Implement futex_wait and futex_wake.

src/shims/posix/linux/foreign_items.rs

@@ -3,6 +3,7 @@ use rustc_middle::mir;
 use crate::*;
 use crate::helpers::check_arg_count;
 use shims::posix::fs::EvalContextExt as _;
+use shims::posix::linux::sync::futex;
 use shims::posix::sync::EvalContextExt as _;
 use shims::posix::thread::EvalContextExt as _;
 
@@ -120,6 +121,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                     .eval_libc("SYS_statx")?
                     .to_machine_usize(this)?;
 
+                let sys_futex = this
+                    .eval_libc("SYS_futex")?
+                    .to_machine_usize(this)?;
+
                 if args.is_empty() {
                     throw_ub_format!("incorrect number of arguments for syscall: got 0, expected at least 1");
                 }
@@ -139,6 +144,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                         let result = this.linux_statx(dirfd, pathname, flags, mask, statxbuf)?;
                         this.write_scalar(Scalar::from_machine_isize(result.into(), this), dest)?;
                     }
+                    // `futex` is used by some synchonization primitives.
+                    id if id == sys_futex => {
+                        futex(this, args, dest)?;
+                    }
                     id => throw_unsup_format!("miri does not support syscall ID {}", id),
                 }
             }

src/shims/posix/linux/mod.rs

@@ -1,2 +1,3 @@
 pub mod foreign_items;
 pub mod dlsym;
+pub mod sync;

src/shims/posix/linux/sync.rs

@@ -0,0 +1,88 @@
+use crate::*;
+use rustc_target::abi::{Align, Size};
+
+/// Implementation of the SYS_futex syscall.
+pub fn futex<'tcx>(
+    this: &mut MiriEvalContext<'_, 'tcx>,
+    args: &[OpTy<'tcx, Tag>],
+    dest: PlaceTy<'tcx, Tag>,
+) -> InterpResult<'tcx> {
+    // The amount of arguments used depends on the type of futex operation.
+    // Some users always pass all arguments, even the unused ones, due to how they wrap this syscall in their code base.
+    // Some other users pass only the arguments the operation actually needs. So we don't use `check_arg_count` here.
+    if !(4..=7).contains(&args.len()) {
+        throw_ub_format!("incorrect number of arguments for futex syscall: got {}, expected between 4 and 7 (inclusive)", args.len());
+    }
+
+    // The first three arguments (after the syscall number itself) are the same to all futex operations:
+    //     (int *addr, int op, int val).
+    // Although note that the first one is often passed as a different pointer type, e.g. `*const AtomicU32` or `*mut u32`.
+    let addr = this.deref_operand(args[1])?;
+    let op = this.read_scalar(args[2])?.to_i32()?;
+    let val = this.read_scalar(args[3])?.to_i32()?;
+
+    // The raw pointer value is used to identify the mutex.
+    // Not all mutex operations actually read from this address or even require this address to exist.
+    let futex_ptr = addr.ptr.assert_ptr();
+
+    let thread = this.get_active_thread();
+
+    let futex_private = this.eval_libc_i32("FUTEX_PRIVATE_FLAG")?;
+    let futex_wait = this.eval_libc_i32("FUTEX_WAIT")?;
+    let futex_wake = this.eval_libc_i32("FUTEX_WAKE")?;
+
+    // FUTEX_PRIVATE enables an optimization that stops it from working across processes.
+    // Miri doesn't support that anyway, so we ignore that flag.
+    match op & !futex_private {
+        // FUTEX_WAIT: (int *addr, int op = FUTEX_WAIT, int val, const timespec *timeout)
+        // Blocks the thread if *addr still equals val. Wakes up when FUTEX_WAKE is called on the same address,
+        // or *timeout expires. `timeout == null` for an infinite timeout.
+        op if op == futex_wait => {
+            if args.len() < 5 {
+                throw_ub_format!("incorrect number of arguments for FUTEX_WAIT syscall: got {}, expected at least 5", args.len());
+            }
+            let timeout = this.read_scalar(args[4])?.check_init()?;
+            if !this.is_null(timeout)? {
+                // FIXME: Implement timeouts. The condvar waiting code is probably a good example to start with.
+                // Note that a triggered timeout should have this syscall return with -1 and errno set to ETIMEOUT.
+                throw_ub_format!("miri does not support timeouts for futex operations");
+            }
+            // Check the pointer for alignment. Atomic operations are only available for fully aligned values.
+            this.memory.check_ptr_access(addr.ptr.into(), Size::from_bytes(4), Align::from_bytes(4).unwrap())?;
+            // Read an `i32` through the pointer, regardless of any wrapper types (e.g. `AtomicI32`).
+            let futex_val = this.read_scalar(addr.offset(Size::ZERO, MemPlaceMeta::None, this.machine.layouts.i32, this)?.into())?.to_i32()?;
+            if val == futex_val {
+                // The value still matches, so we block the trait make it wait for FUTEX_WAKE.
+                this.block_thread(thread);
+                this.futex_wait(futex_ptr, thread);
+                // Succesfully waking up from FUTEX_WAIT always returns zero.
+                this.write_scalar(Scalar::from_i32(0), dest)?;
+            } else {
+                // The futex value doesn't match the expected value, so we return failure
+                // right away without sleeping: -1 and errno set to EAGAIN.
+                let eagain = this.eval_libc("EAGAIN")?;
+                this.set_last_error(eagain)?;
+                this.write_scalar(Scalar::from_i32(-1), dest)?;
+            }
+        }
+        // FUTEX_WAKE: (int *addr, int op = FUTEX_WAKE, int val)
+        // Wakes at most `val` threads waiting on the futex at `addr`.
+        // Returns the amount of threads woken up.
+        // Does not access the futex value at *addr.
+        op if op == futex_wake => {
+            let mut n = 0;
+            for _ in 0..val {
+                if let Some(thread) = this.futex_wake(futex_ptr) {
+                    this.unblock_thread(thread);
+                    n += 1;
+                } else {
+                    break;
+                }
+            }
+            this.write_scalar(Scalar::from_i32(n), dest)?;
+        }
+        op => throw_unsup_format!("miri does not support SYS_futex operation {}", op),
+    }
+
+    Ok(())
+}

src/sync.rs

@@ -96,12 +96,26 @@ struct Condvar {
     waiters: VecDeque<CondvarWaiter>,
 }
 
+/// The futex state.
+#[derive(Default, Debug)]
+struct Futex {
+    waiters: VecDeque<FutexWaiter>,
+}
+
+/// A thread waiting on a futex.
+#[derive(Debug)]
+struct FutexWaiter {
+    /// The thread that is waiting on this futex.
+    thread: ThreadId,
+}
+
 /// The state of all synchronization variables.
 #[derive(Default, Debug)]
 pub(super) struct SynchronizationState {
     mutexes: IndexVec<MutexId, Mutex>,
     rwlocks: IndexVec<RwLockId, RwLock>,
     condvars: IndexVec<CondvarId, Condvar>,
+    futexes: HashMap<Pointer, Futex>,
 }
 
 // Private extension trait for local helper methods
@@ -403,4 +417,17 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         let this = self.eval_context_mut();
         this.machine.threads.sync.condvars[id].waiters.retain(|waiter| waiter.thread != thread);
     }
+
+    fn futex_wait(&mut self, addr: Pointer<stacked_borrows::Tag>, thread: ThreadId) {
+        let this = self.eval_context_mut();
+        let waiters = &mut this.machine.threads.sync.futexes.entry(addr.erase_tag()).or_default().waiters;
+        assert!(waiters.iter().all(|waiter| waiter.thread != thread), "thread is already waiting");
+        waiters.push_back(FutexWaiter { thread });
+    }
+
+    fn futex_wake(&mut self, addr: Pointer<stacked_borrows::Tag>) -> Option<ThreadId> {
+        let this = self.eval_context_mut();
+        let waiters = &mut this.machine.threads.sync.futexes.get_mut(&addr.erase_tag())?.waiters;
+        waiters.pop_front().map(|waiter| waiter.thread)
+    }
 }