vim-patch:8.2.4763: using invalid pointer with "V:" in Ex mode

Problem:    Using invalid pointer with "V:" in Ex mode.
Solution:   Correctly handle the command being changed to "+".

f50808ed13

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/ex_docmd.c

@@ -2511,7 +2511,9 @@ static char *ex_range_without_command(exarg_T *eap)
 /// @return  FAIL when the command is not to be executed.
 int parse_command_modifiers(exarg_T *eap, const char **errormsg, cmdmod_T *cmod, bool skip_only)
 {
+  char *orig_cmd = eap->cmd;
   char *cmd_start = NULL;
+  bool did_plus_cmd = false;
   bool has_visual_range = false;
   CLEAR_POINTER(cmod);
 
@@ -2537,6 +2539,7 @@ int parse_command_modifiers(exarg_T *eap, const char **errormsg, cmdmod_T *cmod,
         && getline_equal(eap->ea_getline, eap->cookie, getexline)
         && curwin->w_cursor.lnum < curbuf->b_ml.ml_line_count) {
       eap->cmd = exmode_plus;
+      did_plus_cmd = true;
       if (!skip_only) {
         ex_pressedreturn = true;
       }
@@ -2767,12 +2770,26 @@ int parse_command_modifiers(exarg_T *eap, const char **errormsg, cmdmod_T *cmod,
       // Since the modifiers have been parsed put the colon on top of the
       // space: "'<,'>mod cmd" -> "mod:'<,'>cmd
       // Put eap->cmd after the colon.
-      memmove(cmd_start - 5, cmd_start, (size_t)(eap->cmd - cmd_start));
-      eap->cmd -= 5;
-      memmove(eap->cmd - 1, ":'<,'>", 6);
+      if (did_plus_cmd) {
+        size_t len = strlen(cmd_start);
+
+        // Special case: empty command may have been changed to "+":
+        //  "'<,'>mod" -> "mod'<,'>+
+        memmove(orig_cmd, cmd_start, len);
+        strcpy(orig_cmd + len, "'<,'>+");
+      } else {
+        memmove(cmd_start - 5, cmd_start, (size_t)(eap->cmd - cmd_start));
+        eap->cmd -= 5;
+        memmove(eap->cmd - 1, ":'<,'>", 6);
+      }
     } else {
-      // no modifiers, move the pointer back
-      eap->cmd -= 5;
+      // No modifiers, move the pointer back.
+      // Special case: empty command may have been changed to "+".
+      if (did_plus_cmd) {
+        eap->cmd = "'<,'>+";
+      } else {
+        eap->cmd = orig_cmd;
+      }
     }
   }
 

test/old/testdir/test_ex_mode.vim

@@ -287,6 +287,18 @@ func Test_ex_mode_large_indent()
   bwipe!
 endfunc
 
+" This was accessing illegal memory when using "+" for eap->cmd.
+func Test_empty_command_visual_mode()
+  let lines =<< trim END
+      r<sfile>
+      0norm0V:
+      :qall!
+  END
+  call writefile(lines, 'Xexmodescript')
+  call assert_equal(1, RunVim([], [], '-u NONE -e -s -S Xexmodescript'))
+
+  call delete('Xexmodescript')
+endfunc
 
 " Testing implicit print command
 func Test_implicit_print()