From cc66f3601ff68b38489ca84cb582dbbe3ea804ef Mon Sep 17 00:00:00 2001
From: bch <brad.harder@gmail.com>
Date: Fri, 28 Jan 2022 23:28:24 +0000
Subject: rejig argv/argc handling in response to investigation prompted by
 [https://arstechnica.com/information-technology/2022/01/a-bug-lurking-for-   
       12-years-gives-attackers-root-on-every-major-linux-distro/|this "polkit
 issue"] and some experimenting w/ execve() (ab)use. Essentially port of
 [0e1d2702ab] and its parent; discussed at length on IRC

---
 generic/tclMain.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/generic/tclMain.c b/generic/tclMain.c
index bb48dbb..f1b1ae2 100644
--- a/generic/tclMain.c
+++ b/generic/tclMain.c
@@ -288,6 +288,8 @@ Tcl_MainEx(
 				 * but before starting to execute commands. */
     Tcl_Interp *interp)
 {
+    char *progname = NULL;	/* may/may-not be able to use argv[0] */
+    int i=0;			/* argv[i] index */
     Tcl_Obj *path, *resultPtr, *argvPtr, *appName;
     const char *encodingName = NULL;
     int code, exitCode = 0;
@@ -296,7 +298,14 @@ Tcl_MainEx(
     InteractiveState is;
 
     TclpSetInitialEncodings();
-    TclpFindExecutable((const char *)argv[0]);
+    if (0 < argc) {
+	progname = argv[0];
+	--argc;			/* consume argv[0] */
+	++i;
+    }
+    TclpFindExecutable ((const char *)progname);	/* nb: this could be NULL
+							 * w/ (eg) a malformed
+							 * execve() */
 
     Tcl_InitMemory(interp);
 
@@ -318,36 +327,35 @@ Tcl_MainEx(
 	 *  FILENAME
 	 */
 
-	if ((argc > 3) && (0 == _tcscmp(TEXT("-encoding"), argv[1]))
+	/* mind argc is being adjusted as we proceed */
+	if ((argc >= 3) && (0 == _tcscmp(TEXT("-encoding"), argv[1]))
 		&& ('-' != argv[3][0])) {
 	    Tcl_Obj *value = NewNativeObj(argv[2]);
 	    Tcl_SetStartupScript(NewNativeObj(argv[3]),
 		    Tcl_GetString(value));
 	    Tcl_DecrRefCount(value);
 	    argc -= 3;
-	    argv += 3;
-	} else if ((argc > 1) && ('-' != argv[1][0])) {
+	    i += 3;
+	} else if ((argc >= 1) && ('-' != argv[1][0])) {
 	    Tcl_SetStartupScript(NewNativeObj(argv[1]), NULL);
 	    argc--;
-	    argv++;
+	    i++;
 	}
     }
 
     path = Tcl_GetStartupScript(&encodingName);
     if (path == NULL) {
-	appName = NewNativeObj(argv[0]);
+	appName = NewNativeObj(progname);
     } else {
 	appName = path;
     }
     Tcl_SetVar2Ex(interp, "argv0", NULL, appName, TCL_GLOBAL_ONLY);
-    argc--;
-    argv++;
 
     Tcl_SetVar2Ex(interp, "argc", NULL, Tcl_NewWideIntObj(argc), TCL_GLOBAL_ONLY);
 
     argvPtr = Tcl_NewListObj(0, NULL);
     while (argc--) {
-	Tcl_ListObjAppendElement(NULL, argvPtr, NewNativeObj(*argv++));
+	Tcl_ListObjAppendElement(NULL, argvPtr, NewNativeObj(argv[i++]));
     }
     Tcl_SetVar2Ex(interp, "argv", NULL, argvPtr, TCL_GLOBAL_ONLY);
 
-- 
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