parse.c: fix use-after-free probs related to funstaks()

When the funstaks() function deletes a stack, other code could
still reference that stack's pointer, at least if a script's DEBUG
trap changed the function context by assigning to ${.sh.level}.
This crashed @ormaaj's funcname.ksh script in certain contexts, at
least when run as a dot script or in a virtual subshell.

This allows that script to run in all contexts by making
funstaks(s) set the stack pointer to NULL after deleting the stack
and making various other points in the code check for a null
pointer before dereferencing it.

This may not be the most elegant fix but (in my testing) it does
work, even when compiling ksh with AddressSanitiser.

Thanks to @JohnoKing for help researching this problem.

Resolves: #212

src/cmd/ksh93/sh/name.c

@@ -2501,7 +2501,11 @@ void	_nv_unset(register Namval_t *np,int flags)
 				}
 				dtclose(rp->sdict);
 			}
-			stakdelete(slp->slptr);
+			if(slp->slptr)
+			{
+				stakdelete(slp->slptr);
+				slp->slptr = NIL(Stak_t*);
+			}
 			free((void*)np->nvalue.ip);
 			np->nvalue.ip = 0;
 		}

src/cmd/ksh93/sh/parse.c

@@ -527,10 +527,16 @@ void sh_funstaks(register struct slnod *slp,int flag)
 		if(slp->slchild)
 			sh_funstaks(slp->slchild,flag);
 		slp = slp->slnext;
-		if(flag<=0)
-			stakdelete(slpold->slptr);
-		else
-			staklink(slpold->slptr);
+		if(slpold->slptr)
+		{
+			if(flag<=0)
+			{
+				stakdelete(slpold->slptr);
+				slpold->slptr = NIL(Stak_t*);
+			}
+			else
+				staklink(slpold->slptr);
+		}
 	}
 }
 /*
@@ -953,6 +959,7 @@ static Shnode_t *funct(Lex_t *lexp)
 		{
 			sh.st.staklist = slp->slnext;
 			stakdelete(slp->slptr);
+			slp->slptr = NIL(Stak_t*);
 		}
 		siglongjmp(*sh.jmplist,jmpval);
 	}

src/cmd/ksh93/sh/xec.c

@@ -1427,7 +1427,8 @@ int sh_exec(register const Shnode_t *t, int flags)
 					/* increase refcnt for unset */
 					slp = (struct slnod*)np->nvenv;
 					sh_funstaks(slp->slchild,1);
-					staklink(slp->slptr);
+					if(slp->slptr)
+						staklink(slp->slptr);
 					if(nq)
 					{
 						Namval_t *mp=0;
@@ -1475,7 +1476,11 @@ int sh_exec(register const Shnode_t *t, int flags)
 					if(nq)
 						unset_instance(nq,&node,&nr,mode);
 					sh_funstaks(slp->slchild,-1);
-					stakdelete(slp->slptr);
+					if(slp->slptr)
+					{
+						stakdelete(slp->slptr);
+						slp->slptr = NIL(Stak_t*);
+					}
 					if(jmpval > SH_JMPFUN || (io && jmpval > SH_JMPIO))
 						siglongjmp(*sh.jmplist,jmpval);
 					goto setexit;
@@ -2486,7 +2491,11 @@ int sh_exec(register const Shnode_t *t, int flags)
 				struct Ufunction *rp = np->nvalue.rp;
 				slp = (struct slnod*)np->nvenv;
 				sh_funstaks(slp->slchild,-1);
-				stakdelete(slp->slptr);
+				if(slp->slptr)
+				{
+					stakdelete(slp->slptr);
+					slp->slptr = NIL(Stak_t*);
+				}
 				if(rp->sdict)
 				{
 					Namval_t *mp, *nq;
@@ -2522,7 +2531,8 @@ int sh_exec(register const Shnode_t *t, int flags)
 				struct comnod *ac = t->funct.functargs;
 				slp = t->funct.functstak;
 				sh_funstaks(slp->slchild,1);
-				staklink(slp->slptr);
+				if(slp->slptr)
+					staklink(slp->slptr);
 				np->nvenv = (char*)slp;
 				nv_funtree(np) = (int*)(t->funct.functtre);
 				np->nvalue.rp->hoffset = t->funct.functloc;

src/cmd/ksh93/tests/functions.sh

@@ -1400,5 +1400,100 @@ else
 	unset exp
 fi
 
+# ======
+# funcname.ksh crashed
+# https://github.com/ksh93/ksh/issues/212
+cat >$tmp/funcname.ksh <<-'EOF'
+# tweaked version of funname.ksh by Daniel Douglas
+# https://gist.github.com/ormaaj/12874b68acd06ee98b59
+# Used by permission: "Consider all my gists MIT / do whatever."
+# https://github.com/ksh93/ksh/issues/212#issuecomment-807915937
+
+# run in subshell for additional regression testing
+(
+	IFS='-'  # element separator for "$*" etc., incl. "${FUNCNAME[*]}"
+	typeset -a FUNCNAME
+
+	function FUNCNAME.get {
+	    nameref self=${.sh.name}
+	    if (( .sh.subscript < .sh.level )); then
+		trap "(( .sh.level -= .sh.subscript + 1 )); eval '(( .sh.level = ${.sh.level} ))' \; _=\${.sh.fun}" DEBUG
+		trap - DEBUG;
+	    fi
+
+	    (( .sh.subscript < .sh.level - 2 )) && self[.sh.subscript + 1]=
+	}
+
+	function f {
+	    if (($1 < 10)); then
+		print -r -- "${FUNCNAME[*]}"
+		g $(($1 + 1))
+	    fi
+	    print -r -- "${FUNCNAME[*]}"
+	}
+
+	function g {
+	    if (($1 < 10)); then
+		print -r -- "${FUNCNAME[*]}"
+		f $(($1 + 1))
+	    fi
+	    print -r -- "${FUNCNAME[*]}"
+	}
+
+	#typeset -ft FUNCNAME.get f g
+	f 0
+)
+: do not optimize out the subshell
+EOF
+exp='f
+g-f
+f-g-f
+g-f-g-f
+f-g-f-g-f
+g-f-g-f-g-f
+f-g-f-g-f-g-f
+g-f-g-f-g-f-g-f
+f-g-f-g-f-g-f-g-f
+g-f-g-f-g-f-g-f-g-f
+f-g-f-g-f-g-f-g-f-g-f
+g-f-g-f-g-f-g-f-g-f-
+f-g-f-g-f-g-f-g-f--
+g-f-g-f-g-f-g-f---
+f-g-f-g-f-g-f----
+g-f-g-f-g-f-----
+f-g-f-g-f------
+g-f-g-f-------
+f-g-f--------
+g-f---------
+f----------'
+got=$(set +x; { "$SHELL" funcname.ksh ;} 2>&1)
+[[ $got == "$exp" ]] || err_exit 'funcname.ksh crash (direct run)' \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+# dotting it slightly changes the output
+exp='f-
+g-f-
+f-g-f-
+g-f-g-f-
+f-g-f-g-f-
+g-f-g-f-g-f-
+f-g-f-g-f-g-f-
+g-f-g-f-g-f-g-f-
+f-g-f-g-f-g-f-g-f-
+g-f-g-f-g-f-g-f-g-f-
+f-g-f-g-f-g-f-g-f-g-f-
+g-f-g-f-g-f-g-f-g-f--
+f-g-f-g-f-g-f-g-f---
+g-f-g-f-g-f-g-f----
+f-g-f-g-f-g-f-----
+g-f-g-f-g-f------
+f-g-f-g-f-------
+g-f-g-f--------
+f-g-f---------
+g-f----------
+f-----------'
+got=$(set +x; { "$SHELL" -c '. ./funcname.ksh' ;} 2>&1)
+[[ $got == "$exp" ]] || err_exit 'funcname.ksh crash (dot)' \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))