[libc] implement sys/getauxval

libc/config/linux/aarch64/entrypoints.txt

@@ -168,6 +168,9 @@ set(TARGET_LIBC_ENTRYPOINTS
     # sys/prctl.h entrypoints
     libc.src.sys.prctl.prctl
 
+    # sys/auxv.h entrypoints
+    libc.src.sys.auxv.getauxval
+
     # termios.h entrypoints
     libc.src.termios.cfgetispeed
     libc.src.termios.cfgetospeed

libc/config/linux/app.h

@@ -93,7 +93,7 @@ struct AppProperties {
   AuxEntry *auxv_ptr;
 };
 
-extern AppProperties app;
+[[gnu::weak]] extern AppProperties app;
 
 // The descriptor of a thread's TLS area.
 struct TLSDescriptor {

libc/config/linux/arm/entrypoints.txt

@@ -95,6 +95,9 @@ set(TARGET_LIBC_ENTRYPOINTS
 
     # sys/prctl.h entrypoints
     libc.src.sys.prctl.prctl
+
+    # sys/auxv.h entrypoints
+    libc.src.sys.auxv.getauxval
 )
 
 set(TARGET_LIBM_ENTRYPOINTS

libc/config/linux/riscv/entrypoints.txt

@@ -174,6 +174,9 @@ set(TARGET_LIBC_ENTRYPOINTS
     # sys/prctl.h entrypoints
     libc.src.sys.prctl.prctl
 
+    # sys/auxv.h entrypoints
+    libc.src.sys.auxv.getauxval
+
     # termios.h entrypoints
     libc.src.termios.cfgetispeed
     libc.src.termios.cfgetospeed

libc/config/linux/x86_64/entrypoints.txt

@@ -174,6 +174,9 @@ set(TARGET_LIBC_ENTRYPOINTS
     # sys/prctl.h entrypoints
     libc.src.sys.prctl.prctl
 
+    # sys/auxv.h entrypoints
+    libc.src.sys.auxv.getauxval
+
     # termios.h entrypoints
     libc.src.termios.cfgetispeed
     libc.src.termios.cfgetospeed

libc/src/sys/CMakeLists.txt

@@ -1,3 +1,4 @@
+add_subdirectory(auxv)
 add_subdirectory(mman)
 add_subdirectory(random)
 add_subdirectory(resource)

libc/src/sys/auxv/CMakeLists.txt

@@ -0,0 +1,10 @@
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
+  add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
+endif()
+
+add_entrypoint_object(
+  getauxval
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.getauxval
+)

libc/src/sys/auxv/getauxval.h

@@ -0,0 +1,20 @@
+//===-- Implementation header 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SYS_AUXV_GETAUXVAL_H
+#define LLVM_LIBC_SRC_SYS_AUXV_GETAUXVAL_H
+
+#include <sys/auxv.h>
+
+namespace LIBC_NAMESPACE {
+
+unsigned long getauxval(unsigned long id);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_SYS_AUXV_GETAUXVAL_H

libc/src/sys/auxv/linux/CMakeLists.txt

@@ -0,0 +1,18 @@
+add_entrypoint_object(
+  getauxval
+  SRCS
+  getauxval.cpp
+  HDRS
+    ../getauxval.h
+  DEPENDS
+    libc.src.sys.prctl.prctl
+    libc.src.sys.mman.mmap
+    libc.src.sys.mman.munmap
+    libc.src.__support.threads.callonce
+    libc.src.__support.common
+    libc.src.errno.errno
+    libc.config.linux.app_h
+    libc.src.fcntl.open
+    libc.src.unistd.read
+    libc.src.unistd.close
+)

libc/src/sys/auxv/linux/getauxval.cpp

@@ -0,0 +1,217 @@
+//===-- 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

libc/test/src/sys/CMakeLists.txt

@@ -8,3 +8,4 @@ add_subdirectory(stat)
 add_subdirectory(utsname)
 add_subdirectory(wait)
 add_subdirectory(prctl)
+add_subdirectory(auxv)

libc/test/src/sys/auxv/CMakeLists.txt

@@ -0,0 +1,3 @@
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
+  add_subdirectory(${LIBC_TARGET_OS})
+endif()

libc/test/src/sys/auxv/linux/CMakeLists.txt

@@ -0,0 +1,14 @@
+add_custom_target(libc_sys_auxv_unittests)
+add_libc_unittest(
+  getauxval_test
+  SUITE
+    libc_sys_auxv_unittests
+  SRCS
+    getauxval_test.cpp
+  DEPENDS
+    libc.include.sys_auxv
+    libc.src.errno.errno
+    libc.src.sys.auxv.getauxval
+    libc.test.UnitTest.ErrnoSetterMatcher
+    libc.src.string.strstr
+)

libc/test/src/sys/auxv/linux/getauxval_test.cpp

@@ -0,0 +1,28 @@
+//===-- Unittests for getaxuval -------------------------------------------===//
+//
+// 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/errno/libc_errno.h"
+#include "src/sys/auxv/getauxval.h"
+#include "test/UnitTest/ErrnoSetterMatcher.h"
+#include "test/UnitTest/Test.h"
+#include <src/string/strstr.h>
+#include <sys/auxv.h>
+
+using namespace LIBC_NAMESPACE::testing::ErrnoSetterMatcher;
+
+TEST(LlvmLibcGetauxvalTest, Basic) {
+  EXPECT_THAT(LIBC_NAMESPACE::getauxval(AT_PAGESZ),
+              returns(GT(0ul)).with_errno(EQ(0)));
+  const char *filename;
+  auto getfilename = [&filename]() {
+    auto value = LIBC_NAMESPACE::getauxval(AT_EXECFN);
+    filename = reinterpret_cast<const char *>(value);
+    return value;
+  };
+  EXPECT_THAT(getfilename(), returns(NE(0ul)).with_errno(EQ(0)));
+  ASSERT_TRUE(LIBC_NAMESPACE::strstr(filename, "getauxval_test") != nullptr);
+}