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SchrodingerZhuSchrodingerZhu
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[libc] implement sys/getauxval (#78493)
This PR implements `sys/getauxval` that can be used in both overlay builds and full builds.
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14 files changed

+325
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lines changed

‎libc/config/linux/aarch64/entrypoints.txt

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

‎libc/config/linux/app.h

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

‎libc/config/linux/arm/entrypoints.txt

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

‎libc/config/linux/riscv/entrypoints.txt

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

‎libc/config/linux/x86_64/entrypoints.txt

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

‎libc/src/sys/CMakeLists.txt

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

‎libc/src/sys/auxv/CMakeLists.txt

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if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
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add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_OS})
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endif()
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add_entrypoint_object(
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getauxval
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ALIAS
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DEPENDS
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.${LIBC_TARGET_OS}.getauxval
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)

‎libc/src/sys/auxv/getauxval.h

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//===-- Implementation header for getauxval function ------------*- C++ -*-===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_LIBC_SRC_SYS_AUXV_GETAUXVAL_H
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#define LLVM_LIBC_SRC_SYS_AUXV_GETAUXVAL_H
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#include <sys/auxv.h>
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namespace LIBC_NAMESPACE {
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unsigned long getauxval(unsigned long id);
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} // namespace LIBC_NAMESPACE
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#endif // LLVM_LIBC_SRC_SYS_AUXV_GETAUXVAL_H

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

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

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

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//===-- Implementation file for getauxval function --------------*- C++ -*-===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include "src/sys/auxv/getauxval.h"
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#include "config/linux/app.h"
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#include "src/__support/common.h"
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#include "src/errno/libc_errno.h"
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#include <linux/auxvec.h>
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// for guarded initialization
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#include "src/__support/threads/callonce.h"
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#include "src/__support/threads/linux/futex_word.h"
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// for mallocing the global auxv
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#include "src/sys/mman/mmap.h"
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#include "src/sys/mman/munmap.h"
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// for reading /proc/self/auxv
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#include "src/fcntl/open.h"
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#include "src/sys/prctl/prctl.h"
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#include "src/unistd/close.h"
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#include "src/unistd/read.h"
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// getauxval will work either with or without __cxa_atexit support.
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// In order to detect if __cxa_atexit is supported, we define a weak symbol.
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// We prefer __cxa_atexit as it is always defined as a C symbol whileas atexit
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// may not be created via objcopy yet. Also, for glibc, atexit is provided via
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// libc_nonshared.a rather than libc.so. So, it is may not be made ready for
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// overlay builds.
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extern "C" [[gnu::weak]] int __cxa_atexit(void (*callback)(void *),
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void *payload, void *);
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namespace LIBC_NAMESPACE {
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constexpr static size_t MAX_AUXV_ENTRIES = 64;
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// Helper to recover or set errno
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class AuxvErrnoGuard {
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public:
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AuxvErrnoGuard() : saved(libc_errno), failure(false) {}
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~AuxvErrnoGuard() { libc_errno = failure ? ENOENT : saved; }
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void mark_failure() { failure = true; }
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private:
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int saved;
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bool failure;
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};
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// Helper to manage the memory
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static AuxEntry *auxv = nullptr;
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class AuxvMMapGuard {
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public:
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constexpr static size_t AUXV_MMAP_SIZE = sizeof(AuxEntry) * MAX_AUXV_ENTRIES;
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AuxvMMapGuard()
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: ptr(mmap(nullptr, AUXV_MMAP_SIZE, PROT_READ | PROT_WRITE,
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MAP_PRIVATE | MAP_ANONYMOUS, -1, 0)) {}
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~AuxvMMapGuard() {
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if (ptr != MAP_FAILED)
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munmap(ptr, AUXV_MMAP_SIZE);
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}
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void submit_to_global() {
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// atexit may fail, we do not set it to global in that case.
70+
int ret = __cxa_atexit(
71+
[](void *) {
72+
munmap(auxv, AUXV_MMAP_SIZE);
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auxv = nullptr;
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},
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nullptr, nullptr);
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if (ret != 0)
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return;
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auxv = reinterpret_cast<AuxEntry *>(ptr);
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ptr = MAP_FAILED;
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}
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bool allocated() const { return ptr != MAP_FAILED; }
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void *get() const { return ptr; }
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private:
87+
void *ptr;
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};
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class AuxvFdGuard {
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public:
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AuxvFdGuard() : fd(open("/proc/self/auxv", O_RDONLY | O_CLOEXEC)) {}
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~AuxvFdGuard() {
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if (fd != -1)
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close(fd);
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}
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bool valid() const { return fd != -1; }
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int get() const { return fd; }
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private:
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int fd;
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};
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static void initialize_auxv_once(void) {
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// If we cannot get atexit, we cannot register the cleanup function.
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if (&__cxa_atexit == nullptr)
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return;
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AuxvMMapGuard mmap_guard;
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if (!mmap_guard.allocated())
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return;
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auto *ptr = reinterpret_cast<AuxEntry *>(mmap_guard.get());
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// We get one less than the max size to make sure the search always
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// terminates. MMAP private pages are zeroed out already.
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size_t available_size = AuxvMMapGuard::AUXV_MMAP_SIZE - sizeof(AuxEntryType);
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// PR_GET_AUXV is only available on Linux kernel 6.1 and above. If this is not
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// defined, we direcly fall back to reading /proc/self/auxv. In case the libc
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// is compiled and run on separate kernels, we also check the return value of
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// prctl.
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#ifdef PR_GET_AUXV
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int ret = prctl(PR_GET_AUXV, reinterpret_cast<unsigned long>(ptr),
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available_size, 0, 0);
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if (ret >= 0) {
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mmap_guard.submit_to_global();
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return;
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}
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#endif
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AuxvFdGuard fd_guard;
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if (!fd_guard.valid())
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return;
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auto *buf = reinterpret_cast<char *>(ptr);
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libc_errno = 0;
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bool error_detected = false;
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// Read until we use up all the available space or we finish reading the file.
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while (available_size != 0) {
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ssize_t bytes_read = read(fd_guard.get(), buf, available_size);
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if (bytes_read <= 0) {
139+
if (libc_errno == EINTR)
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continue;
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// Now, we either have an non-recoverable error or we have reached the end
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// of the file. Mark `error_detected` accordingly.
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if (bytes_read == -1)
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error_detected = true;
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break;
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}
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buf += bytes_read;
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available_size -= bytes_read;
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}
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// If we get out of the loop without an error, the auxv is ready.
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if (!error_detected)
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mmap_guard.submit_to_global();
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}
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static AuxEntry read_entry(int fd) {
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AuxEntry buf;
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ssize_t size = sizeof(AuxEntry);
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char *ptr = reinterpret_cast<char *>(&buf);
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while (size > 0) {
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ssize_t ret = read(fd, ptr, size);
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if (ret < 0) {
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if (libc_errno == EINTR)
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continue;
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// Error detected, return AT_NULL
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buf.id = AT_NULL;
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buf.value = AT_NULL;
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break;
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}
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ptr += ret;
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size -= ret;
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}
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return buf;
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}
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LLVM_LIBC_FUNCTION(unsigned long, getauxval, (unsigned long id)) {
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// Fast path when libc is loaded by its own initialization code. In this case,
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// app.auxv_ptr is already set to the auxv passed on the initial stack of the
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// process.
179+
AuxvErrnoGuard errno_guard;
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auto search_auxv = [&errno_guard](AuxEntry *auxv,
182+
unsigned long id) -> AuxEntryType {
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for (auto *ptr = auxv; ptr->id != AT_NULL; ptr++)
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if (ptr->id == id)
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return ptr->value;
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errno_guard.mark_failure();
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return AT_NULL;
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};
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// App is a weak symbol that is only defined if libc is linked to its own
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// initialization routine. We need to check if it is null.
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if (&app != nullptr)
194+
return search_auxv(app.auxv_ptr, id);
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static FutexWordType once_flag;
197+
callonce(reinterpret_cast<CallOnceFlag *>(&once_flag), initialize_auxv_once);
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if (auxv != nullptr)
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return search_auxv(auxv, id);
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// Fallback to use read without mmap
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AuxvFdGuard fd_guard;
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if (fd_guard.valid()) {
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while (true) {
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AuxEntry buf = read_entry(fd_guard.get());
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if (buf.id == AT_NULL)
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break;
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if (buf.id == id)
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return buf.value;
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}
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}
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// cannot find the entry after all methods, mark failure and return 0
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errno_guard.mark_failure();
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return AT_NULL;
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}
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} // namespace LIBC_NAMESPACE

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