1/* Startup code compliant to the ELF x86-64 ABI.
2 Copyright (C) 2001-2024 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
127

source code of glibc/sysdeps/x86_64/start.S