Implement sync_file_range()

sync_file_range - sync a file segment with disk

In Linux, sync_file_range() accepts three flags:

       SYNC_FILE_RANGE_WAIT_BEFORE
              Wait  upon  write-out  of  all pages in the specified range that
              have already been submitted to the device driver  for  write-out
              before performing any write.

       SYNC_FILE_RANGE_WRITE
              Initiate  write-out  of  all  dirty pages in the specified range
              which are not presently submitted  write-out.   Note  that  even
              this  may  block if you attempt to write more than request queue
              size.

       SYNC_FILE_RANGE_WAIT_AFTER
              Wait upon write-out of all pages in the range  after  performing
              any write.

In this implementation:

SYNC_FILE_RANGE_WAIT_BEFORE without SYNC_FILE_RANGE_WAIT_AFTER isn't
supported right now.

SYNC_FILE_RANGE_WRITE is skipped. It should initiate write-out of  all
dirty pages, but it doesn't wait, so it should be safe to do nothing
while nobody uses SYNC_FILE_RANGE_WAIT_BEFORE.

SYNC_FILE_RANGE_WAIT_AFTER is equal to fdatasync(). In Linux,
sync_file_range() doesn't writes out the  file's  meta-data, but
fdatasync() does if a file size is changed.

PiperOrigin-RevId: 220730840
Change-Id: Iae5dfb23c2c916967d67cf1a1ad32f25eb3f6286

pkg/abi/linux/fs.go

@@ -73,3 +73,10 @@ type Statfs struct {
 	// Spare is unused.
 	Spare [4]uint64
 }
+
+// Sync_file_range flags, from include/uapi/linux/fs.h
+const (
+	SYNC_FILE_RANGE_WAIT_BEFORE = 1
+	SYNC_FILE_RANGE_WRITE       = 2
+	SYNC_FILE_RANGE_WAIT_AFTER  = 4
+)

pkg/sentry/syscalls/linux/linux64.go

@@ -325,7 +325,7 @@ var AMD64 = &kernel.SyscallTable{
 		274: syscalls.Error(syscall.ENOSYS), // GetRobustList, obsolete
 		//     275: Splice, TODO
 		//     276: Tee, TODO
-		//     277: SyncFileRange, TODO
+		277: SyncFileRange,
 		//     278: Vmsplice, TODO
 		279: syscalls.CapError(linux.CAP_SYS_NICE), // MovePages, requires cap_sys_nice (mostly)
 		280: Utimensat,

pkg/sentry/syscalls/linux/sys_sync.go

@@ -15,6 +15,7 @@
 package linux
 
 import (
+	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/fs"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/kernel"
@@ -73,3 +74,65 @@ func Fdatasync(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sys
 	err := file.Fsync(t, 0, fs.FileMaxOffset, fs.SyncData)
 	return 0, nil, syserror.ConvertIntr(err, kernel.ERESTARTSYS)
 }
+
+// SyncFileRange implements linux syscall sync_file_rage(2)
+func SyncFileRange(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
+	var err error
+
+	offset := args[1].Int64()
+	nbytes := args[2].Int64()
+	uflags := args[3].Uint()
+
+	if offset < 0 || offset+nbytes < offset {
+		return 0, nil, syserror.EINVAL
+	}
+
+	if uflags&^(linux.SYNC_FILE_RANGE_WAIT_BEFORE|
+		linux.SYNC_FILE_RANGE_WRITE|
+		linux.SYNC_FILE_RANGE_WAIT_AFTER) != 0 {
+		return 0, nil, syserror.EINVAL
+	}
+
+	if nbytes == 0 {
+		nbytes = fs.FileMaxOffset
+	}
+
+	fd := kdefs.FD(args[0].Int())
+	file := t.FDMap().GetFile(fd)
+	if file == nil {
+		return 0, nil, syserror.EBADF
+	}
+	defer file.DecRef()
+
+	// SYNC_FILE_RANGE_WAIT_BEFORE waits upon write-out of all pages in the
+	// specified range that have already been submitted to the device
+	// driver for write-out before performing any write.
+	if uflags&linux.SYNC_FILE_RANGE_WAIT_BEFORE != 0 &&
+		uflags&linux.SYNC_FILE_RANGE_WAIT_AFTER == 0 {
+		t.Kernel().EmitUnimplementedEvent(t)
+		return 0, nil, syserror.ENOSYS
+	}
+
+	// SYNC_FILE_RANGE_WRITE initiates write-out of all dirty pages in the
+	// specified range which are not presently submitted write-out.
+	//
+	// It looks impossible to implement this functionality without a
+	// massive rework of the vfs subsystem. file.Fsync() take a file lock
+	// for the entire operation, so even if it is running in a go routing,
+	// it blocks other file operations instead of flushing data in the
+	// background.
+	//
+	// It should be safe to skipped this flag while nobody uses
+	// SYNC_FILE_RANGE_WAIT_BEFORE.
+
+	// SYNC_FILE_RANGE_WAIT_AFTER waits upon write-out of all pages in the
+	// range after performing any write.
+	//
+	// In Linux, sync_file_range() doesn't writes out the  file's
+	// meta-data, but fdatasync() does if a file size is changed.
+	if uflags&linux.SYNC_FILE_RANGE_WAIT_AFTER != 0 {
+		err = file.Fsync(t, offset, fs.FileMaxOffset, fs.SyncData)
+	}
+
+	return 0, nil, syserror.ConvertIntr(err, kernel.ERESTARTSYS)
+}