c18n: Augment dummy stack to contain full metadata

Previously, the dummy stack used to fill the Restricted stack pointer
when running Executive mode code is just a capability pointing to
itself. This commit augments the dummy stack to contain full metadata
like all other Restricted stacks.

libexec/rtld-elf/aarch64/rtld_c18n_asm.S

@@ -171,7 +171,8 @@ ENTRY(allocate_rstk)
 	 */
 
 #ifndef __ARM_MORELLO_PURECAP_BENCHMARK_ABI
-	adr	c20, (dummy_stack + CAP_WIDTH * 2)
+	gclim	x20, csp
+	scvalue	c20, csp, x20
 #endif
 	ldr	c20, [c20, #-CAP_WIDTH]
 #ifdef __ARM_MORELLO_PURECAP_BENCHMARK_ABI

libexec/rtld-elf/rtld_c18n.c

@@ -521,23 +521,27 @@ c18n_init_rtld_stack(uintptr_t ret, void *base)
 	/*
 	 * This function does very different things under the two ABIs.
 	 */
-#ifdef __ARM_MORELLO_PURECAP_BENCHMARK_ABI
-	/*
-	 * Under the benchmark ABI, it initialises RTLD's stack as a regular
-	 * compartment's stack.
-	 */
-	init_compart_stack(base, C18N_RTLD_COMPART_ID);
-#else
+#ifndef __ARM_MORELLO_PURECAP_BENCHMARK_ABI
 	struct stk_bottom *stk = base;
 	--stk;
+
 	/*
-	 * Under the purecap ABI, it repurposes the trusted stack into a dummy
-	 * stack to be filled in the Restricted stack register when running
-	 * Executive mode code. The reduction of bounds is merely defensive. It
-	 * should in theory be unnecessary.
+	 * Under the purecap ABI, it repurposes the bottom of the trusted stack
+	 * into a dummy stack to be filled in the Restricted stack register when
+	 * running Executive mode code so that trampolines do not need to test
+	 * if the Restricted stack is valid. The reduction of bounds is merely
+	 * defensive. It should in theory be unnecessary.
 	 */
-	stk->top = cheri_setboundsexact(&stk->top, sizeof(stk->top));
+	stk = cheri_setboundsexact(stk, sizeof(*stk));
+	untrusted_stk_set(stk);
+
+	base = stk + 1;
 #endif
+	/*
+	 * Under the benchmark ABI, it initialises RTLD's stack as a regular
+	 * compartment's stack.
+	 */
+	init_compart_stack(base, C18N_RTLD_COMPART_ID);
 
 	return (ret);
 }
@@ -554,15 +558,6 @@ init_stk_table(struct stk_table *table)
 	table->stacks[cid_to_table_index(C18N_RTLD_COMPART_ID)].bottom =
 	    cheri_setoffset(sp, cheri_getlen(sp));
 }
-#else
-/*
- * Set a dummy Restricted stack so that trampolines do not need to test if the
- * Restricted stack is valid.
- */
-extern struct stk_bottom dummy_stack;
-struct stk_bottom dummy_stack = {
-	.top = &dummy_stack
-};
 #endif
 
 static _Atomic(struct stk_table *) free_stk_tables;
@@ -1414,8 +1409,6 @@ c18n_init(void)
 	 */
 	trusted_stk_set(stk_create(C18N_STACK_SIZE));
 	init_stk_table(table);
-#else
-	untrusted_stk_set(&dummy_stack);
 #endif
 
 	stk_table_set(table);
@@ -1478,8 +1471,6 @@ _rtld_thread_start_impl(struct pthread *curthread)
 #ifdef __ARM_MORELLO_PURECAP_BENCHMARK_ABI
 	trusted_stk_set(stk_create(C18N_STACK_SIZE));
 	init_stk_table(table);
-#else
-	untrusted_stk_set(&dummy_stack);
 #endif
 
 	stk_table_set(table);