1//===-- Implementation file of do_start -----------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8#include "startup/linux/do_start.h"
9#include "config/linux/app.h"
10#include "include/llvm-libc-macros/link-macros.h"
11#include "src/__support/OSUtil/syscall.h"
12#include "src/__support/macros/config.h"
13#include "src/__support/threads/thread.h"
14#include "src/stdlib/atexit.h"
15#include "src/stdlib/exit.h"
16#include "src/unistd/environ.h"
17
18#include <linux/auxvec.h>
19#include <linux/elf.h>
20#include <stdint.h>
21#include <sys/mman.h>
22#include <sys/syscall.h>
23
24extern "C" int main(int argc, char **argv, char **envp);
25
26extern "C" {
27// These arrays are present in the .init_array and .fini_array sections.
28// The symbols are inserted by linker when it sees references to them.
29extern uintptr_t __preinit_array_start[];
30extern uintptr_t __preinit_array_end[];
31extern uintptr_t __init_array_start[];
32extern uintptr_t __init_array_end[];
33extern uintptr_t __fini_array_start[];
34extern uintptr_t __fini_array_end[];
35// https://refspecs.linuxbase.org/elf/gabi4+/ch5.dynamic.html#dynamic_section
36// This symbol is provided by the dynamic linker. It can be undefined depending
37// on how the program is loaded exactly.
38[[gnu::weak,
39 gnu::visibility("hidden")]] extern const Elf64_Dyn _DYNAMIC[]; // NOLINT
40}
41
42namespace LIBC_NAMESPACE_DECL {
43AppProperties app;
44
45using InitCallback = void(int, char **, char **);
46using FiniCallback = void(void);
47
48static void call_init_array_callbacks(int argc, char **argv, char **env) {
49 size_t preinit_array_size = __preinit_array_end - __preinit_array_start;
50 for (size_t i = 0; i < preinit_array_size; ++i)
51 reinterpret_cast<InitCallback *>(__preinit_array_start[i])(argc, argv, env);
52 size_t init_array_size = __init_array_end - __init_array_start;
53 for (size_t i = 0; i < init_array_size; ++i)
54 reinterpret_cast<InitCallback *>(__init_array_start[i])(argc, argv, env);
55}
56
57static void call_fini_array_callbacks() {
58 size_t fini_array_size = __fini_array_end - __fini_array_start;
59 for (size_t i = fini_array_size; i > 0; --i)
60 reinterpret_cast<FiniCallback *>(__fini_array_start[i - 1])();
61}
62
63static ThreadAttributes main_thread_attrib;
64static TLSDescriptor tls;
65// We separate teardown_main_tls from callbacks as callback function themselves
66// may require TLS.
67void teardown_main_tls() { cleanup_tls(tls.addr, tls.size); }
68
69[[noreturn]] void do_start() {
70 auto tid = syscall_impl<long>(SYS_gettid);
71 if (tid <= 0)
72 syscall_impl<long>(SYS_exit, 1);
73 main_thread_attrib.tid = static_cast<int>(tid);
74
75 // After the argv array, is a 8-byte long NULL value before the array of env
76 // values. The end of the env values is marked by another 8-byte long NULL
77 // value. We step over it (the "+ 1" below) to get to the env values.
78 uintptr_t *env_ptr = app.args->argv + app.args->argc + 1;
79 uintptr_t *env_end_marker = env_ptr;
80 app.env_ptr = env_ptr;
81 while (*env_end_marker)
82 ++env_end_marker;
83
84 // Initialize the POSIX global declared in unistd.h
85 environ = reinterpret_cast<char **>(env_ptr);
86
87 // After the env array, is the aux-vector. The end of the aux-vector is
88 // denoted by an AT_NULL entry.
89 ElfW(Phdr) *program_hdr_table = nullptr;
90 uintptr_t program_hdr_count = 0;
91 app.auxv_ptr = reinterpret_cast<AuxEntry *>(env_end_marker + 1);
92 for (auto *aux_entry = app.auxv_ptr; aux_entry->id != AT_NULL; ++aux_entry) {
93 switch (aux_entry->id) {
94 case AT_PHDR:
95 program_hdr_table = reinterpret_cast<ElfW(Phdr) *>(aux_entry->value);
96 break;
97 case AT_PHNUM:
98 program_hdr_count = aux_entry->value;
99 break;
100 case AT_PAGESZ:
101 app.page_size = aux_entry->value;
102 break;
103 default:
104 break; // TODO: Read other useful entries from the aux vector.
105 }
106 }
107
108 ptrdiff_t base = 0;
109 app.tls.size = 0;
110 ElfW(Phdr) *tls_phdr = nullptr;
111
112 for (uintptr_t i = 0; i < program_hdr_count; ++i) {
113 ElfW(Phdr) &phdr = program_hdr_table[i];
114 if (phdr.p_type == PT_PHDR)
115 base = reinterpret_cast<ptrdiff_t>(program_hdr_table) - phdr.p_vaddr;
116 if (phdr.p_type == PT_DYNAMIC && _DYNAMIC)
117 base = reinterpret_cast<ptrdiff_t>(_DYNAMIC) - phdr.p_vaddr;
118 if (phdr.p_type == PT_TLS)
119 tls_phdr = &phdr;
120 // TODO: adjust PT_GNU_STACK
121 }
122
123 app.tls.address = tls_phdr->p_vaddr + base;
124 app.tls.size = tls_phdr->p_memsz;
125 app.tls.init_size = tls_phdr->p_filesz;
126 app.tls.align = tls_phdr->p_align;
127
128 // This descriptor has to be static since its cleanup function cannot
129 // capture the context.
130 init_tls(tls);
131 if (tls.size != 0 && !set_thread_ptr(tls.tp))
132 syscall_impl<long>(SYS_exit, 1);
133
134 self.attrib = &main_thread_attrib;
135 main_thread_attrib.atexit_callback_mgr =
136 internal::get_thread_atexit_callback_mgr();
137
138 // We want the fini array callbacks to be run after other atexit
139 // callbacks are run. So, we register them before running the init
140 // array callbacks as they can potentially register their own atexit
141 // callbacks.
142 atexit(&call_fini_array_callbacks);
143
144 call_init_array_callbacks(argc: static_cast<int>(app.args->argc),
145 argv: reinterpret_cast<char **>(app.args->argv),
146 env: reinterpret_cast<char **>(env_ptr));
147
148 int retval = main(argc: static_cast<int>(app.args->argc),
149 argv: reinterpret_cast<char **>(app.args->argv),
150 envp: reinterpret_cast<char **>(env_ptr));
151
152 exit(retval);
153}
154
155} // namespace LIBC_NAMESPACE_DECL
156

source code of libc/startup/linux/do_start.cpp