csu: Implement _start using as to satisfy unwinders on x86_64

The right unwinding stop indicator should be CFI-undefined PC.
https://dwarfstd.org/doc/Dwarf3.pdf - page 118:
If a Return Address register is defined in the virtual unwind table,
and its rule is undefined (for example, by DW_CFA_undefined), then
there is no return address and no call address, and the virtual
unwind of stack activations is complete.

This requires the crt code be built with unwind tables, for that remove
-fno-asynchronous-unwind-tables to enable unwind tables generation.

PR:			241562, 246322, 246537
Reviewed by:		kib
Differential Revision:	https://reviews.freebsd.org/D40780

lib/csu/Makefile.inc

@@ -20,7 +20,6 @@ CRT1OBJS+=	crtbrand.o feature_note.o ignore_init_note.o
 ACFLAGS+=	-DLOCORE
 
 CFLAGS+=	-DSTRIP_FBSDID
-CFLAGS+=	-fno-asynchronous-unwind-tables
 CFLAGS+=	-fno-omit-frame-pointer
 CFLAGS+=	-I${.CURDIR:H}/common \
 		-I${SRCTOP}/lib/libc/include
@@ -35,25 +34,28 @@ FILESDIR=	${LIBDIR}
 # These FILES qualify as libraries for the purpose of LIBRARIES_ONLY.
 .undef LIBRARIES_ONLY
 
-CLEANFILES+=	${OBJS} ${CRT1OBJS} crt1_c.o gcrt1_c.o Scrt1_c.o
-CLEANFILES+=	crti_s.o
+CRT1SRC?=	crt1_c.c
+CRT1OBJ?=
 
-crt1.o:	crt1_c.o ${CRT1OBJS}
+CLEANFILES+=	${OBJS} ${CRT1OBJS} crt1_c.o gcrt1_c.o Scrt1_c.o
+CLEANFILES+=	crti_s.o ${CRT1SRC:C/.[S|c]$/.o/}
+
+crt1.o:	${CRT1SRC:C/.[S|c]$/.o/} ${CRT1OBJS} ${CRT1OBJ}
 	${LD} ${_LDFLAGS} -o ${.TARGET} -r ${.ALLSRC:M*.o}
 .if ${MACHINE_ARCH} == "i386"
 	${OBJCOPY} --localize-symbol _start1 ${.TARGET}
 .endif
 
-gcrt1_c.o: crt1_c.c
-	${CC} ${CFLAGS} -DGCRT -c -o ${.TARGET} ${.CURDIR}/crt1_c.c
+gcrt1_c.o: ${CRT1SRC}
+	${CC} ${CFLAGS} -DGCRT -c -o ${.TARGET} ${.CURDIR}/${CRT1SRC}
 
-gcrt1.o: gcrt1_c.o ${CRT1OBJS}
+gcrt1.o: gcrt1_c.o ${CRT1OBJS} ${CRT1OBJ}
 	${LD} ${_LDFLAGS} -o ${.TARGET} -r ${.ALLSRC:M*.o}
 
-Scrt1_c.o: crt1_c.c
-	${CC} ${CFLAGS} -fPIC -DPIC -c -o ${.TARGET} ${.CURDIR}/crt1_c.c
+Scrt1_c.o: ${CRT1SRC}
+	${CC} ${CFLAGS} -fPIC -DPIC -c -o ${.TARGET} ${.CURDIR}/${CRT1SRC}
 
-Scrt1.o: Scrt1_c.o ${CRT1OBJS}
+Scrt1.o: Scrt1_c.o ${CRT1OBJS} ${CRT1OBJ}
 	${LD} ${_LDFLAGS} -o ${.TARGET} -r ${.ALLSRC:M*.o}
 .if ${MACHINE_ARCH} == "i386"
 	${OBJCOPY} --localize-symbol _start1 ${.TARGET}

lib/csu/amd64/Makefile

@@ -5,4 +5,7 @@
 CFLAGS+=	-I${.CURDIR}
 CFLAGS+=	-fno-omit-frame-pointer
 
+CRT1SRC=	crt1_s.S
+CRT1OBJ=	crt1_c.o
+
 .include <bsd.lib.mk>

lib/csu/amd64/crt1_c.c

@@ -29,26 +29,4 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
-#include "libc_private.h"
 #include "csu_common.h"
-
-void _start(char **, void (*)(void)) __dead2;
-
-/* The entry function. */
-void
-_start(char **ap, void (*cleanup)(void))
-{
-	int argc;
-	char **argv;
-	char **env;
-
-	argc = *(long *)(void *)ap;
-	argv = ap + 1;
-	env = ap + 2 + argc;
-#ifdef GCRT
-	__libc_start1_gcrt(argc, argv, env, cleanup, main, &eprol, &etext);
-__asm__("eprol:");
-#else
-	__libc_start1(argc, argv, env, cleanup, main);
-#endif
-}

lib/csu/amd64/crt1_s.S

@@ -0,0 +1,88 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (C) 2023 Dmitry Chagin <dchagin@FreeBSD.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <machine/asm.h>
+
+
+	.text
+	.align 8
+
+	/*
+	 * The program entry point
+	 *		  %rdi		%rsi
+	 * void _start(char **ap, void (*cleanup)(void)) __dead2
+	 */
+	.globl	_start
+	.type	_start, @function
+_start:
+	.cfi_startproc
+	.cfi_undefined %rip		/* Terminate call chain. */
+	pushq	%rbp			/* Align stack, terminate call chain. */
+	.cfi_def_cfa_offset 8
+	movq	%rsp, %rbp
+	.cfi_offset %rbp, -16
+	.cfi_def_cfa_register %rbp
+#ifdef GCRT
+	subq	$16, %rsp
+#endif
+	movq	%rsi, %rcx
+	movq	%rdi, %rsi		/* argv = ap */
+	addq	$8, %rsi		/* argv += 1 */
+	movq	%rdi, %rdx		/* env = ap */
+	addq	$16, %rdx		/* env += 2 */
+	movslq	(%rdi), %rax
+	movl	%eax, %edi		/* argc = *(long *)(void *)ap */
+	shlq	$3, %rax
+	addq	%rax, %rdx		/* env += argc */
+#ifdef PIC
+	/*
+	 * XXX. %rip relative addressing does not intended to use in the
+	 * large memory model due to offset from %rip is limited to 32 bits.
+	 */
+	leaq	main(%rip), %r8
+#else
+	movabsq	$main, %r8
+#endif
+#ifdef GCRT
+	movabsq	$eprol, %r9
+	movabsq	$etext, %rax
+	movq    %rax, (%rsp)
+	/*
+	 *		      %edi  %rsi  %rdx  %rcx    %r8    %r9    (%rsp)
+	 * __libc_start1_gcrt(argc, argv, env, cleanup, main, &eprol, &etext)
+	 */
+	callq	__libc_start1_gcrt
+eprol:
+#else
+	/* __libc_start1(argc, argv, env, cleanup, main) */
+	callq	__libc_start1
+#endif
+	int3
+	.cfi_endproc
+	.size   _start, . - _start
+
+	.section .note.GNU-stack,"",%progbits