Only set up FAKECHROOT_CMD_SUBST for paths in PATH containing the original binary

If FAKECHROOT_CMD_SUBST sets up wrong substitutions, then binaries
cannot be found. For example if /usr/bin/chroot is listed in
FAKECHROOT_CMD_SUBST but /usr/sbin (the actual location of the chroot
binary) is not in PATH, the command fails

coverage.txt

@@ -312,3 +312,6 @@ Skip-If:
  hostarch != "amd64"
  mode == "fakechroot" and not run_ma_same_tests
  not have_binfmt
+
+Test: no-sbin-in-path
+Modes: fakechroot

mmdebstrap

@@ -2346,17 +2346,24 @@ sub run_prepare {
         # /etc/fakechroot/debootstrap.env and
         # /etc/fakechroot/chroot.env
         {
-            my $ldconfig = getcwd() . '/ldconfig.fakechroot';
-            if (!-x $ldconfig) {
-                $ldconfig = '/usr/libexec/mmdebstrap/ldconfig.fakechroot';
+            my %subst = (
+                chroot   => "/usr/sbin/chroot.fakechroot",
+                mkfifo   => "/bin/true",
+                ldconfig => (getcwd() . '/ldconfig.fakechroot'),
+                ldd      => "/usr/bin/ldd.fakechroot",
+                ischroot => "/bin/true"
+            );
+            if (!-x $subst{ldconfig}) {
+                $subst{ldconfig}
+                  = '/usr/libexec/mmdebstrap/ldconfig.fakechroot';
             }
             my @fakechrootsubst = ();
-            foreach my $d ('/usr/sbin', '/usr/bin', '/sbin', '/bin') {
-                push @fakechrootsubst, "$d/chroot=/usr/sbin/chroot.fakechroot";
-                push @fakechrootsubst, "$d/mkfifo=/bin/true";
-                push @fakechrootsubst, "$d/ldconfig=$ldconfig";
-                push @fakechrootsubst, "$d/ldd=/usr/bin/ldd.fakechroot";
-                push @fakechrootsubst, "$d/ischroot=/bin/true";
+            foreach my $d (split ':', $ENV{PATH}) {
+                foreach my $k (sort keys %subst) {
+                    if (-e "$d/$k") {
+                        push @fakechrootsubst, "$d/$k=$subst{$k}";
+                    }
+                }
             }
             if (defined $ENV{FAKECHROOT_CMD_SUBST}
                 && $ENV{FAKECHROOT_CMD_SUBST} ne "") {

tests/no-sbin-in-path

@@ -0,0 +1,22 @@
+#!/bin/sh
+#
+# If FAKECHROOT_CMD_SUBST sets up wrong substitutions, then binaries cannot be
+# found. For example if /usr/bin/chroot is listed in FAKECHROOT_CMD_SUBST but
+# /usr/sbin (the actual location of the chroot binary) is not in PATH, the
+# command fails
+
+set -eu
+export LC_ALL=C.UTF-8
+[ "{{ MODE }}" = "fakechroot" ]
+trap "rm -f /tmp/debian-chroot.tar" EXIT INT TERM
+if [ "$(id -u)" -eq 0 ] && ! id -u user > /dev/null 2>&1; then
+	if [ ! -e /mmdebstrap-testenv ]; then
+		echo "this test modifies the system and should only be run inside a container" >&2
+		exit 1
+	fi
+	adduser --gecos user --disabled-password user
+fi
+prefix=
+[ "$(id -u)" -eq 0 ] && prefix="runuser -u user --"
+$prefix env PATH=/usr/bin:/bin fakechroot fakeroot {{ CMD }} --mode=fakechroot --variant=apt {{ DIST }} /tmp/debian-chroot.tar {{ MIRROR }}
+tar -tf /tmp/debian-chroot.tar | sort | diff -u tar1.txt -