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.
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.
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.)
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
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.
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/>. */
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:
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.
45 %rsp The stack contains the arguments and environment:
49 (LP_SIZE*argc)(%rsp) NULL
50 (LP_SIZE*(argc+1))(%rsp) envp[0]
58 /* Clearing frame pointer is insufficient, use CFI. */
60 /* Clear the frame pointer. The ABI suggests this be done, to mark
61 the outermost frame obviously. */
64 /* Extract the arguments as encoded on the stack and set up
65 the arguments for __libc_start_main (int (*main) (int, char **, char **),
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:
78 mov %RDX_LP, %R9_LP /* Address of the shared library termination
81 mov (%rsp), %esi /* Simulate popping 4-byte argument count. */
84 popq %rsi /* Pop the argument count. */
86 /* argv starts just at the current stack top. */
88 /* Align the stack to a 16 byte boundary to follow the ABI. */
91 /* Push garbage because we push 8 more bytes. */
94 /* Provide the highest stack address to the user code (for stacks
95 which grow downwards). */
98 /* These used to be the addresses of .fini and .init. */
103 mov main@GOTPCREL(%rip), %RDI_LP
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
115 call *__libc_start_main@GOTPCREL(%rip)
117 hlt /* Crash if somehow `exit' does return. */
120 /* Define a symbol for the first piece of initialized data. */
126 data_start = __data_start