[libc-commits] [libc] [libc] implement sys/getauxval (PR #78493)

[Nick Desaulniers via libc-commits]

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@@ -0,0 +1,198 @@
+//===-- 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
+struct AuxvErrnoGuard {
+  int saved;
+  bool failure;
+  AuxvErrnoGuard() : saved(libc_errno), failure(false) {}
+  ~AuxvErrnoGuard() { libc_errno = failure ? ENOENT : saved; }
+  void mark_failure() { failure = true; }
+};
+
+// Helper to manage the memory
+static AuxEntry *auxv = nullptr;
+
+struct AuxvMMapGuard {
+  constexpr static size_t AUXV_MMAP_SIZE = sizeof(AuxEntry) * MAX_AUXV_ENTRIES;
+  void *ptr;
+  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() { return ptr != MAP_FAILED; }
+};
+
+struct AuxvFdGuard {
+  int fd;
+  AuxvFdGuard() : fd(open("/proc/self/auxv", O_RDONLY | O_CLOEXEC)) {}
+  ~AuxvFdGuard() {
+    if (fd != -1) {
+      close(fd);
+    }
+  }
+  bool valid() { return fd != -1; }
+};
+
+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.ptr);
+
+  // 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);
+#if defined(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;
+  while (available_size != 0) {
+    ssize_t bytes_read = read(fd_guard.fd, buf, available_size);
+    if (bytes_read <= 0) {
+      if (libc_errno == EINTR)
+        continue;
+      error_detected = bytes_read < 0;
+      break;
+    }
+    available_size -= bytes_read;
+  }
----------------
nickdesaulniers wrote:

I don't follow what this loop is doing.  Mind adding some comments?

https://github.com/llvm/llvm-project/pull/78493