Remove linux.EpollEvent.Fd.

glibc defines struct epoll_event in such a way that epoll_event.data.fd exists.
However, the kernel's definition of struct epoll_event makes epoll_event.data
an opaque uint64, so naming half of it "fd" just introduces confusion. Remove
the Fd field, and make Data a [2]int32 to compensate.

Also add required padding to linux.EpollEvent on ARM64.

PiperOrigin-RevId: 295250424

pkg/abi/linux/BUILD

@@ -17,6 +17,8 @@ go_library(
         "dev.go",
         "elf.go",
         "epoll.go",
+        "epoll_amd64.go",
+        "epoll_arm64.go",
         "errors.go",
         "eventfd.go",
         "exec.go",

pkg/abi/linux/epoll.go

@@ -14,13 +14,6 @@
 
 package linux
 
-// EpollEvent is equivalent to struct epoll_event from epoll(2).
-type EpollEvent struct {
-	Events uint32
-	Fd     int32
-	Data   int32
-}
-
 // Event masks.
 const (
 	EPOLLIN     = 0x1

pkg/abi/linux/epoll_amd64.go

@@ -0,0 +1,25 @@
+// Copyright 2019 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 linux
+
+// EpollEvent is equivalent to struct epoll_event from epoll(2).
+type EpollEvent struct {
+	Events uint32
+	// Linux makes struct epoll_event::data a __u64. We represent it as
+	// [2]int32 because, on amd64, Linux also makes struct epoll_event
+	// __attribute__((packed)), such that there is no padding between Events
+	// and Data.
+	Data [2]int32
+}

pkg/abi/linux/epoll_arm64.go

@@ -0,0 +1,24 @@
+// Copyright 2020 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 linux
+
+// EpollEvent is equivalent to struct epoll_event from epoll(2).
+type EpollEvent struct {
+	Events uint32
+	// Linux makes struct epoll_event a __u64, necessitating 4 bytes of padding
+	// here.
+	_    int32
+	Data [2]int32
+}

pkg/sentry/syscalls/linux/sys_epoll.go

@@ -83,8 +83,7 @@ func EpollCtl(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sysc
 		}
 
 		mask = waiter.EventMaskFromLinux(e.Events)
-		data[0] = e.Fd
-		data[1] = e.Data
+		data = e.Data
 	}
 
 	// Perform the requested operations.

pkg/sentry/vfs/epoll.go

@@ -85,8 +85,8 @@ type epollInterest struct {
 	ready bool
 	epollInterestEntry
 
-	// userData is the epoll_data_t associated with this epollInterest.
-	// userData is protected by epoll.mu.
+	// userData is the struct epoll_event::data associated with this
+	// epollInterest. userData is protected by epoll.mu.
 	userData [2]int32
 }
 
@@ -157,7 +157,7 @@ func (ep *EpollInstance) Seek(ctx context.Context, offset int64, whence int32) (
 // AddInterest implements the semantics of EPOLL_CTL_ADD.
 //
 // Preconditions: A reference must be held on file.
-func (ep *EpollInstance) AddInterest(file *FileDescription, num int32, mask uint32, userData [2]int32) error {
+func (ep *EpollInstance) AddInterest(file *FileDescription, num int32, event linux.EpollEvent) error {
 	// Check for cyclic polling if necessary.
 	subep, _ := file.impl.(*EpollInstance)
 	if subep != nil {
@@ -183,12 +183,12 @@ func (ep *EpollInstance) AddInterest(file *FileDescription, num int32, mask uint
 	}
 
 	// Register interest in file.
-	mask |= linux.EPOLLERR | linux.EPOLLRDHUP
+	mask := event.Events | linux.EPOLLERR | linux.EPOLLRDHUP
 	epi := &epollInterest{
 		epoll:    ep,
 		key:      key,
 		mask:     mask,
-		userData: userData,
+		userData: event.Data,
 	}
 	ep.interest[key] = epi
 	wmask := waiter.EventMaskFromLinux(mask)
@@ -236,7 +236,7 @@ func (ep *EpollInstance) mightPollRecursive(ep2 *EpollInstance, remainingRecursi
 // ModifyInterest implements the semantics of EPOLL_CTL_MOD.
 //
 // Preconditions: A reference must be held on file.
-func (ep *EpollInstance) ModifyInterest(file *FileDescription, num int32, mask uint32, userData [2]int32) error {
+func (ep *EpollInstance) ModifyInterest(file *FileDescription, num int32, event linux.EpollEvent) error {
 	ep.interestMu.Lock()
 	defer ep.interestMu.Unlock()
 
@@ -250,13 +250,13 @@ func (ep *EpollInstance) ModifyInterest(file *FileDescription, num int32, mask u
 	}
 
 	// Update epi for the next call to ep.ReadEvents().
+	mask := event.Events | linux.EPOLLERR | linux.EPOLLRDHUP
 	ep.mu.Lock()
 	epi.mask = mask
-	epi.userData = userData
+	epi.userData = event.Data
 	ep.mu.Unlock()
 
 	// Re-register with the new mask.
-	mask |= linux.EPOLLERR | linux.EPOLLRDHUP
 	file.EventUnregister(&epi.waiter)
 	wmask := waiter.EventMaskFromLinux(mask)
 	file.EventRegister(&epi.waiter, wmask)
@@ -363,8 +363,7 @@ func (ep *EpollInstance) ReadEvents(events []linux.EpollEvent) int {
 		// Report ievents.
 		events[i] = linux.EpollEvent{
 			Events: ievents.ToLinux(),
-			Fd:     epi.userData[0],
-			Data:   epi.userData[1],
+			Data:   epi.userData,
 		}
 		i++
 		if i == len(events) {