,7 @@ The following commands are always allowed:
  - "btrfs subvolume show" (not affected by "--restrict-path")
  - "btrfs subvolume list" (not affected by "--restrict-path")
  - "readlink"
+ - "test -d" (only if "compat busybox" configuration option is set)
  - "cat /proc/self/mountinfo"
  - pipes through "gzip", "pigz", "bzip2", "pbzip2", "xz", "lzop",
    "lz4" (stream_compress)

ssh_filter_btrbk.sh

@@ -164,9 +164,10 @@ done
 # NOTE: subvolume queries are NOT affected by "--restrict-path":
 # btrbk also calls show/list on the mount point of the subvolume
 allow_exact_cmd "${sudo_prefix}btrfs subvolume (show|list)( ${option_match})* ${file_match}";
-allow_cmd "${sudo_prefix}readlink"              # used to resolve mountpoints
-allow_exact_cmd "cat /proc/self/mountinfo"      # used to resolve mountpoints
-allow_exact_cmd "cat /proc/self/mounts"         # legacy, for btrbk < 0.27.0
+allow_cmd "${sudo_prefix}readlink"                    # resolve symlink
+allow_exact_cmd "${sudo_prefix}test -d ${file_match}" # check directory (only for compat=busybox)
+allow_exact_cmd "cat /proc/self/mountinfo"            # resolve mountpoints
+allow_exact_cmd "cat /proc/self/mounts"               # legacy, for btrbk < 0.27.0
 
 # remove leading "|" on alternation lists
 allow_list=${allow_list#\|}