blob 6cb661bf (4431B) - Raw
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 57 ENTRY (_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. */ 118 END (_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