1 files changed, 604 insertions, 0 deletions
diff --git a/pkg/sentry/mm/io.go b/pkg/sentry/mm/io.go
new file mode 100644
index 000000000..cac81a59d
--- /dev/null
+++ b/pkg/sentry/mm/io.go
@@ -0,0 +1,604 @@
+// Copyright 2018 Google Inc.
+//
+// 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 mm
+
+import (
+	"gvisor.googlesource.com/gvisor/pkg/sentry/context"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/platform"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/safemem"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
+	"gvisor.googlesource.com/gvisor/pkg/syserror"
+)
+
+// There are two supported ways to copy data to/from application virtual
+// memory:
+//
+// 1. Internally-mapped copying: Determine the platform.File that backs the
+// copied-to/from virtual address, obtain a mapping of its pages, and read or
+// write to the mapping.
+//
+// 2. AddressSpace copying: If platform.Platform.SupportsAddressSpaceIO() is
+// true, AddressSpace permissions are applicable, and an AddressSpace is
+// available, copy directly through the AddressSpace, handling faults as
+// needed.
+//
+// (Given that internally-mapped copying requires that backing memory is always
+// implemented using a host file descriptor, we could also preadv/pwritev to it
+// instead. But this would incur a host syscall for each use of the mapped
+// page, whereas mmap is a one-time cost.)
+//
+// The fixed overhead of internally-mapped copying is expected to be higher
+// than that of AddressSpace copying since the former always needs to translate
+// addresses, whereas the latter only needs to do so when faults occur.
+// However, the throughput of internally-mapped copying is expected to be
+// somewhat higher than that of AddressSpace copying due to the high cost of
+// page faults and because implementations of the latter usually rely on
+// safecopy, which doesn't use AVX registers. So we prefer to use AddressSpace
+// copying (when available) for smaller copies, and switch to internally-mapped
+// copying once a size threshold is exceeded.
+const (
+	// copyMapMinBytes is the size threshold for switching to internally-mapped
+	// copying in CopyOut, CopyIn, and ZeroOut.
+	copyMapMinBytes = 32 << 10 // 32 KB
+
+	// rwMapMinBytes is the size threshold for switching to internally-mapped
+	// copying in CopyOutFrom and CopyInTo. It's lower than copyMapMinBytes
+	// since AddressSpace copying in this case requires additional buffering;
+	// see CopyOutFrom for details.
+	rwMapMinBytes = 512
+)
+
+// checkIORange is similar to usermem.Addr.ToRange, but applies bounds checks
+// consistent with Linux's arch/x86/include/asm/uaccess.h:access_ok().
+//
+// Preconditions: length >= 0.
+func (mm *MemoryManager) checkIORange(addr usermem.Addr, length int64) (usermem.AddrRange, bool) {
+	// Note that access_ok() constrains end even if length == 0.
+	ar, ok := addr.ToRange(uint64(length))
+	return ar, (ok && ar.End <= mm.layout.MaxAddr)
+}
+
+// checkIOVec applies bound checks consistent with Linux's
+// arch/x86/include/asm/uaccess.h:access_ok() to ars.
+func (mm *MemoryManager) checkIOVec(ars usermem.AddrRangeSeq) bool {
+	for !ars.IsEmpty() {
+		ar := ars.Head()
+		if _, ok := mm.checkIORange(ar.Start, int64(ar.Length())); !ok {
+			return false
+		}
+		ars = ars.Tail()
+	}
+	return true
+}
+
+func (mm *MemoryManager) asioEnabled(opts usermem.IOOpts) bool {
+	return mm.haveASIO && !opts.IgnorePermissions && opts.AddressSpaceActive
+}
+
+// translateIOError converts errors to EFAULT, as is usually reported for all
+// I/O errors originating from MM in Linux.
+func translateIOError(ctx context.Context, err error) error {
+	if err == nil {
+		return nil
+	}
+	if logIOErrors {
+		ctx.Debugf("MM I/O error: %v", err)
+	}
+	return syserror.EFAULT
+}
+
+// CopyOut implements usermem.IO.CopyOut.
+func (mm *MemoryManager) CopyOut(ctx context.Context, addr usermem.Addr, src []byte, opts usermem.IOOpts) (int, error) {
+	ar, ok := mm.checkIORange(addr, int64(len(src)))
+	if !ok {
+		return 0, syserror.EFAULT
+	}
+
+	if len(src) == 0 {
+		return 0, nil
+	}
+
+	// Do AddressSpace IO if applicable.
+	if mm.asioEnabled(opts) && len(src) < copyMapMinBytes {
+		return mm.asCopyOut(ctx, addr, src)
+	}
+
+	// Go through internal mappings.
+	n64, err := mm.withInternalMappings(ctx, ar, usermem.Write, opts.IgnorePermissions, func(ims safemem.BlockSeq) (uint64, error) {
+		n, err := safemem.CopySeq(ims, safemem.BlockSeqOf(safemem.BlockFromSafeSlice(src)))
+		return n, translateIOError(ctx, err)
+	})
+	return int(n64), err
+}
+
+func (mm *MemoryManager) asCopyOut(ctx context.Context, addr usermem.Addr, src []byte) (int, error) {
+	var done int
+	for {
+		n, err := mm.as.CopyOut(addr+usermem.Addr(done), src[done:])
+		done += n
+		if err == nil {
+			return done, nil
+		}
+		if f, ok := err.(platform.SegmentationFault); ok {
+			ar, _ := addr.ToRange(uint64(len(src)))
+			if err := mm.handleASIOFault(ctx, f.Addr, ar, usermem.Write); err != nil {
+				return done, err
+			}
+			continue
+		}
+		return done, translateIOError(ctx, err)
+	}
+}
+
+// CopyIn implements usermem.IO.CopyIn.
+func (mm *MemoryManager) CopyIn(ctx context.Context, addr usermem.Addr, dst []byte, opts usermem.IOOpts) (int, error) {
+	ar, ok := mm.checkIORange(addr, int64(len(dst)))
+	if !ok {
+		return 0, syserror.EFAULT
+	}
+
+	if len(dst) == 0 {
+		return 0, nil
+	}
+
+	// Do AddressSpace IO if applicable.
+	if mm.asioEnabled(opts) && len(dst) < copyMapMinBytes {
+		return mm.asCopyIn(ctx, addr, dst)
+	}
+
+	// Go through internal mappings.
+	n64, err := mm.withInternalMappings(ctx, ar, usermem.Read, opts.IgnorePermissions, func(ims safemem.BlockSeq) (uint64, error) {
+		n, err := safemem.CopySeq(safemem.BlockSeqOf(safemem.BlockFromSafeSlice(dst)), ims)
+		return n, translateIOError(ctx, err)
+	})
+	return int(n64), err
+}
+
+func (mm *MemoryManager) asCopyIn(ctx context.Context, addr usermem.Addr, dst []byte) (int, error) {
+	var done int
+	for {
+		n, err := mm.as.CopyIn(addr+usermem.Addr(done), dst[done:])
+		done += n
+		if err == nil {
+			return done, nil
+		}
+		if f, ok := err.(platform.SegmentationFault); ok {
+			ar, _ := addr.ToRange(uint64(len(dst)))
+			if err := mm.handleASIOFault(ctx, f.Addr, ar, usermem.Read); err != nil {
+				return done, err
+			}
+			continue
+		}
+		return done, translateIOError(ctx, err)
+	}
+}
+
+// ZeroOut implements usermem.IO.ZeroOut.
+func (mm *MemoryManager) ZeroOut(ctx context.Context, addr usermem.Addr, toZero int64, opts usermem.IOOpts) (int64, error) {
+	ar, ok := mm.checkIORange(addr, toZero)
+	if !ok {
+		return 0, syserror.EFAULT
+	}
+
+	if toZero == 0 {
+		return 0, nil
+	}
+
+	// Do AddressSpace IO if applicable.
+	if mm.asioEnabled(opts) && toZero < copyMapMinBytes {
+		return mm.asZeroOut(ctx, addr, toZero)
+	}
+
+	// Go through internal mappings.
+	return mm.withInternalMappings(ctx, ar, usermem.Write, opts.IgnorePermissions, func(dsts safemem.BlockSeq) (uint64, error) {
+		n, err := safemem.ZeroSeq(dsts)
+		return n, translateIOError(ctx, err)
+	})
+}
+
+func (mm *MemoryManager) asZeroOut(ctx context.Context, addr usermem.Addr, toZero int64) (int64, error) {
+	var done int64
+	for {
+		n, err := mm.as.ZeroOut(addr+usermem.Addr(done), uintptr(toZero-done))
+		done += int64(n)
+		if err == nil {
+			return done, nil
+		}
+		if f, ok := err.(platform.SegmentationFault); ok {
+			ar, _ := addr.ToRange(uint64(toZero))
+			if err := mm.handleASIOFault(ctx, f.Addr, ar, usermem.Write); err != nil {
+				return done, err
+			}
+			continue
+		}
+		return done, translateIOError(ctx, err)
+	}
+}
+
+// CopyOutFrom implements usermem.IO.CopyOutFrom.
+func (mm *MemoryManager) CopyOutFrom(ctx context.Context, ars usermem.AddrRangeSeq, src safemem.Reader, opts usermem.IOOpts) (int64, error) {
+	if !mm.checkIOVec(ars) {
+		return 0, syserror.EFAULT
+	}
+
+	if ars.NumBytes() == 0 {
+		return 0, nil
+	}
+
+	// Do AddressSpace IO if applicable.
+	if mm.asioEnabled(opts) && ars.NumBytes() < rwMapMinBytes {
+		// We have to introduce a buffered copy, instead of just passing a
+		// safemem.BlockSeq representing addresses in the AddressSpace to src.
+		// This is because usermem.IO.CopyOutFrom() guarantees that it calls
+		// src.ReadToBlocks() at most once, which is incompatible with handling
+		// faults between calls. In the future, this is probably best resolved
+		// by introducing a CopyOutFrom variant or option that allows it to
+		// call src.ReadToBlocks() any number of times.
+		//
+		// This issue applies to CopyInTo as well.
+		buf := make([]byte, int(ars.NumBytes()))
+		bufN, bufErr := src.ReadToBlocks(safemem.BlockSeqOf(safemem.BlockFromSafeSlice(buf)))
+		var done int64
+		for done < int64(bufN) {
+			ar := ars.Head()
+			cplen := int64(ar.Length())
+			if cplen > int64(bufN)-done {
+				cplen = int64(bufN) - done
+			}
+			n, err := mm.asCopyOut(ctx, ar.Start, buf[int(done):int(done+cplen)])
+			done += int64(n)
+			if err != nil {
+				return done, err
+			}
+			ars = ars.Tail()
+		}
+		// Do not convert errors returned by src to EFAULT.
+		return done, bufErr
+	}
+
+	// Go through internal mappings.
+	return mm.withVecInternalMappings(ctx, ars, usermem.Write, opts.IgnorePermissions, src.ReadToBlocks)
+}
+
+// CopyInTo implements usermem.IO.CopyInTo.
+func (mm *MemoryManager) CopyInTo(ctx context.Context, ars usermem.AddrRangeSeq, dst safemem.Writer, opts usermem.IOOpts) (int64, error) {
+	if !mm.checkIOVec(ars) {
+		return 0, syserror.EFAULT
+	}
+
+	if ars.NumBytes() == 0 {
+		return 0, nil
+	}
+
+	// Do AddressSpace IO if applicable.
+	if mm.asioEnabled(opts) && ars.NumBytes() < rwMapMinBytes {
+		buf := make([]byte, int(ars.NumBytes()))
+		var done int
+		var bufErr error
+		for !ars.IsEmpty() {
+			ar := ars.Head()
+			var n int
+			n, bufErr = mm.asCopyIn(ctx, ar.Start, buf[done:done+int(ar.Length())])
+			done += n
+			if bufErr != nil {
+				break
+			}
+			ars = ars.Tail()
+		}
+		n, err := dst.WriteFromBlocks(safemem.BlockSeqOf(safemem.BlockFromSafeSlice(buf[:done])))
+		if err != nil {
+			return int64(n), err
+		}
+		// Do not convert errors returned by dst to EFAULT.
+		return int64(n), bufErr
+	}
+
+	// Go through internal mappings.
+	return mm.withVecInternalMappings(ctx, ars, usermem.Read, opts.IgnorePermissions, dst.WriteFromBlocks)
+}
+
+// SwapUint32 implements usermem.IO.SwapUint32.
+func (mm *MemoryManager) SwapUint32(ctx context.Context, addr usermem.Addr, new uint32, opts usermem.IOOpts) (uint32, error) {
+	ar, ok := mm.checkIORange(addr, 4)
+	if !ok {
+		return 0, syserror.EFAULT
+	}
+
+	// Do AddressSpace IO if applicable.
+	if mm.haveASIO && opts.AddressSpaceActive && !opts.IgnorePermissions {
+		for {
+			old, err := mm.as.SwapUint32(addr, new)
+			if err == nil {
+				return old, nil
+			}
+			if f, ok := err.(platform.SegmentationFault); ok {
+				if err := mm.handleASIOFault(ctx, f.Addr, ar, usermem.ReadWrite); err != nil {
+					return 0, err
+				}
+				continue
+			}
+			return 0, translateIOError(ctx, err)
+		}
+	}
+
+	// Go through internal mappings.
+	var old uint32
+	_, err := mm.withInternalMappings(ctx, ar, usermem.ReadWrite, opts.IgnorePermissions, func(ims safemem.BlockSeq) (uint64, error) {
+		if ims.NumBlocks() != 1 || ims.NumBytes() != 4 {
+			// Atomicity is unachievable across mappings.
+			return 0, syserror.EFAULT
+		}
+		im := ims.Head()
+		var err error
+		old, err = safemem.SwapUint32(im, new)
+		if err != nil {
+			return 0, translateIOError(ctx, err)
+		}
+		return 4, nil
+	})
+	return old, err
+}
+
+// CompareAndSwapUint32 implements usermem.IO.CompareAndSwapUint32.
+func (mm *MemoryManager) CompareAndSwapUint32(ctx context.Context, addr usermem.Addr, old, new uint32, opts usermem.IOOpts) (uint32, error) {
+	ar, ok := mm.checkIORange(addr, 4)
+	if !ok {
+		return 0, syserror.EFAULT
+	}
+
+	// Do AddressSpace IO if applicable.
+	if mm.haveASIO && opts.AddressSpaceActive && !opts.IgnorePermissions {
+		for {
+			prev, err := mm.as.CompareAndSwapUint32(addr, old, new)
+			if err == nil {
+				return prev, nil
+			}
+			if f, ok := err.(platform.SegmentationFault); ok {
+				if err := mm.handleASIOFault(ctx, f.Addr, ar, usermem.ReadWrite); err != nil {
+					return 0, err
+				}
+				continue
+			}
+			return 0, translateIOError(ctx, err)
+		}
+	}
+
+	// Go through internal mappings.
+	var prev uint32
+	_, err := mm.withInternalMappings(ctx, ar, usermem.ReadWrite, opts.IgnorePermissions, func(ims safemem.BlockSeq) (uint64, error) {
+		if ims.NumBlocks() != 1 || ims.NumBytes() != 4 {
+			// Atomicity is unachievable across mappings.
+			return 0, syserror.EFAULT
+		}
+		im := ims.Head()
+		var err error
+		prev, err = safemem.CompareAndSwapUint32(im, old, new)
+		if err != nil {
+			return 0, translateIOError(ctx, err)
+		}
+		return 4, nil
+	})
+	return prev, err
+}
+
+// handleASIOFault handles a page fault at address addr for an AddressSpaceIO
+// operation spanning ioar.
+//
+// Preconditions: mm.as != nil. ioar.Length() != 0. ioar.Contains(addr).
+func (mm *MemoryManager) handleASIOFault(ctx context.Context, addr usermem.Addr, ioar usermem.AddrRange, at usermem.AccessType) error {
+	// Try to map all remaining pages in the I/O operation. This RoundUp can't
+	// overflow because otherwise it would have been caught by checkIORange.
+	end, _ := ioar.End.RoundUp()
+	ar := usermem.AddrRange{addr.RoundDown(), end}
+
+	// Don't bother trying existingPMAsLocked; in most cases, if we did have
+	// existing pmas, we wouldn't have faulted.
+
+	// Ensure that we have usable vmas. Here and below, only return early if we
+	// can't map the first (faulting) page; failure to map later pages are
+	// silently ignored. This maximizes partial success.
+	mm.mappingMu.RLock()
+	vseg, vend, err := mm.getVMAsLocked(ctx, ar, at, false)
+	if vendaddr := vend.Start(); vendaddr < ar.End {
+		if vendaddr <= ar.Start {
+			mm.mappingMu.RUnlock()
+			return translateIOError(ctx, err)
+		}
+		ar.End = vendaddr
+	}
+
+	// Ensure that we have usable pmas.
+	mm.activeMu.Lock()
+	pseg, pend, err := mm.getPMAsLocked(ctx, vseg, ar, pmaOpts{
+		breakCOW: at.Write,
+	})
+	mm.mappingMu.RUnlock()
+	if pendaddr := pend.Start(); pendaddr < ar.End {
+		if pendaddr <= ar.Start {
+			mm.activeMu.Unlock()
+			return translateIOError(ctx, err)
+		}
+		ar.End = pendaddr
+	}
+
+	// Downgrade to a read-lock on activeMu since we don't need to mutate pmas
+	// anymore.
+	mm.activeMu.DowngradeLock()
+
+	err = mm.mapASLocked(pseg, ar, false)
+	mm.activeMu.RUnlock()
+	return translateIOError(ctx, err)
+}
+
+// withInternalMappings ensures that pmas exist for all addresses in ar,
+// support access of type (at, ignorePermissions), and have internal mappings
+// cached. It then calls f with mm.activeMu locked for reading, passing
+// internal mappings for the subrange of ar for which this property holds.
+//
+// withInternalMappings takes a function returning uint64 since many safemem
+// functions have this property, but returns an int64 since this is usually
+// more useful for usermem.IO methods.
+//
+// Preconditions: 0 < ar.Length() <= math.MaxInt64.
+func (mm *MemoryManager) withInternalMappings(ctx context.Context, ar usermem.AddrRange, at usermem.AccessType, ignorePermissions bool, f func(safemem.BlockSeq) (uint64, error)) (int64, error) {
+	po := pmaOpts{
+		breakCOW: at.Write,
+	}
+
+	// If pmas are already available, we can do IO without touching mm.vmas or
+	// mm.mappingMu.
+	mm.activeMu.RLock()
+	if pseg := mm.existingPMAsLocked(ar, at, ignorePermissions, po, true /* needInternalMappings */); pseg.Ok() {
+		n, err := f(mm.internalMappingsLocked(pseg, ar))
+		mm.activeMu.RUnlock()
+		// Do not convert errors returned by f to EFAULT.
+		return int64(n), err
+	}
+	mm.activeMu.RUnlock()
+
+	// Ensure that we have usable vmas.
+	mm.mappingMu.RLock()
+	vseg, vend, verr := mm.getVMAsLocked(ctx, ar, at, ignorePermissions)
+	if vendaddr := vend.Start(); vendaddr < ar.End {
+		if vendaddr <= ar.Start {
+			mm.mappingMu.RUnlock()
+			return 0, translateIOError(ctx, verr)
+		}
+		ar.End = vendaddr
+	}
+
+	// Ensure that we have usable pmas.
+	mm.activeMu.Lock()
+	pseg, pend, perr := mm.getPMAsLocked(ctx, vseg, ar, po)
+	mm.mappingMu.RUnlock()
+	if pendaddr := pend.Start(); pendaddr < ar.End {
+		if pendaddr <= ar.Start {
+			mm.activeMu.Unlock()
+			return 0, translateIOError(ctx, perr)
+		}
+		ar.End = pendaddr
+	}
+	imend, imerr := mm.getPMAInternalMappingsLocked(pseg, ar)
+	mm.activeMu.DowngradeLock()
+	if imendaddr := imend.Start(); imendaddr < ar.End {
+		if imendaddr <= ar.Start {
+			mm.activeMu.RUnlock()
+			return 0, translateIOError(ctx, imerr)
+		}
+		ar.End = imendaddr
+	}
+
+	// Do I/O.
+	un, err := f(mm.internalMappingsLocked(pseg, ar))
+	mm.activeMu.RUnlock()
+	n := int64(un)
+
+	// Return the first error in order of progress through ar.
+	if err != nil {
+		// Do not convert errors returned by f to EFAULT.
+		return n, err
+	}
+	if imerr != nil {
+		return n, translateIOError(ctx, imerr)
+	}
+	if perr != nil {
+		return n, translateIOError(ctx, perr)
+	}
+	return n, translateIOError(ctx, verr)
+}
+
+// withVecInternalMappings ensures that pmas exist for all addresses in ars,
+// support access of type (at, ignorePermissions), and have internal mappings
+// cached. It then calls f with mm.activeMu locked for reading, passing
+// internal mappings for the subset of ars for which this property holds.
+//
+// Preconditions: !ars.IsEmpty().
+func (mm *MemoryManager) withVecInternalMappings(ctx context.Context, ars usermem.AddrRangeSeq, at usermem.AccessType, ignorePermissions bool, f func(safemem.BlockSeq) (uint64, error)) (int64, error) {
+	// withInternalMappings is faster than withVecInternalMappings because of
+	// iterator plumbing (this isn't generally practical in the vector case due
+	// to iterator invalidation between AddrRanges). Use it if possible.
+	if ars.NumRanges() == 1 {
+		return mm.withInternalMappings(ctx, ars.Head(), at, ignorePermissions, f)
+	}
+
+	po := pmaOpts{
+		breakCOW: at.Write,
+	}
+
+	// If pmas are already available, we can do IO without touching mm.vmas or
+	// mm.mappingMu.
+	mm.activeMu.RLock()
+	if mm.existingVecPMAsLocked(ars, at, ignorePermissions, po, true /* needInternalMappings */) {
+		n, err := f(mm.vecInternalMappingsLocked(ars))
+		mm.activeMu.RUnlock()
+		// Do not convert errors returned by f to EFAULT.
+		return int64(n), err
+	}
+	mm.activeMu.RUnlock()
+
+	// Ensure that we have usable vmas.
+	mm.mappingMu.RLock()
+	vars, verr := mm.getVecVMAsLocked(ctx, ars, at, ignorePermissions)
+	if vars.NumBytes() == 0 {
+		mm.mappingMu.RUnlock()
+		return 0, translateIOError(ctx, verr)
+	}
+
+	// Ensure that we have usable pmas.
+	mm.activeMu.Lock()
+	pars, perr := mm.getVecPMAsLocked(ctx, vars, po)
+	mm.mappingMu.RUnlock()
+	if pars.NumBytes() == 0 {
+		mm.activeMu.Unlock()
+		return 0, translateIOError(ctx, perr)
+	}
+	imars, imerr := mm.getVecPMAInternalMappingsLocked(pars)
+	mm.activeMu.DowngradeLock()
+	if imars.NumBytes() == 0 {
+		mm.activeMu.RUnlock()
+		return 0, translateIOError(ctx, imerr)
+	}
+
+	// Do I/O.
+	un, err := f(mm.vecInternalMappingsLocked(imars))
+	mm.activeMu.RUnlock()
+	n := int64(un)
+
+	// Return the first error in order of progress through ars.
+	if err != nil {
+		// Do not convert errors from f to EFAULT.
+		return n, err
+	}
+	if imerr != nil {
+		return n, translateIOError(ctx, imerr)
+	}
+	if perr != nil {
+		return n, translateIOError(ctx, perr)
+	}
+	return n, translateIOError(ctx, verr)
+}
+
+// truncatedAddrRangeSeq returns a copy of ars, but with the end truncated to
+// at most address end on AddrRange arsit.Head(). It is used in vector I/O paths to
+// truncate usermem.AddrRangeSeq when errors occur.
+//
+// Preconditions: !arsit.IsEmpty(). end <= arsit.Head().End.
+func truncatedAddrRangeSeq(ars, arsit usermem.AddrRangeSeq, end usermem.Addr) usermem.AddrRangeSeq {
+	ar := arsit.Head()
+	if end <= ar.Start {
+		return ars.TakeFirst64(ars.NumBytes() - arsit.NumBytes())
+	}
+	return ars.TakeFirst64(ars.NumBytes() - arsit.NumBytes() + int64(end-ar.Start))
+}