1/* Startup code compliant to the ELF x86-64 ABI.
2 Copyright (C) 2001-2021 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4 Contributed by Andreas Jaeger <aj@suse.de>, 2001.
5
6 The GNU C Library is free software; you can redistribute it and/or
7 modify it under the terms of the GNU Lesser General Public
8 License as published by the Free Software Foundation; either
9 version 2.1 of the License, or (at your option) any later version.
10
11 In addition to the permissions in the GNU Lesser General Public
12 License, the Free Software Foundation gives you unlimited
13 permission to link the compiled version of this file with other
14 programs, and to distribute those programs without any restriction
15 coming from the use of this file. (The GNU Lesser General Public
16 License restrictions do apply in other respects; for example, they
17 cover modification of the file, and distribution when not linked
18 into another program.)
19
20 Note that people who make modified versions of this file are not
21 obligated to grant this special exception for their modified
22 versions; it is their choice whether to do so. The GNU Lesser
23 General Public License gives permission to release a modified
24 version without this exception; this exception also makes it
25 possible to release a modified version which carries forward this
26 exception.
27
28 The GNU C Library is distributed in the hope that it will be useful,
29 but WITHOUT ANY WARRANTY; without even the implied warranty of
30 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
31 Lesser General Public License for more details.
32
33 You should have received a copy of the GNU Lesser General Public
34 License along with the GNU C Library; if not, see
35 <https://www.gnu.org/licenses/>. */
36
37/* This is the canonical entry point, usually the first thing in the text
38 segment. The SVR4/i386 ABI (pages 3-31, 3-32) says that when the entry
39 point runs, most registers' values are unspecified, except for:
40
41 %rdx Contains a function pointer to be registered with `atexit'.
42 This is how the dynamic linker arranges to have DT_FINI
43 functions called for shared libraries that have been loaded
44 before this code runs.
45
46 %rsp The stack contains the arguments and environment:
47 0(%rsp) argc
48 LP_SIZE(%rsp) argv[0]
49 ...
50 (LP_SIZE*argc)(%rsp) NULL
51 (LP_SIZE*(argc+1))(%rsp) envp[0]
52 ...
53 NULL
54*/
55
56#include <sysdep.h>
57
58ENTRY (_start)
59 /* Clearing frame pointer is insufficient, use CFI. */
60 cfi_undefined (rip)
61 /* Clear the frame pointer. The ABI suggests this be done, to mark
62 the outermost frame obviously. */
63 xorl %ebp, %ebp
64
65 /* Extract the arguments as encoded on the stack and set up
66 the arguments for __libc_start_main (int (*main) (int, char **, char **),
67 int argc, char *argv,
68 void (*init) (void), void (*fini) (void),
69 void (*rtld_fini) (void), void *stack_end).
70 The arguments are passed via registers and on the stack:
71 main: %rdi
72 argc: %rsi
73 argv: %rdx
74 init: %rcx
75 fini: %r8
76 rtld_fini: %r9
77 stack_end: stack. */
78
79 mov %RDX_LP, %R9_LP /* Address of the shared library termination
80 function. */
81#ifdef __ILP32__
82 mov (%rsp), %esi /* Simulate popping 4-byte argument count. */
83 add $4, %esp
84#else
85 popq %rsi /* Pop the argument count. */
86#endif
87 /* argv starts just at the current stack top. */
88 mov %RSP_LP, %RDX_LP
89 /* Align the stack to a 16 byte boundary to follow the ABI. */
90 and $~15, %RSP_LP
91
92 /* Push garbage because we push 8 more bytes. */
93 pushq %rax
94
95 /* Provide the highest stack address to the user code (for stacks
96 which grow downwards). */
97 pushq %rsp
98
99 /* These used to be the addresses of .fini and .init. */
100 xorl %r8d, %r8d
101 xorl %ecx, %ecx
102
103#ifdef PIC
104 mov main@GOTPCREL(%rip), %RDI_LP
105#else
106 mov $main, %RDI_LP
107#endif
108
109 /* Call the user's main function, and exit with its value.
110 But let the libc call main. Since __libc_start_main in
111 libc.so is called very early, lazy binding isn't relevant
112 here. Use indirect branch via GOT to avoid extra branch
113 to PLT slot. In case of static executable, ld in binutils
114 2.26 or above can convert indirect branch into direct
115 branch. */
116 call *__libc_start_main@GOTPCREL(%rip)
117
118 hlt /* Crash if somehow `exit' does return. */
119END (_start)
120
121/* Define a symbol for the first piece of initialized data. */
122 .data
123 .globl __data_start
124__data_start:
125 .long 0
126 .weak data_start
127 data_start = __data_start
128