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//===-- 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