core: apply UnsetEnvironment= after command line expansion

UnsetEnvironment= was applied to the environment block before the
command line was expanded. Since the expansion looks the variables up
in that same block, any variable listed in UnsetEnvironment= was
already gone when $VAR in ExecStart= and friends was expanded, and a
whole-word $VAR reference was dropped from the command line.

Let the expansion see the full environment block, and apply the
unsetting only to the environment block passed to the executed
process. This way UnsetEnvironment= can hide variables from the
executed process without breaking command lines that reference them.

Extend the exec-unsetenvironment test accordingly: the whole-word
$VAR6 argument must be expanded into a positional parameter for the
shell, while ${VAR6-X} must evaluate to X in the executed process.

Also document in both the UnsetEnvironment= description and the
"Environment Variables in Spawned Processes" section that the
unsetting happens after command line variable expansion.

Fixes: #43343

man/systemd.exec.xml

@@ -3366,7 +3366,9 @@ SystemCallErrorNumber=EPERM</programlisting>
         assignments made through <varname>Environment=</varname> or <varname>EnvironmentFile=</varname>, inherited from
         the system manager's global set of environment variables, inherited via <varname>PassEnvironment=</varname>,
         set by the service manager itself (such as <varname>$NOTIFY_SOCKET</varname> and such), or set by a PAM module
-        (in case <varname>PAMName=</varname> is used).</para>
+        (in case <varname>PAMName=</varname> is used). Note that variable expansion in the command lines of
+        <varname>ExecStart=</varname> and friends is not affected by this option, as it happens before the
+        unsetting is applied.</para>
 
         <para>See "Environment Variables in Spawned Processes" below for a description of how those
         settings combine to form the inherited environment. See <citerefentry
@@ -4165,7 +4167,9 @@ StandardInputData=V2XigLJyZSBubyBzdHJhbmdlcnMgdG8gbG92ZQpZb3Uga25vdyB0aGUgcnVsZX
     <para>If the same environment variable is set by multiple of these sources, the later source — according
     to the order of the list above — wins. Note that as the final step all variables listed in
     <varname>UnsetEnvironment=</varname> are removed from the compiled environment variable list, immediately
-    before it is passed to the executed process.</para>
+    before it is passed to the executed process. This unsetting happens after variable expansion in
+    the command lines of <varname>ExecStart=</varname> and friends is performed, hence command line
+    expansion is not affected by <varname>UnsetEnvironment=</varname>.</para>
 
     <para>The general philosophy is to expose a small curated list of environment variables to processes.
     Services started by the system manager (PID 1) will be started, without additional service-specific

src/core/exec-invoke.c

@@ -6665,18 +6665,6 @@ int exec_invoke(
 #endif
         }
 
-        if (!strv_isempty(context->unset_environment)) {
-                char **ee = NULL;
-
-                ee = strv_env_delete(accum_env, 1, context->unset_environment);
-                if (!ee) {
-                        *exit_status = EXIT_MEMORY;
-                        return log_oom();
-                }
-
-                strv_free_and_replace(accum_env, ee);
-        }
-
         _cleanup_strv_free_ char **replaced_argv = NULL, **argv_via_shell = NULL;
         char **final_argv = FLAGS_SET(command->flags, EXEC_COMMAND_VIA_SHELL) ? strv_skip(command->argv, 1) : command->argv;
 
@@ -6701,6 +6689,21 @@ int exec_invoke(
                 }
         }
 
+        /* Apply UnsetEnvironment= after the command line has been expanded, so that the variables it
+         * removes are still available for expansion. Only the environment block passed to the executed
+         * process is affected, not the expansion. */
+        if (!strv_isempty(context->unset_environment)) {
+                char **ee = NULL;
+
+                ee = strv_env_delete(accum_env, 1, context->unset_environment);
+                if (!ee) {
+                        *exit_status = EXIT_MEMORY;
+                        return log_oom();
+                }
+
+                strv_free_and_replace(accum_env, ee);
+        }
+
         if (FLAGS_SET(command->flags, EXEC_COMMAND_VIA_SHELL)) {
                 r = strv_extendf(&argv_via_shell, "%s%s", command->argv[0][0] == '-' ? "-" : "", path);
                 if (r < 0) {

test/test-execute/exec-unsetenvironment.service

@@ -3,7 +3,9 @@
 Description=Test for UnsetEnvironment
 
 [Service]
-ExecStart=bash -x -c 'test "$$FOO" = "bar" && test "$${QUUX-X}" = "X" && test "$$VAR3" = "value3" && test "$${VAR4-X}" = "X" && test "$$VAR5" = "value5" && test "$${X%b-X}" = "X"'
+# Variable expansion in the command line is done by the service manager before UnsetEnvironment= is
+# applied, so $VAR6 is expanded, but the variable is not part of the environment of the executed bash.
+ExecStart=bash -x -c 'test "$$FOO" = "bar" && test "$${QUUX-X}" = "X" && test "$$VAR3" = "value3" && test "$${VAR4-X}" = "X" && test "$$VAR5" = "value5" && test "$${X%b-X}" = "X" && test "$$1" = "value6" && test "$${VAR6-X}" = "X"' x $VAR6
 Type=oneshot
-Environment=FOO=bar QUUX=waldo VAR3=value3 VAR4=value4 VAR5=value5 X%b=%U
-UnsetEnvironment=QUUX=waldo VAR3=somethingelse VAR4 X%b=%U
+Environment=FOO=bar QUUX=waldo VAR3=value3 VAR4=value4 VAR5=value5 X%b=%U VAR6=value6
+UnsetEnvironment=QUUX=waldo VAR3=somethingelse VAR4 X%b=%U VAR6