| //===-- Implementation file for getauxval function --------------*- C++ -*-===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "src/sys/auxv/getauxval.h" |
| #include "config/linux/app.h" |
| #include "src/__support/common.h" |
| #include "src/errno/libc_errno.h" |
| #include <linux/auxvec.h> |
| |
| // for guarded initialization |
| #include "src/__support/threads/callonce.h" |
| #include "src/__support/threads/linux/futex_word.h" |
| |
| // for mallocing the global auxv |
| #include "src/sys/mman/mmap.h" |
| #include "src/sys/mman/munmap.h" |
| |
| // for reading /proc/self/auxv |
| #include "src/fcntl/open.h" |
| #include "src/sys/prctl/prctl.h" |
| #include "src/unistd/close.h" |
| #include "src/unistd/read.h" |
| |
| // getauxval will work either with or without __cxa_atexit support. |
| // In order to detect if __cxa_atexit is supported, we define a weak symbol. |
| // We prefer __cxa_atexit as it is always defined as a C symbol whileas atexit |
| // may not be created via objcopy yet. Also, for glibc, atexit is provided via |
| // libc_nonshared.a rather than libc.so. So, it is may not be made ready for |
| // overlay builds. |
| extern "C" [[gnu::weak]] int __cxa_atexit(void (*callback)(void *), |
| void *payload, void *); |
| |
| namespace LIBC_NAMESPACE { |
| |
| constexpr static size_t MAX_AUXV_ENTRIES = 64; |
| |
| // Helper to recover or set errno |
| class AuxvErrnoGuard { |
| public: |
| AuxvErrnoGuard() : saved(libc_errno), failure(false) {} |
| ~AuxvErrnoGuard() { libc_errno = failure ? ENOENT : saved; } |
| void mark_failure() { failure = true; } |
| |
| private: |
| int saved; |
| bool failure; |
| }; |
| |
| // Helper to manage the memory |
| static AuxEntry *auxv = nullptr; |
| |
| class AuxvMMapGuard { |
| public: |
| constexpr static size_t AUXV_MMAP_SIZE = sizeof(AuxEntry) * MAX_AUXV_ENTRIES; |
| |
| AuxvMMapGuard() |
| : ptr(mmap(nullptr, AUXV_MMAP_SIZE, PROT_READ | PROT_WRITE, |
| MAP_PRIVATE | MAP_ANONYMOUS, -1, 0)) {} |
| ~AuxvMMapGuard() { |
| if (ptr != MAP_FAILED) |
| munmap(ptr, AUXV_MMAP_SIZE); |
| } |
| void submit_to_global() { |
| // atexit may fail, we do not set it to global in that case. |
| int ret = __cxa_atexit( |
| [](void *) { |
| munmap(auxv, AUXV_MMAP_SIZE); |
| auxv = nullptr; |
| }, |
| nullptr, nullptr); |
| |
| if (ret != 0) |
| return; |
| |
| auxv = reinterpret_cast<AuxEntry *>(ptr); |
| ptr = MAP_FAILED; |
| } |
| bool allocated() const { return ptr != MAP_FAILED; } |
| void *get() const { return ptr; } |
| |
| private: |
| void *ptr; |
| }; |
| |
| class AuxvFdGuard { |
| public: |
| AuxvFdGuard() : fd(open("/proc/self/auxv", O_RDONLY | O_CLOEXEC)) {} |
| ~AuxvFdGuard() { |
| if (fd != -1) |
| close(fd); |
| } |
| bool valid() const { return fd != -1; } |
| int get() const { return fd; } |
| |
| private: |
| int fd; |
| }; |
| |
| static void initialize_auxv_once(void) { |
| // If we cannot get atexit, we cannot register the cleanup function. |
| if (&__cxa_atexit == nullptr) |
| return; |
| |
| AuxvMMapGuard mmap_guard; |
| if (!mmap_guard.allocated()) |
| return; |
| auto *ptr = reinterpret_cast<AuxEntry *>(mmap_guard.get()); |
| |
| // We get one less than the max size to make sure the search always |
| // terminates. MMAP private pages are zeroed out already. |
| size_t available_size = AuxvMMapGuard::AUXV_MMAP_SIZE - sizeof(AuxEntryType); |
| // PR_GET_AUXV is only available on Linux kernel 6.1 and above. If this is not |
| // defined, we direcly fall back to reading /proc/self/auxv. In case the libc |
| // is compiled and run on separate kernels, we also check the return value of |
| // prctl. |
| #ifdef PR_GET_AUXV |
| int ret = prctl(PR_GET_AUXV, reinterpret_cast<unsigned long>(ptr), |
| available_size, 0, 0); |
| if (ret >= 0) { |
| mmap_guard.submit_to_global(); |
| return; |
| } |
| #endif |
| AuxvFdGuard fd_guard; |
| if (!fd_guard.valid()) |
| return; |
| auto *buf = reinterpret_cast<char *>(ptr); |
| libc_errno = 0; |
| bool error_detected = false; |
| // Read until we use up all the available space or we finish reading the file. |
| while (available_size != 0) { |
| ssize_t bytes_read = read(fd_guard.get(), buf, available_size); |
| if (bytes_read <= 0) { |
| if (libc_errno == EINTR) |
| continue; |
| // Now, we either have an non-recoverable error or we have reached the end |
| // of the file. Mark `error_detected` accordingly. |
| if (bytes_read == -1) |
| error_detected = true; |
| break; |
| } |
| buf += bytes_read; |
| available_size -= bytes_read; |
| } |
| // If we get out of the loop without an error, the auxv is ready. |
| if (!error_detected) |
| mmap_guard.submit_to_global(); |
| } |
| |
| static AuxEntry read_entry(int fd) { |
| AuxEntry buf; |
| ssize_t size = sizeof(AuxEntry); |
| char *ptr = reinterpret_cast<char *>(&buf); |
| while (size > 0) { |
| ssize_t ret = read(fd, ptr, size); |
| if (ret < 0) { |
| if (libc_errno == EINTR) |
| continue; |
| // Error detected, return AT_NULL |
| buf.id = AT_NULL; |
| buf.value = AT_NULL; |
| break; |
| } |
| ptr += ret; |
| size -= ret; |
| } |
| return buf; |
| } |
| |
| LLVM_LIBC_FUNCTION(unsigned long, getauxval, (unsigned long id)) { |
| // Fast path when libc is loaded by its own initialization code. In this case, |
| // app.auxv_ptr is already set to the auxv passed on the initial stack of the |
| // process. |
| AuxvErrnoGuard errno_guard; |
| |
| auto search_auxv = [&errno_guard](AuxEntry *auxv, |
| unsigned long id) -> AuxEntryType { |
| for (auto *ptr = auxv; ptr->id != AT_NULL; ptr++) |
| if (ptr->id == id) |
| return ptr->value; |
| |
| errno_guard.mark_failure(); |
| return AT_NULL; |
| }; |
| |
| // App is a weak symbol that is only defined if libc is linked to its own |
| // initialization routine. We need to check if it is null. |
| if (&app != nullptr) |
| return search_auxv(app.auxv_ptr, id); |
| |
| static FutexWordType once_flag; |
| callonce(reinterpret_cast<CallOnceFlag *>(&once_flag), initialize_auxv_once); |
| if (auxv != nullptr) |
| return search_auxv(auxv, id); |
| |
| // Fallback to use read without mmap |
| AuxvFdGuard fd_guard; |
| if (fd_guard.valid()) { |
| while (true) { |
| AuxEntry buf = read_entry(fd_guard.get()); |
| if (buf.id == AT_NULL) |
| break; |
| if (buf.id == id) |
| return buf.value; |
| } |
| } |
| |
| // cannot find the entry after all methods, mark failure and return 0 |
| errno_guard.mark_failure(); |
| return AT_NULL; |
| } |
| } // namespace LIBC_NAMESPACE |