1//===-- Implementation file for getauxval function --------------*- C++ -*-===//
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
9#include "src/sys/auxv/getauxval.h"
10#include "config/app.h"
11#include "src/__support/common.h"
12#include "src/__support/libc_errno.h"
13#include "src/__support/macros/config.h"
14#include <linux/auxvec.h>
15
16// for guarded initialization
17#include "src/__support/threads/callonce.h"
18#include "src/__support/threads/linux/futex_word.h"
19
20// for mallocing the global auxv
21#include "src/sys/mman/mmap.h"
22#include "src/sys/mman/munmap.h"
23
24// for reading /proc/self/auxv
25#include "src/fcntl/open.h"
26#include "src/sys/prctl/prctl.h"
27#include "src/unistd/close.h"
28#include "src/unistd/read.h"
29
30// getauxval will work either with or without __cxa_atexit support.
31// In order to detect if __cxa_atexit is supported, we define a weak symbol.
32// We prefer __cxa_atexit as it is always defined as a C symbol whileas atexit
33// may not be created via objcopy yet. Also, for glibc, atexit is provided via
34// libc_nonshared.a rather than libc.so. So, it is may not be made ready for
35// overlay builds.
36extern "C" [[gnu::weak]] int __cxa_atexit(void (*callback)(void *),
37 void *payload, void *);
38
39namespace LIBC_NAMESPACE_DECL {
40
41constexpr static size_t MAX_AUXV_ENTRIES = 64;
42
43// Helper to recover or set errno
44class AuxvErrnoGuard {
45public:
46 AuxvErrnoGuard() : saved(libc_errno), failure(false) {}
47 ~AuxvErrnoGuard() { libc_errno = failure ? ENOENT : saved; }
48 void mark_failure() { failure = true; }
49
50private:
51 int saved;
52 bool failure;
53};
54
55// Helper to manage the memory
56static AuxEntry *auxv = nullptr;
57
58class AuxvMMapGuard {
59public:
60 constexpr static size_t AUXV_MMAP_SIZE = sizeof(AuxEntry) * MAX_AUXV_ENTRIES;
61
62 AuxvMMapGuard()
63 : ptr(mmap(nullptr, AUXV_MMAP_SIZE, PROT_READ | PROT_WRITE,
64 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0)) {}
65 ~AuxvMMapGuard() {
66 if (ptr != MAP_FAILED)
67 munmap(ptr, AUXV_MMAP_SIZE);
68 }
69 void submit_to_global() {
70 // atexit may fail, we do not set it to global in that case.
71 int ret = __cxa_atexit(
72 callback: [](void *) {
73 munmap(auxv, AUXV_MMAP_SIZE);
74 auxv = nullptr;
75 },
76 payload: nullptr, nullptr);
77
78 if (ret != 0)
79 return;
80
81 auxv = reinterpret_cast<AuxEntry *>(ptr);
82 ptr = MAP_FAILED;
83 }
84 bool allocated() const { return ptr != MAP_FAILED; }
85 void *get() const { return ptr; }
86
87private:
88 void *ptr;
89};
90
91class AuxvFdGuard {
92public:
93 AuxvFdGuard() : fd(open("/proc/self/auxv", O_RDONLY | O_CLOEXEC)) {}
94 ~AuxvFdGuard() {
95 if (fd != -1)
96 close(fd);
97 }
98 bool valid() const { return fd != -1; }
99 int get() const { return fd; }
100
101private:
102 int fd;
103};
104
105static void initialize_auxv_once(void) {
106 // If we cannot get atexit, we cannot register the cleanup function.
107 if (&__cxa_atexit == nullptr)
108 return;
109
110 AuxvMMapGuard mmap_guard;
111 if (!mmap_guard.allocated())
112 return;
113 auto *ptr = reinterpret_cast<AuxEntry *>(mmap_guard.get());
114
115 // We get one less than the max size to make sure the search always
116 // terminates. MMAP private pages are zeroed out already.
117 size_t available_size = AuxvMMapGuard::AUXV_MMAP_SIZE - sizeof(AuxEntryType);
118 // PR_GET_AUXV is only available on Linux kernel 6.1 and above. If this is not
119 // defined, we direcly fall back to reading /proc/self/auxv. In case the libc
120 // is compiled and run on separate kernels, we also check the return value of
121 // prctl.
122#ifdef PR_GET_AUXV
123 int ret = prctl(PR_GET_AUXV, reinterpret_cast<unsigned long>(ptr),
124 available_size, 0, 0);
125 if (ret >= 0) {
126 mmap_guard.submit_to_global();
127 return;
128 }
129#endif
130 AuxvFdGuard fd_guard;
131 if (!fd_guard.valid())
132 return;
133 auto *buf = reinterpret_cast<char *>(ptr);
134 libc_errno = 0;
135 bool error_detected = false;
136 // Read until we use up all the available space or we finish reading the file.
137 while (available_size != 0) {
138 ssize_t bytes_read = read(fd_guard.get(), buf, available_size);
139 if (bytes_read <= 0) {
140 if (libc_errno == EINTR)
141 continue;
142 // Now, we either have an non-recoverable error or we have reached the end
143 // of the file. Mark `error_detected` accordingly.
144 if (bytes_read == -1)
145 error_detected = true;
146 break;
147 }
148 buf += bytes_read;
149 available_size -= bytes_read;
150 }
151 // If we get out of the loop without an error, the auxv is ready.
152 if (!error_detected)
153 mmap_guard.submit_to_global();
154}
155
156static AuxEntry read_entry(int fd) {
157 AuxEntry buf;
158 size_t size = sizeof(AuxEntry);
159 char *ptr = reinterpret_cast<char *>(&buf);
160 while (size > 0) {
161 ssize_t ret = read(fd, ptr, size);
162 if (ret < 0) {
163 if (libc_errno == EINTR)
164 continue;
165 // Error detected, return AT_NULL
166 buf.id = AT_NULL;
167 buf.value = AT_NULL;
168 break;
169 }
170 ptr += ret;
171 size -= ret;
172 }
173 return buf;
174}
175
176LLVM_LIBC_FUNCTION(unsigned long, getauxval, (unsigned long id)) {
177 // Fast path when libc is loaded by its own initialization code. In this case,
178 // app.auxv_ptr is already set to the auxv passed on the initial stack of the
179 // process.
180 AuxvErrnoGuard errno_guard;
181
182 auto search_auxv = [&errno_guard](AuxEntry *auxv,
183 unsigned long id) -> AuxEntryType {
184 for (auto *ptr = auxv; ptr->id != AT_NULL; ptr++)
185 if (ptr->id == id)
186 return ptr->value;
187
188 errno_guard.mark_failure();
189 return AT_NULL;
190 };
191
192 // App is a weak symbol that is only defined if libc is linked to its own
193 // initialization routine. We need to check if it is null.
194 if (&app != nullptr)
195 return search_auxv(app.auxv_ptr, id);
196
197 static FutexWordType once_flag;
198 callonce(reinterpret_cast<CallOnceFlag *>(&once_flag), initialize_auxv_once);
199 if (auxv != nullptr)
200 return search_auxv(auxv, id);
201
202 // Fallback to use read without mmap
203 AuxvFdGuard fd_guard;
204 if (fd_guard.valid()) {
205 while (true) {
206 AuxEntry buf = read_entry(fd_guard.get());
207 if (buf.id == AT_NULL)
208 break;
209 if (buf.id == id)
210 return buf.value;
211 }
212 }
213
214 // cannot find the entry after all methods, mark failure and return 0
215 errno_guard.mark_failure();
216 return AT_NULL;
217}
218} // namespace LIBC_NAMESPACE_DECL
219

source code of libc/src/sys/auxv/linux/getauxval.cpp