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c9cf6dde | 1 | /* Startup code compliant to the ELF x86-64 ABI. |
6d7e8eda | 2 | Copyright (C) 2001-2023 Free Software Foundation, Inc. |
c9cf6dde | 3 | This file is part of the GNU C Library. |
c9cf6dde AJ |
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 | ||
92c6ccd1 RM |
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 | ||
c9cf6dde AJ |
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 | |
59ba27a6 | 33 | License along with the GNU C Library; if not, see |
5a82c748 | 34 | <https://www.gnu.org/licenses/>. */ |
c9cf6dde AJ |
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: | |
6cdef1ab L |
46 | 0(%rsp) argc |
47 | LP_SIZE(%rsp) argv[0] | |
c9cf6dde | 48 | ... |
6cdef1ab L |
49 | (LP_SIZE*argc)(%rsp) NULL |
50 | (LP_SIZE*(argc+1))(%rsp) envp[0] | |
c9cf6dde | 51 | ... |
6cdef1ab | 52 | NULL |
c9cf6dde AJ |
53 | */ |
54 | ||
6a1bd2a1 | 55 | #include <sysdep.h> |
c9cf6dde | 56 | |
467fec0b | 57 | ENTRY (_start) |
6a1bd2a1 JK |
58 | /* Clearing frame pointer is insufficient, use CFI. */ |
59 | cfi_undefined (rip) | |
c9cf6dde AJ |
60 | /* Clear the frame pointer. The ABI suggests this be done, to mark |
61 | the outermost frame obviously. */ | |
ee618985 | 62 | xorl %ebp, %ebp |
c9cf6dde AJ |
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 | ||
6cdef1ab | 78 | mov %RDX_LP, %R9_LP /* Address of the shared library termination |
c9cf6dde | 79 | function. */ |
6cdef1ab L |
80 | #ifdef __ILP32__ |
81 | mov (%rsp), %esi /* Simulate popping 4-byte argument count. */ | |
82 | add $4, %esp | |
83 | #else | |
c9cf6dde | 84 | popq %rsi /* Pop the argument count. */ |
6cdef1ab L |
85 | #endif |
86 | /* argv starts just at the current stack top. */ | |
87 | mov %RSP_LP, %RDX_LP | |
c9cf6dde | 88 | /* Align the stack to a 16 byte boundary to follow the ABI. */ |
6cdef1ab | 89 | and $~15, %RSP_LP |
c9cf6dde | 90 | |
6cdef1ab L |
91 | /* Push garbage because we push 8 more bytes. */ |
92 | pushq %rax | |
c9cf6dde AJ |
93 | |
94 | /* Provide the highest stack address to the user code (for stacks | |
95 | which grow downwards). */ | |
96 | pushq %rsp | |
97 | ||
035c012e FW |
98 | /* These used to be the addresses of .fini and .init. */ |
99 | xorl %r8d, %r8d | |
100 | xorl %ecx, %ecx | |
d49c05e0 | 101 | |
035c012e | 102 | #ifdef PIC |
29691210 | 103 | mov main@GOTPCREL(%rip), %RDI_LP |
d49c05e0 | 104 | #else |
29691210 | 105 | mov $main, %RDI_LP |
ac187dc4 | 106 | #endif |
c9cf6dde AJ |
107 | |
108 | /* Call the user's main function, and exit with its value. | |
ac187dc4 L |
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) | |
c9cf6dde AJ |
116 | |
117 | hlt /* Crash if somehow `exit' does return. */ | |
467fec0b | 118 | END (_start) |
c9cf6dde AJ |
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 |