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// Copyright 2018 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package kernel
import (
"gvisor.dev/gvisor/pkg/abi/linux"
"gvisor.dev/gvisor/pkg/sentry/kernel/futex"
"gvisor.dev/gvisor/pkg/usermem"
)
// Futex returns t's futex manager.
//
// Preconditions: The caller must be running on the task goroutine, or t.mu
// must be locked.
func (t *Task) Futex() *futex.Manager {
return t.tc.fu
}
// SwapUint32 implements futex.Target.SwapUint32.
func (t *Task) SwapUint32(addr usermem.Addr, new uint32) (uint32, error) {
return t.MemoryManager().SwapUint32(t, addr, new, usermem.IOOpts{
AddressSpaceActive: true,
})
}
// CompareAndSwapUint32 implements futex.Target.CompareAndSwapUint32.
func (t *Task) CompareAndSwapUint32(addr usermem.Addr, old, new uint32) (uint32, error) {
return t.MemoryManager().CompareAndSwapUint32(t, addr, old, new, usermem.IOOpts{
AddressSpaceActive: true,
})
}
// LoadUint32 implements futex.Target.LoadUint32.
func (t *Task) LoadUint32(addr usermem.Addr) (uint32, error) {
return t.MemoryManager().LoadUint32(t, addr, usermem.IOOpts{
AddressSpaceActive: true,
})
}
// GetSharedKey implements futex.Target.GetSharedKey.
func (t *Task) GetSharedKey(addr usermem.Addr) (futex.Key, error) {
return t.MemoryManager().GetSharedFutexKey(t, addr)
}
// GetRobustList sets the robust futex list for the task.
func (t *Task) GetRobustList() usermem.Addr {
t.mu.Lock()
addr := t.robustList
t.mu.Unlock()
return addr
}
// SetRobustList sets the robust futex list for the task.
func (t *Task) SetRobustList(addr usermem.Addr) {
t.mu.Lock()
t.robustList = addr
t.mu.Unlock()
}
// exitRobustList walks the robust futex list, marking locks dead and notifying
// wakers. It corresponds to Linux's exit_robust_list(). Following Linux,
// errors are silently ignored.
func (t *Task) exitRobustList() {
t.mu.Lock()
addr := t.robustList
t.robustList = 0
t.mu.Unlock()
if addr == 0 {
return
}
var rl linux.RobustListHead
if _, err := rl.CopyIn(t, usermem.Addr(addr)); err != nil {
return
}
next := rl.List
done := 0
var pendingLockAddr usermem.Addr
if rl.ListOpPending != 0 {
pendingLockAddr = usermem.Addr(rl.ListOpPending + rl.FutexOffset)
}
// Wake up normal elements.
for usermem.Addr(next) != addr {
// We traverse to the next element of the list before we
// actually wake anything. This prevents the race where waking
// this futex causes a modification of the list.
thisLockAddr := usermem.Addr(next + rl.FutexOffset)
// Try to decode the next element in the list before waking the
// current futex. But don't check the error until after we've
// woken the current futex. Linux does it in this order too
_, nextErr := t.CopyIn(usermem.Addr(next), &next)
// Wakeup the current futex if it's not pending.
if thisLockAddr != pendingLockAddr {
t.wakeRobustListOne(thisLockAddr)
}
// If there was an error copying the next futex, we must bail.
if nextErr != nil {
break
}
// This is a user structure, so it could be a massive list, or
// even contain a loop if they are trying to mess with us. We
// cap traversal to prevent that.
done++
if done >= linux.ROBUST_LIST_LIMIT {
break
}
}
// Is there a pending entry to wake?
if pendingLockAddr != 0 {
t.wakeRobustListOne(pendingLockAddr)
}
}
// wakeRobustListOne wakes a single futex from the robust list.
func (t *Task) wakeRobustListOne(addr usermem.Addr) {
// Bit 0 in address signals PI futex.
pi := addr&1 == 1
addr = addr &^ 1
// Load the futex.
f, err := t.LoadUint32(addr)
if err != nil {
// Can't read this single value? Ignore the problem.
// We can wake the other futexes in the list.
return
}
tid := uint32(t.ThreadID())
for {
// Is this held by someone else?
if f&linux.FUTEX_TID_MASK != tid {
return
}
// This thread is dying and it's holding this futex. We need to
// set the owner died bit and wake up any waiters.
newF := (f & linux.FUTEX_WAITERS) | linux.FUTEX_OWNER_DIED
if curF, err := t.CompareAndSwapUint32(addr, f, newF); err != nil {
return
} else if curF != f {
// Futex changed out from under us. Try again...
f = curF
continue
}
// Wake waiters if there are any.
if f&linux.FUTEX_WAITERS != 0 {
private := f&linux.FUTEX_PRIVATE_FLAG != 0
if pi {
t.Futex().UnlockPI(t, addr, tid, private)
return
}
t.Futex().Wake(t, addr, private, linux.FUTEX_BITSET_MATCH_ANY, 1)
}
// Done.
return
}
}
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