1/* Startup code compliant to the 64 bit S/390 ELF 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#include <sysdep.h>
37
38/*
39 This is the canonical entry point, usually the first thing in the text
40 segment. Most registers' values are unspecified, except for:
41
42 %r14 Contains a function pointer to be registered with `atexit'.
43 This is how the dynamic linker arranges to have DT_FINI
44 functions called for shared libraries that have been loaded
45 before this code runs.
46
47 %r15 The stack contains the arguments and environment:
48 0(%r15) argc
49 8(%r15) argv[0]
50 ...
51 (8*argc)(%r15) NULL
52 (8*(argc+1))(%r15) envp[0]
53 ...
54 NULL
55*/
56
57 .text
58 .globl _start
59 .type _start,@function
60_start:
61 cfi_startproc
62 /* Mark r14 as undefined in order to stop unwinding here! */
63 cfi_undefined (r14)
64 /* Load argc and argv from stack. */
65 la %r4,8(%r15) # get argv
66 lg %r3,0(%r15) # get argc
67
68 /* Align the stack to a double word boundary. */
69 lghi %r0,-16
70 ngr %r15,%r0
71
72 /* Setup a stack frame and a parameter area. */
73 aghi %r15,-176 # make room on stack
74 xc 0(8,%r15),0(%r15) # clear back-chain
75
76 /* Set up arguments for __libc_start_main:
77 main, argc, argv, envp, _init, _fini, rtld_fini, stack_end
78 Note that envp will be determined later in __libc_start_main.
79 */
80 stmg %r14,%r15,160(%r15) # store rtld_fini/stack_end to parameter area
81 la %r7,160(%r15)
82 lghi %r6,0 # Used to be fini.
83 lghi %r5,0 # Used to be init.
84
85 /* Ok, now branch to the libc main routine. */
86#ifdef PIC
87# ifdef SHARED
88 /* Used for dynamic linked position independent executable.
89 => Scrt1.o */
90 larl %r2,main@GOTENT # load pointer to main
91 lg %r2,0(%r2)
92# else
93 /* Used for dynamic linked position dependent executable.
94 => crt1.o (glibc configured without --disable-default-pie:
95 PIC is defined)
96 Or for static linked position independent executable.
97 => rcrt1.o (only available if glibc configured without
98 --disable-default-pie: PIC is defined) */
99 larl %r2,__wrap_main
100# endif
101 brasl %r14,__libc_start_main@plt
102#else
103 /* Used for dynamic/static linked position dependent executable.
104 => crt1.o (glibc configured with --disable-default-pie:
105 PIC and SHARED are not defined) */
106 larl %r2,main # load pointer to main
107 brasl %r14,__libc_start_main
108#endif
109
110 /* Crash if __libc_start_main returns. */
111 .word 0
112
113 cfi_endproc
114
115#if defined PIC && !defined SHARED
116 /* When main is not defined in the executable but in a shared library
117 then a wrapper is needed in crt1.o of the static-pie enabled libc,
118 because crt1.o and rcrt1.o share code and the later must avoid the
119 use of GOT relocations before __libc_start_main is called. */
120__wrap_main:
121 cfi_startproc
122 larl %r1,main@GOTENT # load pointer to main
123 lg %r1,0(%r1)
124 br %r1
125 cfi_endproc
126#endif
127
128 /* Define a symbol for the first piece of initialized data. */
129 .data
130 .globl __data_start
131__data_start:
132 .long 0
133 .weak data_start
134 data_start = __data_start
135

source code of glibc/sysdeps/s390/s390-64/start.S