[libc] implement sys/getauxval #78493

SchrodingerZhu · 2024-01-17T19:31:45Z

As there are usages in overlay mode. This PR implements sys/getauxval that can be used in both overlay builds and full builds.

SchrodingerZhu · 2024-01-17T19:53:27Z

@nickdesaulniers there is a complication that internal atexit may not exist. Should I try calling extern "C" version instead?

nickdesaulniers · 2024-01-18T17:42:07Z

cc @@llvm/pr-subscribers-libc (perhaps there's a better list)

I suspect that for baremetal targets, we should document a shortlist of functions that their runtime MUST provide. @Prabhuk or @petrhosek might have some thoughts about this, too.

@michaelrj-google is in the middle of some design discussions around abstracting I/O that might also be relevant here.

michaelrj-google · 2024-01-19T23:03:41Z

The I/O discussions aren't super relevant to this change, though I appreciate asking. The I/O discussions mostly centered on enabling a mode with no function pointers (discussed here: https://discourse.llvm.org/t/rfc-modernizing-and-formalizing-the-file-abstraction-interface/70605/6) and the other pieces of the FILE abstraction that need to be finished (i.e. wide chars, buffer flushing when reading)

petrhosek · 2024-01-19T23:12:54Z

Once this lands, I'd like to use it in the implementation of #78804.

llvmbot · 2024-01-22T19:28:09Z

@llvm/pr-subscribers-libc

Author: Schrodinger ZHU Yifan (SchrodingerZhu)

Changes

WIP, tests are to be added.

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

14 Files Affected:

(modified) libc/config/linux/aarch64/entrypoints.txt (+3)
(modified) libc/config/linux/app.h (+1-1)
(modified) libc/config/linux/arm/entrypoints.txt (+3)
(modified) libc/config/linux/riscv/entrypoints.txt (+3)
(modified) libc/config/linux/x86_64/entrypoints.txt (+3)
(modified) libc/src/sys/CMakeLists.txt (+1)
(added) libc/src/sys/auxv/CMakeLists.txt (+10)
(added) libc/src/sys/auxv/getauxval.h (+20)
(added) libc/src/sys/auxv/linux/CMakeLists.txt (+18)
(added) libc/src/sys/auxv/linux/getauxval.cpp (+196)
(modified) libc/test/src/sys/CMakeLists.txt (+1)
(added) libc/test/src/sys/auxv/CMakeLists.txt (+3)
(added) libc/test/src/sys/auxv/linux/CMakeLists.txt (+14)
(added) libc/test/src/sys/auxv/linux/getauxval_test.cpp (+27)

diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
index 625fa6bffe63c65..3f66a582f5e3ee3 100644
--- a/libc/config/linux/aarch64/entrypoints.txt
+++ b/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
diff --git a/libc/config/linux/app.h b/libc/config/linux/app.h
index 1b3523deb1b23ec..766cd49e88f6f76 100644
--- a/libc/config/linux/app.h
+++ b/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 {
diff --git a/libc/config/linux/arm/entrypoints.txt b/libc/config/linux/arm/entrypoints.txt
index c75ac2302d4ac45..301870d337ca007 100644
--- a/libc/config/linux/arm/entrypoints.txt
+++ b/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
diff --git a/libc/config/linux/riscv/entrypoints.txt b/libc/config/linux/riscv/entrypoints.txt
index ec2a16f5cf473bd..0331ef782cf74a8 100644
--- a/libc/config/linux/riscv/entrypoints.txt
+++ b/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
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
index 094bdde2e1589cb..d5ab891674a2d86 100644
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/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
diff --git a/libc/src/sys/CMakeLists.txt b/libc/src/sys/CMakeLists.txt
index 12e2020f013ab12..81098294176ad5b 100644
--- a/libc/src/sys/CMakeLists.txt
+++ b/libc/src/sys/CMakeLists.txt
@@ -1,3 +1,4 @@
+add_subdirectory(auxv)
 add_subdirectory(mman)
 add_subdirectory(random)
 add_subdirectory(resource)
diff --git a/libc/src/sys/auxv/CMakeLists.txt b/libc/src/sys/auxv/CMakeLists.txt
new file mode 100644
index 000000000000000..4065761064b129a
--- /dev/null
+++ b/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
+)
diff --git a/libc/src/sys/auxv/getauxval.h b/libc/src/sys/auxv/getauxval.h
new file mode 100644
index 000000000000000..7c9fb846e919841
--- /dev/null
+++ b/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
diff --git a/libc/src/sys/auxv/linux/CMakeLists.txt b/libc/src/sys/auxv/linux/CMakeLists.txt
new file mode 100644
index 000000000000000..b38d63ee0329c79
--- /dev/null
+++ b/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
+)
diff --git a/libc/src/sys/auxv/linux/getauxval.cpp b/libc/src/sys/auxv/linux/getauxval.cpp
new file mode 100644
index 000000000000000..4b2aabe40c39b5a
--- /dev/null
+++ b/libc/src/sys/auxv/linux/getauxval.cpp
@@ -0,0 +1,196 @@
+//===-- 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.
+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;
+  }
+  if (!error_detected) {
+    mmap_guard.submit_to_global();
+  }
+}
+
+static AuxEntry read_entry(int fd) {
+  AuxEntry buf;
+  ssize_t size = sizeof(AuxEntry);
+  while (size > 0) {
+    ssize_t ret = read(fd, &buf, size);
+    if (ret < 0) {
+      if (libc_errno == EINTR)
+        continue;
+      buf.id = AT_NULL;
+      buf.value = AT_NULL;
+      break;
+    }
+    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.fd);
+      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
diff --git a/libc/test/src/sys/CMakeLists.txt b/libc/test/src/sys/CMakeLists.txt
index a87e77da7d2cdb7..c7095456383b300 100644
--- a/libc/test/src/sys/CMakeLists.txt
+++ b/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)
diff --git a/libc/test/src/sys/auxv/CMakeLists.txt b/libc/test/src/sys/auxv/CMakeLists.txt
new file mode 100644
index 000000000000000..b4bbe81c92ff2eb
--- /dev/null
+++ b/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()
diff --git a/libc/test/src/sys/auxv/linux/CMakeLists.txt b/libc/test/src/sys/auxv/linux/CMakeLists.txt
new file mode 100644
index 000000000000000..c1e82a1f0a46c3c
--- /dev/null
+++ b/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
+)
diff --git a/libc/test/src/sys/auxv/linux/getauxval_test.cpp b/libc/test/src/sys/auxv/linux/getauxval_test.cpp
new file mode 100644
index 000000000000000..3b0c4e1b4175f28
--- /dev/null
+++ b/libc/test/src/sys/auxv/linux/getauxval_test.cpp
@@ -0,0 +1,27 @@
+//===-- 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)));
+  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)));
+  ASSERT_TRUE(LIBC_NAMESPACE::strstr(filename, "getauxval_test") != nullptr);
+}

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

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

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

SchrodingerZhu · 2024-01-22T23:09:16Z

I failed to reproduce the error locally. let me check what asan say.

SchrodingerZhu force-pushed the libc/getauxval-impl branch from 0478641 to 613adc2 Compare January 22, 2024 18:20

[libc] implement sys/getauxval

579c5ae

SchrodingerZhu force-pushed the libc/getauxval-impl branch from 613adc2 to 579c5ae Compare January 22, 2024 18:23

fix bugs

0e2fcff

SchrodingerZhu force-pushed the libc/getauxval-impl branch from d8997d3 to 0e2fcff Compare January 22, 2024 19:26

SchrodingerZhu marked this pull request as ready for review January 22, 2024 19:27

llvmbot added the libc label Jan 22, 2024

SchrodingerZhu requested review from petrhosek and michaelrj-google January 22, 2024 19:28

more comments

6ec61d7

nickdesaulniers reviewed Jan 22, 2024

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SchrodingerZhu added 3 commits January 22, 2024 16:29

address CR

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address CR

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nickdesaulniers reviewed Jan 22, 2024

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clarify error_detected

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nickdesaulniers approved these changes Jan 22, 2024

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clean code style

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fix UB

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SchrodingerZhu merged commit 4beb723 into llvm:main Jan 23, 2024

SchrodingerZhu deleted the libc/getauxval-impl branch January 23, 2024 02:47

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