Add --argv0 option

Fixes containers/bubblewrap#91

Add the ability to overwrite argv[0] when starting a process in a
container. Using --argv0 to be consistent with ld.so --argv0.

Overwriting argv[0] is useful as some tools change their behavior based
on the value of argv[0]. For example, when bash is symlinked to sh it
behaves as sh. Similarly, unxz is a symlink to xz and changes the
default from compressing to decompressing. An extreme example is on many
systems, date, df, cat and so on are all symlinks to the coreutils
binary.

Example usage: bwrap --bind / / --argv0 sh bash

Signed-off-by: Jonathan Wright <quaggy@gmail.com>

bubblewrap.c

@@ -72,6 +72,7 @@ static const char *opt_exec_label = NULL;
 static const char *opt_file_label = NULL;
 static bool opt_as_pid_1;
 
+static const char *opt_argv0 = NULL;
 static const char *opt_chdir_path = NULL;
 static bool opt_assert_userns_disabled = FALSE;
 static bool opt_disable_userns = FALSE;
@@ -309,6 +310,7 @@ usage (int ecode, FILE *out)
            "    --help                       Print this help\n"
            "    --version                    Print version\n"
            "    --args FD                    Parse NUL-separated args from FD\n"
+           "    --argv0 VALUE                Set argv[0] to the value VALUE before running the program\n"
            "    --unshare-all                Unshare every namespace we support by default\n"
            "    --share-net                  Retain the network namespace (can only combine with --unshare-all)\n"
            "    --unshare-user               Create new user namespace (may be automatically implied if not setuid)\n"
@@ -1728,6 +1730,18 @@ parse_args_recurse (int          *argcp,
           argv += 1;
           argc -= 1;
         }
+      else if (strcmp (arg, "--argv0") == 0)
+        {
+          if (argc < 2)
+            die ("--argv0 takes one argument");
+
+          if (opt_argv0 != NULL)
+            die ("--argv0 used multiple times");
+
+          opt_argv0 = argv[1];
+          argv++;
+          argc--;
+        }
       else if (strcmp (arg, "--unshare-all") == 0)
         {
           /* Keep this in order with the older (legacy) --unshare arguments,
@@ -2641,6 +2655,7 @@ main (int    argc,
   int res UNUSED;
   cleanup_free char *args_data UNUSED = NULL;
   int intermediate_pids_sockets[2] = {-1, -1};
+  const char *exec_path = NULL;
 
   /* Handle --version early on before we try to acquire/drop
    * any capabilities so it works in a build environment;
@@ -3351,7 +3366,11 @@ main (int    argc,
          we don't want to error out here */
     }
 
-  if (execvp (argv[0], argv) == -1)
+  exec_path = argv[0];
+  if (opt_argv0 != NULL)
+    argv[0] = (char *) opt_argv0;
+
+  if (execvp (exec_path, argv) == -1)
     {
       if (setup_finished_pipe[1] != -1)
         {
@@ -3362,7 +3381,7 @@ main (int    argc,
           /* Ignore res, if e.g. the parent died and closed setup_finished_pipe[0]
              we don't want to error out here */
         }
-      die_with_error ("execvp %s", argv[0]);
+      die_with_error ("execvp %s", exec_path);
     }
 
   return 0;

bwrap.xml

@@ -92,6 +92,10 @@
         multiple sources.
       </para></listitem>
     </varlistentry>
+    <varlistentry>
+      <term><option>--argv0 <arg choice="plain">VALUE</arg></option></term>
+      <listitem><para>Set argv[0] to the value <arg choice="plain">VALUE</arg> before running the program</para></listitem>
+    </varlistentry>
   </variablelist>
   <para>Options related to kernel namespaces:</para>
   <variablelist>

completions/bash/bwrap

@@ -32,6 +32,7 @@ _bwrap() {
 		$boolean_optons
 		--add-seccomp-fd
 		--args
+		--argv0
 		--bind
 		--bind-data
 		--block-fd

completions/zsh/_bwrap

@@ -29,6 +29,7 @@ _bwrap_args=(
     '--add-seccomp-fd[Load and use seccomp rules from FD]: :_guard "[0-9]#" "file descriptor to read seccomp rules from"'
     '--assert-userns-disabled[Fail unless further use of user namespace inside sandbox is disabled]'
     '--args[Parse NUL-separated args from FD]: :_guard "[0-9]#" "file descriptor with NUL-separated arguments"'
+    '--argv0[Set argv0 to the value VALUE before running the program]:value:'
     '--as-pid-1[Do not install a reaper process with PID=1]'
     '--bind-try[Equal to --bind but ignores non-existent SRC]:source:_files:destination:_files'
     '--bind[Bind mount the host path SRC on DEST]:source:_files:destination:_files'

tests/test-run.sh

@@ -8,7 +8,7 @@ srcd=$(cd $(dirname "$0") && pwd)
 
 bn=$(basename "$0")
 
-echo "1..58"
+echo "1..59"
 
 # Test help
 ${BWRAP} --help > help.txt
@@ -532,4 +532,12 @@ echo "PWD=$(pwd -P)" > reference
 assert_files_equal stdout reference
 echo "ok - environment manipulation"
 
+$RUN sh -c 'echo $0' > stdout
+assert_file_has_content stdout sh
+$RUN --argv0 sh sh -c 'echo $0' > stdout
+assert_file_has_content stdout sh
+$RUN --argv0 right sh -c 'echo $0' > stdout
+assert_file_has_content stdout right
+echo "ok - argv0 manipulation"
+
 echo "ok - End of test"