1 use std::time::SystemTime;
3 use crate::concurrency::thread::{MachineCallback, Time};
6 /// Implementation of the SYS_futex syscall.
7 /// `args` is the arguments *after* the syscall number.
9 this: &mut MiriInterpCx<'_, 'tcx>,
10 args: &[OpTy<'tcx, Provenance>],
11 dest: &PlaceTy<'tcx, Provenance>,
12 ) -> InterpResult<'tcx> {
13 // The amount of arguments used depends on the type of futex operation.
14 // The full futex syscall takes six arguments (excluding the syscall
15 // number), which is also the maximum amount of arguments a linux syscall
16 // can take on most architectures.
17 // However, not all futex operations use all six arguments. The unused ones
18 // may or may not be left out from the `syscall()` call.
19 // Therefore we don't use `check_arg_count` here, but only check for the
20 // number of arguments to fall within a range.
23 "incorrect number of arguments for `futex` syscall: got {}, expected at least 3",
28 // The first three arguments (after the syscall number itself) are the same to all futex operations:
29 // (int *addr, int op, int val).
30 // We checked above that these definitely exist.
31 let addr = this.read_pointer(&args[0])?;
32 let op = this.read_scalar(&args[1])?.to_i32()?;
33 let val = this.read_scalar(&args[2])?.to_i32()?;
35 let thread = this.get_active_thread();
36 // This is a vararg function so we have to bring our own type for this pointer.
37 let addr = MPlaceTy::from_aligned_ptr(addr, this.machine.layouts.i32);
38 let addr_usize = addr.ptr.addr().bytes();
40 let futex_private = this.eval_libc_i32("FUTEX_PRIVATE_FLAG")?;
41 let futex_wait = this.eval_libc_i32("FUTEX_WAIT")?;
42 let futex_wait_bitset = this.eval_libc_i32("FUTEX_WAIT_BITSET")?;
43 let futex_wake = this.eval_libc_i32("FUTEX_WAKE")?;
44 let futex_wake_bitset = this.eval_libc_i32("FUTEX_WAKE_BITSET")?;
45 let futex_realtime = this.eval_libc_i32("FUTEX_CLOCK_REALTIME")?;
47 // FUTEX_PRIVATE enables an optimization that stops it from working across processes.
48 // Miri doesn't support that anyway, so we ignore that flag.
49 match op & !futex_private {
50 // FUTEX_WAIT: (int *addr, int op = FUTEX_WAIT, int val, const timespec *timeout)
51 // Blocks the thread if *addr still equals val. Wakes up when FUTEX_WAKE is called on the same address,
52 // or *timeout expires. `timeout == null` for an infinite timeout.
54 // FUTEX_WAIT_BITSET: (int *addr, int op = FUTEX_WAIT_BITSET, int val, const timespec *timeout, int *_ignored, unsigned int bitset)
55 // This is identical to FUTEX_WAIT, except:
56 // - The timeout is absolute rather than relative.
57 // - You can specify the bitset to selecting what WAKE operations to respond to.
58 op if op & !futex_realtime == futex_wait || op & !futex_realtime == futex_wait_bitset => {
59 let wait_bitset = op & !futex_realtime == futex_wait_bitset;
61 let bitset = if wait_bitset {
64 "incorrect number of arguments for `futex` syscall with `op=FUTEX_WAIT_BITSET`: got {}, expected at least 6",
68 let _timeout = this.read_pointer(&args[3])?;
69 let _uaddr2 = this.read_pointer(&args[4])?;
70 this.read_scalar(&args[5])?.to_u32()?
74 "incorrect number of arguments for `futex` syscall with `op=FUTEX_WAIT`: got {}, expected at least 4",
82 let einval = this.eval_libc("EINVAL")?;
83 this.set_last_error(einval)?;
84 this.write_scalar(Scalar::from_machine_isize(-1, this), dest)?;
88 // `deref_operand` but not actually dereferencing the ptr yet (it might be NULL!).
89 let timeout = this.ref_to_mplace(&this.read_immediate(&args[3])?)?;
90 let timeout_time = if this.ptr_is_null(timeout.ptr)? {
93 let realtime = op & futex_realtime == futex_realtime;
95 this.check_no_isolation(
96 "`futex` syscall with `op=FUTEX_WAIT` and non-null timeout with `FUTEX_CLOCK_REALTIME`",
99 let duration = match this.read_timespec(&timeout)? {
100 Some(duration) => duration,
102 let einval = this.eval_libc("EINVAL")?;
103 this.set_last_error(einval)?;
104 this.write_scalar(Scalar::from_machine_isize(-1, this), dest)?;
108 Some(if wait_bitset {
109 // FUTEX_WAIT_BITSET uses an absolute timestamp.
111 Time::RealTime(SystemTime::UNIX_EPOCH.checked_add(duration).unwrap())
113 Time::Monotonic(this.machine.clock.anchor().checked_add(duration).unwrap())
116 // FUTEX_WAIT uses a relative timestamp.
118 Time::RealTime(SystemTime::now().checked_add(duration).unwrap())
120 Time::Monotonic(this.machine.clock.now().checked_add(duration).unwrap())
124 // There may be a concurrent thread changing the value of addr
125 // and then invoking the FUTEX_WAKE syscall. It is critical that the
126 // effects of this and the other thread are correctly observed,
127 // otherwise we will deadlock.
129 // There are two scenarios to consider:
130 // 1. If we (FUTEX_WAIT) execute first, we'll push ourselves into
131 // the waiters queue and go to sleep. They (addr write & FUTEX_WAKE)
132 // will see us in the queue and wake us up.
133 // 2. If they (addr write & FUTEX_WAKE) execute first, we must observe
134 // addr's new value. If we see an outdated value that happens to equal
135 // the expected val, then we'll put ourselves to sleep with no one to wake us
136 // up, so we end up with a deadlock. This is prevented by having a SeqCst
137 // fence inside FUTEX_WAKE syscall, and another SeqCst fence
138 // below, the atomic read on addr after the SeqCst fence is guaranteed
139 // not to see any value older than the addr write immediately before
140 // calling FUTEX_WAKE. We'll see futex_val != val and return without
143 // Note that the fences do not create any happens-before relationship.
144 // The read sees the write immediately before the fence not because
145 // one happens after the other, but is instead due to a guarantee unique
146 // to SeqCst fences that restricts what an atomic read placed AFTER the
147 // fence can see. The read still has to be atomic, otherwise it's a data
148 // race. This guarantee cannot be achieved with acquire-release fences
149 // since they only talk about reads placed BEFORE a fence - and places
150 // no restrictions on what the read itself can see, only that there is
151 // a happens-before between the fences IF the read happens to see the
152 // right value. This is useless to us, since we need the read itself
153 // to see an up-to-date value.
155 // The above case distinction is valid since both FUTEX_WAIT and FUTEX_WAKE
156 // contain a SeqCst fence, therefore inducting a total order between the operations.
157 // It is also critical that the fence, the atomic load, and the comparison in FUTEX_WAIT
158 // altogether happen atomically. If the other thread's fence in FUTEX_WAKE
159 // gets interleaved after our fence, then we lose the guarantee on the
160 // atomic load being up-to-date; if the other thread's write on addr and FUTEX_WAKE
161 // call are interleaved after the load but before the comparison, then we get a TOCTOU
162 // race condition, and go to sleep thinking the other thread will wake us up,
163 // even though they have already finished.
165 // Thankfully, preemptions cannot happen inside a Miri shim, so we do not need to
166 // do anything special to guarantee fence-load-comparison atomicity.
167 this.atomic_fence(AtomicFenceOrd::SeqCst)?;
168 // Read an `i32` through the pointer, regardless of any wrapper types.
169 // It's not uncommon for `addr` to be passed as another type than `*mut i32`, such as `*const AtomicI32`.
170 let futex_val = this.read_scalar_atomic(&addr, AtomicReadOrd::Relaxed)?.to_i32()?;
171 if val == futex_val {
172 // The value still matches, so we block the thread make it wait for FUTEX_WAKE.
173 this.block_thread(thread);
174 this.futex_wait(addr_usize, thread, bitset);
175 // Succesfully waking up from FUTEX_WAIT always returns zero.
176 this.write_scalar(Scalar::from_machine_isize(0, this), dest)?;
177 // Register a timeout callback if a timeout was specified.
178 // This callback will override the return value when the timeout triggers.
179 if let Some(timeout_time) = timeout_time {
180 struct Callback<'tcx> {
183 dest: PlaceTy<'tcx, Provenance>,
186 impl<'tcx> VisitTags for Callback<'tcx> {
187 fn visit_tags(&self, visit: &mut dyn FnMut(BorTag)) {
188 let Callback { thread: _, addr_usize: _, dest } = self;
189 dest.visit_tags(visit);
193 impl<'mir, 'tcx: 'mir> MachineCallback<'mir, 'tcx> for Callback<'tcx> {
194 fn call(&self, this: &mut MiriInterpCx<'mir, 'tcx>) -> InterpResult<'tcx> {
195 this.unblock_thread(self.thread);
196 this.futex_remove_waiter(self.addr_usize, self.thread);
197 let etimedout = this.eval_libc("ETIMEDOUT")?;
198 this.set_last_error(etimedout)?;
199 this.write_scalar(Scalar::from_machine_isize(-1, this), &self.dest)?;
205 this.register_timeout_callback(
208 Box::new(Callback { thread, addr_usize, dest: dest.clone() }),
212 // The futex value doesn't match the expected value, so we return failure
213 // right away without sleeping: -1 and errno set to EAGAIN.
214 let eagain = this.eval_libc("EAGAIN")?;
215 this.set_last_error(eagain)?;
216 this.write_scalar(Scalar::from_machine_isize(-1, this), dest)?;
219 // FUTEX_WAKE: (int *addr, int op = FUTEX_WAKE, int val)
220 // Wakes at most `val` threads waiting on the futex at `addr`.
221 // Returns the amount of threads woken up.
222 // Does not access the futex value at *addr.
223 // FUTEX_WAKE_BITSET: (int *addr, int op = FUTEX_WAKE, int val, const timespect *_unused, int *_unused, unsigned int bitset)
224 // Same as FUTEX_WAKE, but allows you to specify a bitset to select which threads to wake up.
225 op if op == futex_wake || op == futex_wake_bitset => {
226 let bitset = if op == futex_wake_bitset {
229 "incorrect number of arguments for `futex` syscall with `op=FUTEX_WAKE_BITSET`: got {}, expected at least 6",
233 let _timeout = this.read_pointer(&args[3])?;
234 let _uaddr2 = this.read_pointer(&args[4])?;
235 this.read_scalar(&args[5])?.to_u32()?
240 let einval = this.eval_libc("EINVAL")?;
241 this.set_last_error(einval)?;
242 this.write_scalar(Scalar::from_machine_isize(-1, this), dest)?;
245 // Together with the SeqCst fence in futex_wait, this makes sure that futex_wait
246 // will see the latest value on addr which could be changed by our caller
247 // before doing the syscall.
248 this.atomic_fence(AtomicFenceOrd::SeqCst)?;
250 #[allow(clippy::integer_arithmetic)]
252 if let Some(thread) = this.futex_wake(addr_usize, bitset) {
253 this.unblock_thread(thread);
254 this.unregister_timeout_callback_if_exists(thread);
260 this.write_scalar(Scalar::from_machine_isize(n, this), dest)?;
262 op => throw_unsup_format!("Miri does not support `futex` syscall with op={}", op),