crt: make sure stack is aligned (and restored on exit)

crt/Makefile

@@ -23,7 +23,7 @@ CFLAGS += -fms-extensions -Wno-microsoft-anon-tag
 CFLAGS += -O1
 
 OBJ := crt.o syscall.o klog.o nid.o kernel.o rtld.o rtld_so.o rtld_sprx.o \
-       rtld_payload.o rtld_dlfcn.o mdbg.o patch.o kernel_procio.o
+       rtld_payload.o rtld_dlfcn.o mdbg.o patch.o kernel_procio.o _start.o
 
 .PHONY: all clean install
 
@@ -53,6 +53,9 @@ crt1.o: $(OBJ)
 %.o: %.c
 	$(CC) -c $(CFLAGS) -o $@ $^
 
+%.o: %.S
+	$(CC) -c $(CFLAGS) -masm=intel -o $@ $^
+
 clean:
 	rm -f *.o *.a
 

crt/_start.S

@@ -0,0 +1,37 @@
+/* Copyright (C) 2026 John Törnblom
+
+This program is free software; you can redistribute it and/or modify it
+under the terms of the GNU General Public License as published by the
+Free Software Foundation; either version 3, or (at your option) any
+later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; see the file COPYING. If not, see
+<http://www.gnu.org/licenses/>.  */
+
+.intel_syntax noprefix
+
+.data
+.local __crt_reg_backup
+.type  __crt_reg_backup, @object
+__crt_reg_backup:
+	.quad 0
+
+.text
+.extern __crt_start
+.type   __crt_start, @function
+
+.text
+.global _start
+.type   _start, @function
+_start:
+	mov [__crt_reg_backup + rip], rsp
+	and rsp, -16
+	call __crt_start
+	mov rsp, [__crt_reg_backup + rip]
+	ret

crt/crt.c

@@ -194,7 +194,7 @@ payload_get_args(void) {
  * Entry-point invoked by the ELF loader.
  **/
 int
-_start(payload_args_t *args) {
+__crt_start(payload_args_t *args) {
   int err;
 
   // clear .bss section