PR #1433: Fix incorrect timespec definition on 32-bit platforms with 64-bit time_t
Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/1433
Some 32-bit configurations may use 64-bit time_t, which leads to different layout of userspace timespec and the one expected by SYS_futex implementation in kernel. In particular the issue occurs when using musl libc which has switched to unconditional 64-bit time_t definition.
This patch introduces custom struct timespec with two longs when old SYS_futex is used to match the kernel timespec definition.
Merge 2eaca415da825b3f31a90f58a35bdef2b6d2a6c5 into f8bf909108b3604a00590a074f2986c0895514d2
Merging this change closes #1433
COPYBARA_INTEGRATE_REVIEW=https://github.com/abseil/abseil-cpp/pull/1433 from olegartys:futex_time64_bug 2eaca415da825b3f31a90f58a35bdef2b6d2a6c5
PiperOrigin-RevId: 528796119
Change-Id: Idaa952f64bd97c6dc9703a8b44deac43e29ff9ae
diff --git a/absl/synchronization/internal/futex.h b/absl/synchronization/internal/futex.h
index 55078f1..573c01b 100644
--- a/absl/synchronization/internal/futex.h
+++ b/absl/synchronization/internal/futex.h
@@ -32,6 +32,7 @@
#include <atomic>
#include <cstdint>
+#include <limits>
#include "absl/base/optimization.h"
#include "absl/synchronization/internal/kernel_timeout.h"
@@ -79,6 +80,18 @@
#if defined(SYS_futex_time64) && !defined(SYS_futex)
#define SYS_futex SYS_futex_time64
+using FutexTimespec = struct timespec;
+#else
+// Some libc implementations have switched to an unconditional 64-bit `time_t`
+// definition. This means that `struct timespec` may not match the layout
+// expected by the kernel ABI on 32-bit platforms. So we define the
+// FutexTimespec that matches the kernel timespec definition. It should be safe
+// to use this struct for 64-bit userspace builds too, since it will use another
+// SYS_futex kernel call with 64-bit tv_sec inside timespec.
+struct FutexTimespec {
+ long tv_sec; // NOLINT
+ long tv_nsec; // NOLINT
+};
#endif
class FutexImpl {
@@ -93,12 +106,13 @@
// CLOCK_REALTIME reaches `*abs_timeout`, or until woken by `Wake()`.
static int WaitAbsoluteTimeout(std::atomic<int32_t>* v, int32_t val,
const struct timespec* abs_timeout) {
+ FutexTimespec ts;
// https://locklessinc.com/articles/futex_cheat_sheet/
// Unlike FUTEX_WAIT, FUTEX_WAIT_BITSET uses absolute time.
- auto err =
- syscall(SYS_futex, reinterpret_cast<int32_t*>(v),
- FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME,
- val, abs_timeout, nullptr, FUTEX_BITSET_MATCH_ANY);
+ auto err = syscall(
+ SYS_futex, reinterpret_cast<int32_t*>(v),
+ FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME, val,
+ ToFutexTimespec(abs_timeout, &ts), nullptr, FUTEX_BITSET_MATCH_ANY);
if (err != 0) {
return -errno;
}
@@ -109,10 +123,12 @@
// `*rel_timeout` has elapsed, or until woken by `Wake()`.
static int WaitRelativeTimeout(std::atomic<int32_t>* v, int32_t val,
const struct timespec* rel_timeout) {
+ FutexTimespec ts;
// Atomically check that the futex value is still 0, and if it
// is, sleep until abs_timeout or until woken by FUTEX_WAKE.
- auto err = syscall(SYS_futex, reinterpret_cast<int32_t*>(v),
- FUTEX_PRIVATE_FLAG, val, rel_timeout);
+ auto err =
+ syscall(SYS_futex, reinterpret_cast<int32_t*>(v), FUTEX_PRIVATE_FLAG,
+ val, ToFutexTimespec(rel_timeout, &ts));
if (err != 0) {
return -errno;
}
@@ -128,6 +144,26 @@
}
return 0;
}
+
+ private:
+ static FutexTimespec* ToFutexTimespec(const struct timespec* userspace_ts,
+ FutexTimespec* futex_ts) {
+ if (userspace_ts == nullptr) {
+ return nullptr;
+ }
+
+ using FutexSeconds = decltype(futex_ts->tv_sec);
+ using FutexNanoseconds = decltype(futex_ts->tv_nsec);
+
+ constexpr auto kMaxSeconds{(std::numeric_limits<FutexSeconds>::max)()};
+ if (userspace_ts->tv_sec > kMaxSeconds) {
+ futex_ts->tv_sec = kMaxSeconds;
+ } else {
+ futex_ts->tv_sec = static_cast<FutexSeconds>(userspace_ts->tv_sec);
+ }
+ futex_ts->tv_nsec = static_cast<FutexNanoseconds>(userspace_ts->tv_nsec);
+ return futex_ts;
+ }
};
class Futex : public FutexImpl {};
