glibc/sysdeps/i386/start.S

/* Startup code compliant to the ELF i386 ABI.
   Copyright (C) 1995-2022 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   In addition to the permissions in the GNU Lesser General Public
   License, the Free Software Foundation gives you unlimited
   permission to link the compiled version of this file with other
   programs, and to distribute those programs without any restriction
   coming from the use of this file. (The GNU Lesser General Public
   License restrictions do apply in other respects; for example, they
   cover modification of the file, and distribution when not linked
   into another program.)

   Note that people who make modified versions of this file are not
   obligated to grant this special exception for their modified
   versions; it is their choice whether to do so. The GNU Lesser
   General Public License gives permission to release a modified
   version without this exception; this exception also makes it
   possible to release a modified version which carries forward this
   exception.

   The GNU C Library 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
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

/* This is the canonical entry point, usually the first thing in the text
   segment.  The SVR4/i386 ABI (pages 3-31, 3-32) says that when the entry
   point runs, most registers' values are unspecified, except for:

   %edx		Contains a function pointer to be registered with `atexit'.
		This is how the dynamic linker arranges to have DT_FINI
		functions called for shared libraries that have been loaded
		before this code runs.

   %esp		The stack contains the arguments and environment:
		0(%esp)			argc
		4(%esp)			argv[0]
		...
		(4*argc)(%esp)		NULL
		(4*(argc+1))(%esp)	envp[0]
		...
					NULL
*/

#include <sysdep.h>

ENTRY (_start)
	/* Clearing frame pointer is insufficient, use CFI.  */
	cfi_undefined (eip)
	/* Clear the frame pointer.  The ABI suggests this be done, to mark
	   the outermost frame obviously.  */
	xorl %ebp, %ebp

	/* Extract the arguments as encoded on the stack and set up
	   the arguments for `main': argc, argv.  envp will be determined
	   later in __libc_start_main.  */
	popl %esi		/* Pop the argument count.  */
	movl %esp, %ecx		/* argv starts just at the current stack top.*/

	/* Before pushing the arguments align the stack to a 16-byte
	(SSE needs 16-byte alignment) boundary to avoid penalties from
	misaligned accesses.  Thanks to Edward Seidl <seidl@janed.com>
	for pointing this out.  */
	andl $0xfffffff0, %esp
	pushl %eax		/* Push garbage because we allocate
				   28 more bytes.  */

	/* Provide the highest stack address to the user code (for stacks
	   which grow downwards).  */
	pushl %esp

	pushl %edx		/* Push address of the shared library
				   termination function.  */

#ifdef PIC
	/* Load PIC register.  */
	call 1f
	addl $_GLOBAL_OFFSET_TABLE_, %ebx

	/* This used to be the addresses of .fini and .init.  */
	pushl $0
	pushl $0

	pushl %ecx		/* Push second argument: argv.  */
	pushl %esi		/* Push first argument: argc.  */

# ifdef SHARED
	pushl main@GOT(%ebx)
# else
	/* Avoid relocation in static PIE since _start is called before
	   it is relocated.  Don't use "leal main@GOTOFF(%ebx), %eax"
	   since main may be in a shared object.  Linker will convert
	   "movl main@GOT(%ebx), %eax" to "leal main@GOTOFF(%ebx), %eax"
	   if main is defined locally.  */
	movl main@GOT(%ebx), %eax
	pushl %eax
# endif

	/* Call the user's main function, and exit with its value.
	   But let the libc call main.    */
	call __libc_start_main@PLT
#else
	/* This used to be the addresses of .fini and .init.  */
	pushl $0
	pushl $0

	pushl %ecx		/* Push second argument: argv.  */
	pushl %esi		/* Push first argument: argc.  */

	pushl $main

	/* Call the user's main function, and exit with its value.
	   But let the libc call main.    */
	call __libc_start_main
#endif

	hlt			/* Crash if somehow `exit' does return.  */

#ifdef PIC
1:	movl	(%esp), %ebx
	ret
#endif
END (_start)

/* To fulfill the System V/i386 ABI we need this symbol.  Yuck, it's so
   meaningless since we don't support machines < 80386.  */
	.section .rodata
	.globl _fp_hw
_fp_hw:	.long 3
	.size _fp_hw, 4
	.type _fp_hw,@object

/* Define a symbol for the first piece of initialized data.  */
	.data
	.globl __data_start
__data_start:
	.long 0
	.weak data_start
	data_start = __data_start