visor/pkg/sentry/mm/vma.go

// 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 mm

import (
	"fmt"
	"sync/atomic"

	"gvisor.dev/gvisor/pkg/abi/linux"
	"gvisor.dev/gvisor/pkg/context"
	"gvisor.dev/gvisor/pkg/errors/linuxerr"
	"gvisor.dev/gvisor/pkg/hostarch"
	"gvisor.dev/gvisor/pkg/sentry/arch"
	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
	"gvisor.dev/gvisor/pkg/sentry/limits"
	"gvisor.dev/gvisor/pkg/sentry/memmap"
)

// Caller provides the droppedIDs slice to collect dropped mapping
// identities. The caller must drop the references on these identities outside a
// mm.mappingMu critical section. droppedIDs has append-like semantics, multiple
// calls to functions that drop mapping identities within a scope should reuse
// the same slice.
//
// Preconditions:
//   - mm.mappingMu must be locked for writing.
//   - opts must be valid as defined by the checks in MMap.
func (mm *MemoryManager) createVMALocked(ctx context.Context, opts memmap.MMapOpts, droppedIDs []memmap.MappingIdentity) (vmaIterator, hostarch.AddrRange, []memmap.MappingIdentity, error) {
	if opts.MaxPerms != opts.MaxPerms.Effective() {
		panic(fmt.Sprintf("Non-effective MaxPerms %s cannot be enforced", opts.MaxPerms))
	}

	// Find a usable range.
	addr, err := mm.findAvailableLocked(opts.Length, findAvailableOpts{
		Addr:      opts.Addr,
		Fixed:     opts.Fixed,
		GrowsDown: opts.GrowsDown,
		Stack:     opts.Stack,
		Private:   opts.Private,
		Unmap:     opts.Unmap,
		NoReplace: opts.NoReplace,
		Map32Bit:  opts.Map32Bit,
	})
	if err != nil {
		// Can't force without opts.Unmap and opts.Fixed.
		if opts.Force && opts.Unmap && opts.Fixed {
			addr = opts.Addr
		} else {
			return vmaIterator{}, hostarch.AddrRange{}, droppedIDs, err
		}
	}
	ar, _ := addr.ToRange(opts.Length)

	// Check against RLIMIT_AS.
	newUsageAS := mm.usageAS + opts.Length
	if opts.Unmap {
		newUsageAS -= uint64(mm.vmas.SpanRange(ar))
	}
	if limitAS := limits.FromContext(ctx).Get(limits.AS).Cur; newUsageAS > limitAS {
		return vmaIterator{}, hostarch.AddrRange{}, droppedIDs, linuxerr.ENOMEM
	}

	if opts.MLockMode != memmap.MLockNone {
		// Check against RLIMIT_MEMLOCK.
		if !auth.CredentialsFromContext(ctx).HasRootCapability(linux.CAP_IPC_LOCK) {
			mlockLimit := limits.FromContext(ctx).Get(limits.MemoryLocked).Cur
			if mlockLimit == 0 {
				return vmaIterator{}, hostarch.AddrRange{}, droppedIDs, linuxerr.EPERM
			}
			newLockedAS := mm.lockedAS + opts.Length
			if opts.Unmap {
				newLockedAS -= mm.mlockedBytesRangeLocked(ar)
			}
			if newLockedAS > mlockLimit {
				return vmaIterator{}, hostarch.AddrRange{}, droppedIDs, linuxerr.EAGAIN
			}
		}
	}

	// Remove overwritten mappings. This ordering is consistent with Linux:
	// compare Linux's mm/mmap.c:mmap_region() => do_munmap(),
	// file->f_op->mmap().
	var vgap vmaGapIterator
	if opts.Unmap {
		vgap, droppedIDs = mm.unmapLocked(ctx, ar, droppedIDs)
	} else {
		vgap = mm.vmas.FindGap(ar.Start)
	}

	// Inform the Mappable, if any, of the new mapping.
	if opts.Mappable != nil {
		// The expression for writable is vma.canWriteMappableLocked(), but we
		// don't yet have a vma.
		if err := opts.Mappable.AddMapping(ctx, mm, ar, opts.Offset, !opts.Private && opts.MaxPerms.Write); err != nil {
			return vmaIterator{}, hostarch.AddrRange{}, droppedIDs, err
		}
	}

	// Take a reference on opts.MappingIdentity before inserting the vma since
	// vma merging can drop the reference.
	if opts.MappingIdentity != nil {
		opts.MappingIdentity.IncRef()
	}

	// Finally insert the vma.
	v := vma{
		mappable:       opts.Mappable,
		off:            opts.Offset,
		realPerms:      opts.Perms,
		effectivePerms: opts.Perms.Effective(),
		maxPerms:       opts.MaxPerms,
		private:        opts.Private,
		growsDown:      opts.GrowsDown,
		isStack:        opts.Stack,
		mlockMode:      opts.MLockMode,
		numaPolicy:     linux.MPOL_DEFAULT,
		id:             opts.MappingIdentity,
		name:           opts.Name,
		nameMut:        opts.NameMut,
	}

	vseg := mm.vmas.Insert(vgap, ar, v)
	mm.usageAS += opts.Length
	if v.isPrivateDataLocked() {
		mm.dataAS += opts.Length
	}
	if opts.MLockMode != memmap.MLockNone {
		mm.lockedAS += opts.Length
	}

	return vseg, ar, droppedIDs, nil
}

type findAvailableOpts struct {
	// These fields are equivalent to those in memmap.MMapOpts, except that:
	//
	//	- Addr must be page-aligned.
	//
	//	- Unmap allows existing guard pages in the returned range.
	//
	//	- NoReplace allows existing guard pages (but not existing vmas) in the
	//		returned range.

	Addr      hostarch.Addr
	Fixed     bool
	GrowsDown bool
	Stack     bool
	Private   bool
	Unmap     bool
	NoReplace bool
	Map32Bit  bool
}

// map32Start/End are the bounds to which MAP_32BIT mappings are constrained,
// and are equivalent to Linux's MAP32_BASE and MAP32_MAX respectively.
const (
	map32Start = 0x40000000
	map32End   = 0x80000000
)

// findAvailableLocked finds an allocatable range.
//
// Preconditions: mm.mappingMu must be locked.
func (mm *MemoryManager) findAvailableLocked(length uint64, opts findAvailableOpts) (hostarch.Addr, error) {
	if opts.Fixed {
		opts.Map32Bit = false
	}
	allowedAR := mm.applicationAddrRange()
	if opts.Map32Bit {
		allowedAR = allowedAR.Intersect(hostarch.AddrRange{map32Start, map32End})
	}

	// Does the provided suggestion work?
	if ar, ok := opts.Addr.ToRange(length); ok {
		if allowedAR.IsSupersetOf(ar) {
			if opts.Unmap {
				return ar.Start, nil
			}
			vgap := mm.vmas.FindGap(ar.Start)
			if opts.NoReplace {
				// MAP_FIXED_NOREPLACE fails with EEXIST iff the requested
				// range overlaps an existing vma; like Linux, it may map over
				// guard pages of adjacent MAP_GROWSDOWN mappings. Compare
				// Linux's mm/mmap.c:do_mmap() => find_vma_intersection().
				if vgap.Ok() && vgap.Range().IsSupersetOf(ar) {
					return ar.Start, nil
				}
				return 0, linuxerr.EEXIST
			}
			// Check for the presence of an existing vma or guard page.
			if vgap.Ok() && vgap.availableRange().IsSupersetOf(ar) {
				return ar.Start, nil
			}
		}
	}

	// Fixed mappings accept only the requested address.
	if opts.Fixed {
		return 0, linuxerr.ENOMEM
	}

	// Prefer hugepage alignment if length is hugepage-aligned and the vma will
	// actually be eligible for hugepages.
	alignment := uint64(hostarch.PageSize)
	if hostarch.IsHugePageAligned(length) && opts.Private && !opts.GrowsDown && !opts.Stack {
		alignment = hostarch.HugePageSize
	}

	if opts.Map32Bit {
		return mm.findLowestAvailableLocked(length, alignment, allowedAR)
	}
	if mm.layout.DefaultDirection == arch.MmapBottomUp {
		return mm.findLowestAvailableLocked(length, alignment, hostarch.AddrRange{mm.layout.BottomUpBase, mm.layout.MaxAddr})
	}
	return mm.findHighestAvailableLocked(length, alignment, hostarch.AddrRange{mm.layout.MinAddr, mm.layout.TopDownBase})
}

func (mm *MemoryManager) applicationAddrRange() hostarch.AddrRange {
	return hostarch.AddrRange{mm.layout.MinAddr, mm.layout.MaxAddr}
}

// Preconditions: mm.mappingMu must be locked.
func (mm *MemoryManager) findLowestAvailableLocked(length, alignment uint64, bounds hostarch.AddrRange) (hostarch.Addr, error) {
	for gap := mm.vmas.LowerBoundGap(bounds.Start); gap.Ok() && gap.Start() < bounds.End; gap = gap.NextLargeEnoughGap(hostarch.Addr(length)) {
		if gr := gap.availableRange().Intersect(bounds); uint64(gr.Length()) >= length {
			// Can we shift up to match the alignment?
			if offset := uint64(gr.Start) % alignment; offset != 0 {
				if uint64(gr.Length()) >= length+alignment-offset {
					// Yes, we're aligned.
					return gr.Start + hostarch.Addr(alignment-offset), nil
				}
			}

			// Either aligned perfectly, or can't align it.
			return gr.Start, nil
		}
	}
	return 0, linuxerr.ENOMEM
}

// Preconditions: mm.mappingMu must be locked.
func (mm *MemoryManager) findHighestAvailableLocked(length, alignment uint64, bounds hostarch.AddrRange) (hostarch.Addr, error) {
	for gap := mm.vmas.UpperBoundGap(bounds.End); gap.Ok() && gap.End() > bounds.Start; gap = gap.PrevLargeEnoughGap(hostarch.Addr(length)) {
		if gr := gap.availableRange().Intersect(bounds); uint64(gr.Length()) >= length {
			// Can we shift down to match the alignment?
			start := gr.End - hostarch.Addr(length)
			if offset := uint64(start) % alignment; offset != 0 {
				if gr.Start <= start-hostarch.Addr(offset) {
					// Yes, we're aligned.
					return start - hostarch.Addr(offset), nil
				}
			}

			// Either aligned perfectly, or can't align it.
			return start, nil
		}
	}
	return 0, linuxerr.ENOMEM
}

// Preconditions: mm.mappingMu must be locked.
func (mm *MemoryManager) mlockedBytesRangeLocked(ar hostarch.AddrRange) uint64 {
	var total uint64
	for vseg := mm.vmas.LowerBoundSegment(ar.Start); vseg.Ok() && vseg.Start() < ar.End; vseg = vseg.NextSegment() {
		if vseg.ValuePtr().mlockMode != memmap.MLockNone {
			total += uint64(vseg.Range().Intersect(ar).Length())
		}
	}
	return total
}

// getVMAsLocked ensures that vmas exist for all addresses in ar, and support
// access of type (at, ignorePermissions). It returns:
//
//   - An iterator to the vma containing ar.Start. If no vma contains ar.Start,
//     the iterator is unspecified.
//
//   - An iterator to the gap after the last vma containing an address in ar. If
//     vmas exist for no addresses in ar, the iterator is to a gap that begins
//     before ar.Start.
//
//   - An error that is non-nil if vmas exist for only a subset of ar.
//
// Preconditions:
//   - mm.mappingMu must be locked for reading; it may be temporarily unlocked.
//   - ar.Length() != 0.
func (mm *MemoryManager) getVMAsLocked(ctx context.Context, ar hostarch.AddrRange, at hostarch.AccessType, ignorePermissions bool) (vmaIterator, vmaGapIterator, error) {
	if checkInvariants {
		if !ar.WellFormed() || ar.Length() == 0 {
			panic(fmt.Sprintf("invalid ar: %v", ar))
		}
	}

	// Inline mm.vmas.LowerBoundSegment so that we have the preceding gap if
	// !vbegin.Ok().
	vbegin, vgap := mm.vmas.Find(ar.Start)
	if !vbegin.Ok() {
		vbegin = vgap.NextSegment()
		// vseg.Ok() is checked before entering the following loop.
	} else {
		vgap = vbegin.PrevGap()
	}

	addr := ar.Start
	vseg := vbegin
	for vseg.Ok() {
		// Loop invariants: vgap = vseg.PrevGap(); addr < vseg.End().
		vma := vseg.ValuePtr()
		if addr < vseg.Start() {
			// TODO(jamieliu): Implement vma.growsDown here.
			return vbegin, vgap, linuxerr.EFAULT
		}

		perms := vma.effectivePerms
		if ignorePermissions {
			perms = vma.maxPerms
		}
		if !perms.SupersetOf(at) {
			return vbegin, vgap, linuxerr.EPERM
		}

		addr = vseg.End()
		vgap = vseg.NextGap()
		if addr >= ar.End {
			return vbegin, vgap, nil
		}
		vseg = vgap.NextSegment()
	}

	// Ran out of vmas before ar.End.
	return vbegin, vgap, linuxerr.EFAULT
}

// getVecVMAsLocked ensures that vmas exist for all addresses in ars, and
// support access to type of (at, ignorePermissions). It returns the subset of
// ars for which vmas exist. If this is not equal to ars, it returns a non-nil
// error explaining why.
//
// Preconditions: mm.mappingMu must be locked for reading; it may be
// temporarily unlocked.
//
// Postconditions: ars is not mutated.
func (mm *MemoryManager) getVecVMAsLocked(ctx context.Context, ars hostarch.AddrRangeSeq, at hostarch.AccessType, ignorePermissions bool) (hostarch.AddrRangeSeq, error) {
	for arsit := ars; !arsit.IsEmpty(); arsit = arsit.Tail() {
		ar := arsit.Head()
		if ar.Length() == 0 {
			continue
		}
		if _, vend, err := mm.getVMAsLocked(ctx, ar, at, ignorePermissions); err != nil {
			return truncatedAddrRangeSeq(ars, arsit, vend.Start()), err
		}
	}
	return ars, nil
}

// vma extension will not shrink the number of unmapped bytes between the start
// of a growsDown vma and the end of its predecessor non-growsDown vma below
// guardBytes.
//
// guardBytes is equivalent to Linux's stack_guard_gap after upstream
// 1be7107fbe18 "mm: larger stack guard gap, between vmas".
const guardBytes = 256 * hostarch.PageSize

// unmapLocked unmaps all addresses in ar and returns the resulting gap in
// mm.vmas.
//
// Caller provides the droppedIDs slice to collect dropped mapping
// identities. The caller must drop the references on these identities outside a
// mm.mappingMu critical section. droppedIDs has append-like semantics, multiple
// calls to functions that drop mapping identities within a scope should reuse
// the same slice.
//
// Preconditions:
//   - mm.mappingMu must be locked for writing.
//   - ar.Length() != 0.
//   - ar must be page-aligned.
func (mm *MemoryManager) unmapLocked(ctx context.Context, ar hostarch.AddrRange, droppedIDs []memmap.MappingIdentity) (vmaGapIterator, []memmap.MappingIdentity) {
	if checkInvariants {
		if !ar.WellFormed() || ar.Length() == 0 || !ar.IsPageAligned() {
			panic(fmt.Sprintf("invalid ar: %v", ar))
		}
	}

	// AddressSpace mappings and pmas must be invalidated before
	// mm.removeVMAsLocked() => memmap.Mappable.RemoveMapping().
	mm.Invalidate(ar, memmap.InvalidateOpts{InvalidatePrivate: true})
	return mm.removeVMAsLocked(ctx, ar, droppedIDs)
}

// removeVMAsLocked removes vmas for addresses in ar and returns the
// resulting gap in mm.vmas.
//
// Caller provides the droppedIDs slice to collect dropped mapping
// identities. The caller must drop the references on these identities outside a
// mm.mappingMu critical section. droppedIDs has append-like semantics, multiple
// calls to functions that drop mapping identities within a scope should reuse
// the same slice.
//
// Preconditions:
//   - mm.mappingMu must be locked for writing.
//   - ar.Length() != 0.
//   - ar must be page-aligned.
func (mm *MemoryManager) removeVMAsLocked(ctx context.Context, ar hostarch.AddrRange, droppedIDs []memmap.MappingIdentity) (vmaGapIterator, []memmap.MappingIdentity) {
	if checkInvariants {
		if !ar.WellFormed() || ar.Length() == 0 || !ar.IsPageAligned() {
			panic(fmt.Sprintf("invalid ar: %v", ar))
		}
	}
	vgap := mm.vmas.RemoveRangeWith(ar, func(vseg vmaIterator) {
		vmaAR := vseg.Range()
		vma := vseg.ValuePtr()
		if vma.mappable != nil {
			vma.mappable.RemoveMapping(ctx, mm, vmaAR, vma.off, vma.canWriteMappableLocked())
		}
		if vma.id != nil {
			droppedIDs = append(droppedIDs, vma.id)
		}
		mm.usageAS -= uint64(vmaAR.Length())
		if vma.isPrivateDataLocked() {
			mm.dataAS -= uint64(vmaAR.Length())
		}
		if vma.mlockMode != memmap.MLockNone {
			mm.lockedAS -= uint64(vmaAR.Length())
		}
	})
	return vgap, droppedIDs
}

// canWriteMappableLocked returns true if it is possible for vma.mappable to be
// written to via this vma, i.e. if it is possible that
// vma.mappable.Translate(at.Write=true) may be called as a result of this vma.
// This includes via I/O with usermem.IOOpts.IgnorePermissions = true, such as
// PTRACE_POKEDATA.
//
// canWriteMappableLocked is equivalent to Linux's VM_SHARED.
//
// Preconditions: mm.mappingMu must be locked.
func (v *vma) canWriteMappableLocked() bool {
	return !v.private && v.maxPerms.Write
}

// isPrivateDataLocked identify the data segments - private, writable, not stack
//
// Preconditions: mm.mappingMu must be locked.
func (v *vma) isPrivateDataLocked() bool {
	return v.realPerms.Write && v.private && !v.growsDown
}

// vmaSetFunctions implements segment.Functions for vmaSet.
type vmaSetFunctions struct{}

func (vmaSetFunctions) MinKey() hostarch.Addr {
	return 0
}

func (vmaSetFunctions) MaxKey() hostarch.Addr {
	return ^hostarch.Addr(0)
}

func (vmaSetFunctions) ClearValue(vma *vma) {
	vma.mappable = nil
	vma.id = nil
	vma.name = ""
	atomic.StoreUintptr(&vma.lastFault, 0)
}

func (vmaSetFunctions) Merge(ar1 hostarch.AddrRange, vma1 vma, ar2 hostarch.AddrRange, vma2 vma) (vma, bool) {
	if vma1.mappable != vma2.mappable ||
		(vma1.mappable != nil && vma1.off+uint64(ar1.Length()) != vma2.off) ||
		vma1.realPerms != vma2.realPerms ||
		vma1.maxPerms != vma2.maxPerms ||
		vma1.private != vma2.private ||
		vma1.growsDown != vma2.growsDown ||
		vma1.isStack != vma2.isStack ||
		vma1.mlockMode != vma2.mlockMode ||
		vma1.numaPolicy != vma2.numaPolicy ||
		vma1.numaNodemask != vma2.numaNodemask ||
		vma1.dontfork != vma2.dontfork ||
		vma1.id != vma2.id ||
		vma1.name != vma2.name ||
		vma1.nameMut != vma2.nameMut {
		return vma{}, false
	}

	if vma2.id != nil {
		// This DecRef() will never be the final ref, since the vma1 is
		// currently holding a ref to the same mapping identity. Thus, we don't
		// need to worry about whether we're in a mm.mappingMu critical section.
		vma2.id.DecRef(context.Background())
	}

	// If the existing vma (vma2) has non-zero lastFault address,
	// we should preserve it to the resulting merged-VMA
	if vma1.lastFault == 0 {
		vma1.lastFault = vma2.lastFault
	}

	return vma1, true
}

func (vmaSetFunctions) Split(ar hostarch.AddrRange, v vma, split hostarch.Addr) (vma, vma) {
	v2 := v
	if v2.mappable != nil {
		v2.off += uint64(split - ar.Start)
	}
	if v2.id != nil {
		v2.id.IncRef()
	}
	return v, v2
}

// Preconditions:
//   - vseg.ValuePtr().mappable != nil.
//   - vseg.Range().Contains(addr).
func (vseg vmaIterator) mappableOffsetAt(addr hostarch.Addr) uint64 {
	if checkInvariants {
		if !vseg.Ok() {
			panic("terminal vma iterator")
		}
		if vseg.ValuePtr().mappable == nil {
			panic("Mappable offset is meaningless for anonymous vma")
		}
		if !vseg.Range().Contains(addr) {
			panic(fmt.Sprintf("addr %v out of bounds %v", addr, vseg.Range()))
		}
	}

	vma := vseg.ValuePtr()
	vstart := vseg.Start()
	return vma.off + uint64(addr-vstart)
}

// Preconditions: vseg.ValuePtr().mappable != nil.
func (vseg vmaIterator) mappableRange() memmap.MappableRange {
	return vseg.mappableRangeOf(vseg.Range())
}

// Preconditions:
//   - vseg.ValuePtr().mappable != nil.
//   - vseg.Range().IsSupersetOf(ar).
//   - ar.Length() != 0.
func (vseg vmaIterator) mappableRangeOf(ar hostarch.AddrRange) memmap.MappableRange {
	if checkInvariants {
		if !vseg.Ok() {
			panic("terminal vma iterator")
		}
		if vseg.ValuePtr().mappable == nil {
			panic("MappableRange is meaningless for anonymous vma")
		}
		if !ar.WellFormed() || ar.Length() == 0 {
			panic(fmt.Sprintf("invalid ar: %v", ar))
		}
		if !vseg.Range().IsSupersetOf(ar) {
			panic(fmt.Sprintf("ar %v out of bounds %v", ar, vseg.Range()))
		}
	}

	vma := vseg.ValuePtr()
	vstart := vseg.Start()
	return memmap.MappableRange{vma.off + uint64(ar.Start-vstart), vma.off + uint64(ar.End-vstart)}
}

// Preconditions:
//   - vseg.ValuePtr().mappable != nil.
//   - vseg.mappableRange().IsSupersetOf(mr).
//   - mr.Length() != 0.
func (vseg vmaIterator) addrRangeOf(mr memmap.MappableRange) hostarch.AddrRange {
	if checkInvariants {
		if !vseg.Ok() {
			panic("terminal vma iterator")
		}
		if vseg.ValuePtr().mappable == nil {
			panic("MappableRange is meaningless for anonymous vma")
		}
		if !mr.WellFormed() || mr.Length() == 0 {
			panic(fmt.Sprintf("invalid mr: %v", mr))
		}
		if !vseg.mappableRange().IsSupersetOf(mr) {
			panic(fmt.Sprintf("mr %v out of bounds %v", mr, vseg.mappableRange()))
		}
	}

	vma := vseg.ValuePtr()
	vstart := vseg.Start()
	return hostarch.AddrRange{vstart + hostarch.Addr(mr.Start-vma.off), vstart + hostarch.Addr(mr.End-vma.off)}
}

// seekNextLowerBound returns mm.vmas.LowerBoundSegment(addr), but does so by
// scanning linearly forward from vseg.
//
// Preconditions:
//   - mm.mappingMu must be locked.
//   - addr >= vseg.Start().
func (vseg vmaIterator) seekNextLowerBound(addr hostarch.Addr) vmaIterator {
	if checkInvariants {
		if !vseg.Ok() {
			panic("terminal vma iterator")
		}
		if addr < vseg.Start() {
			panic(fmt.Sprintf("can't seek forward to %#x from %#x", addr, vseg.Start()))
		}
	}
	for vseg.Ok() && addr >= vseg.End() {
		vseg = vseg.NextSegment()
	}
	return vseg
}

// availableRange returns the subset of vgap.Range() in which new vmas may be
// created without MMapOpts.Unmap or MMapOpts.NoReplace set.
func (vgap vmaGapIterator) availableRange() hostarch.AddrRange {
	ar := vgap.Range()
	next := vgap.NextSegment()
	if !next.Ok() || !next.ValuePtr().growsDown {
		return ar
	}
	// Exclude guard pages.
	if ar.Length() < guardBytes {
		return hostarch.AddrRange{ar.Start, ar.Start}
	}
	ar.End -= guardBytes
	return ar
}