sysdeps/arm/start.S

   1 /* Startup code for ARM & ELF
   2    Copyright (C) 1995-2019 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    <http://www.gnu.org/licenses/>.  */
  35
  36 /* This is the canonical entry point, usually the first thing in the text
  37    segment.
  38
  39         Note that the code in the .init section has already been run.
  40         This includes _init and _libc_init
  41
  42
  43         At this entry point, most registers' values are unspecified, except:
  44
  45    a1           Contains a function pointer to be registered with `atexit'.
  46                 This is how the dynamic linker arranges to have DT_FINI
  47                 functions called for shared libraries that have been loaded
  48                 before this code runs.
  49
  50    sp           The stack contains the arguments and environment:
  51                 0(sp)                   argc
  52                 4(sp)                   argv[0]
  53                 ...
  54                 (4*argc)(sp)            NULL
  55                 (4*(argc+1))(sp)        envp[0]
  56                 ...
  57                                         NULL
  58 */
  59
  60 /* Tag_ABI_align8_preserved: This code preserves 8-byte
  61    alignment in any callee.  */
  62         .eabi_attribute 25, 1
  63 /* Tag_ABI_align8_needed: This code may require 8-byte alignment from
  64    the caller.  */
  65         .eabi_attribute 24, 1
  66
  67 #if defined(__thumb2__)
  68         .thumb
  69         .syntax unified
  70 #endif
  71
  72         .text
  73         .globl _start
  74         .type _start,#function
  75 _start:
  76        /* Protect against unhandled exceptions.  */
  77        .fnstart
  78         /* Clear the frame pointer and link register since this is the outermost frame. */
  79         mov fp, #0
  80         mov lr, #0
  81
  82         /* Pop argc off the stack and save a pointer to argv */
  83         pop { a2 }
  84         mov a3, sp
  85
  86         /* Push stack limit */
  87         push { a3 }
  88
  89         /* Push rtld_fini */
  90         push { a1 }
  91
  92 #ifdef PIC
  93         ldr sl, .L_GOT
  94         adr a4, .L_GOT
  95         add sl, sl, a4
  96
  97         ldr ip, .L_GOT+4        /* __libc_csu_fini */
  98         ldr ip, [sl, ip]
  99
 100         push { ip }             /* Push __libc_csu_fini */
 101
 102         ldr a4, .L_GOT+8        /* __libc_csu_init */
 103         ldr a4, [sl, a4]
 104
 105         ldr a1, .L_GOT+12       /* main */
 106         ldr a1, [sl, a1]
 107
 108         /* __libc_start_main (main, argc, argv, init, fini, rtld_fini, stack_end) */
 109         /* Let the libc call main and exit with its return code.  */
 110         bl __libc_start_main(PLT)
 111 #else
 112         /* Fetch address of __libc_csu_fini */
 113         ldr ip, =__libc_csu_fini
 114
 115         /* Push __libc_csu_fini */
 116         push { ip }
 117
 118         /* Set up the other arguments in registers */
 119         ldr a1, =main
 120         ldr a4, =__libc_csu_init
 121
 122         /* __libc_start_main (main, argc, argv, init, fini, rtld_fini, stack_end) */
 123         /* Let the libc call main and exit with its return code.  */
 124         bl __libc_start_main
 125 #endif
 126
 127         /* should never get here....*/
 128         bl abort
 129
 130 #ifdef PIC
 131         .align 2
 132 .L_GOT:
 133         .word _GLOBAL_OFFSET_TABLE_ - .L_GOT
 134         .word __libc_csu_fini(GOT)
 135         .word __libc_csu_init(GOT)
 136         .word main(GOT)
 137 #endif
 138
 139        .cantunwind
 140        .fnend
 141
 142 /* Define a symbol for the first piece of initialized data.  */
 143         .data
 144         .globl __data_start
 145 __data_start:
 146         .long 0
 147         .weak data_start
 148         data_start = __data_start