1/* Startup code compliant to the ELF x86-64 ABI.
  2   Copyright (C) 2001-2025 Free Software Foundation, Inc.
  3   This file is part of the GNU C Library.
  4
  5   The GNU C Library is free software; you can redistribute it and/or
  6   modify it under the terms of the GNU Lesser General Public
  7   License as published by the Free Software Foundation; either
  8   version 2.1 of the License, or (at your option) any later version.
  9
 10   In addition to the permissions in the GNU Lesser General Public
 11   License, the Free Software Foundation gives you unlimited
 12   permission to link the compiled version of this file with other
 13   programs, and to distribute those programs without any restriction
 14   coming from the use of this file. (The GNU Lesser General Public
 15   License restrictions do apply in other respects; for example, they
 16   cover modification of the file, and distribution when not linked
 17   into another program.)
 18
 19   Note that people who make modified versions of this file are not
 20   obligated to grant this special exception for their modified
 21   versions; it is their choice whether to do so. The GNU Lesser
 22   General Public License gives permission to release a modified
 23   version without this exception; this exception also makes it
 24   possible to release a modified version which carries forward this
 25   exception.
 26
 27   The GNU C Library is distributed in the hope that it will be useful,
 28   but WITHOUT ANY WARRANTY; without even the implied warranty of
 29   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 30   Lesser General Public License for more details.
 31
 32   You should have received a copy of the GNU Lesser General Public
 33   License along with the GNU C Library; if not, see
 34   <https://www.gnu.org/licenses/>.  */
 35
 36/* This is the canonical entry point, usually the first thing in the text
 37   segment.  The SVR4/i386 ABI (pages 3-31, 3-32) says that when the entry
 38   point runs, most registers' values are unspecified, except for:
 39
 40   %rdx		Contains a function pointer to be registered with `atexit'.
 41		This is how the dynamic linker arranges to have DT_FINI
 42		functions called for shared libraries that have been loaded
 43		before this code runs.
 44
 45   %rsp		The stack contains the arguments and environment:
 46		0(%rsp)				argc
 47		LP_SIZE(%rsp)			argv[0]
 48		...
 49		(LP_SIZE*argc)(%rsp)		NULL
 50		(LP_SIZE*(argc+1))(%rsp)	envp[0]
 51		...
 52						NULL
 53*/
 54
 55#include <sysdep.h>
 56
 57ENTRY (_start)
 58	/* Clearing frame pointer is insufficient, use CFI.  */
 59	cfi_undefined (rip)
 60	/* Clear the frame pointer.  The ABI suggests this be done, to mark
 61	   the outermost frame obviously.  */
 62	xorl %ebp, %ebp
 63
 64	/* Extract the arguments as encoded on the stack and set up
 65	   the arguments for __libc_start_main (int (*main) (int, char **, char **),
 66		   int argc, char *argv,
 67		   void (*init) (void), void (*fini) (void),
 68		   void (*rtld_fini) (void), void *stack_end).
 69	   The arguments are passed via registers and on the stack:
 70	main:		%rdi
 71	argc:		%rsi
 72	argv:		%rdx
 73	init:		%rcx
 74	fini:		%r8
 75	rtld_fini:	%r9
 76	stack_end:	stack.	*/
 77
 78	mov %RDX_LP, %R9_LP	/* Address of the shared library termination
 79				   function.  */
 80#ifdef __ILP32__
 81	mov (%rsp), %esi	/* Simulate popping 4-byte argument count.  */
 82	add $4, %esp
 83#else
 84	popq %rsi		/* Pop the argument count.  */
 85#endif
 86	/* argv starts just at the current stack top.  */
 87	mov %RSP_LP, %RDX_LP
 88	/* Align the stack to a 16 byte boundary to follow the ABI.  */
 89	and  $~15, %RSP_LP
 90
 91	/* Push garbage because we push 8 more bytes.  */
 92	pushq %rax
 93
 94	/* Provide the highest stack address to the user code (for stacks
 95	   which grow downwards).  */
 96	pushq %rsp
 97
 98	/* These used to be the addresses of .fini and .init.  */
 99	xorl %r8d, %r8d
100	xorl %ecx, %ecx
101
102#ifdef PIC
103	mov main@GOTPCREL(%rip), %RDI_LP
104#else
105	mov $main, %RDI_LP
106#endif
107
108	/* Call the user's main function, and exit with its value.
109	   But let the libc call main.  Since __libc_start_main in
110	   libc.so is called very early, lazy binding isn't relevant
111	   here.  Use indirect branch via GOT to avoid extra branch
112	   to PLT slot.  In case of static executable, ld in binutils
113	   2.26 or above can convert indirect branch into direct
114	   branch.  */
115	call *__libc_start_main@GOTPCREL(%rip)
116
117	hlt			/* Crash if somehow `exit' does return.	 */
118END (_start)
119
120/* Define a symbol for the first piece of initialized data.  */
121	.data
122	.globl __data_start
123__data_start:
124	.long 0
125	.weak data_start
126	data_start = __data_start