[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);
+}