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xreftable.go
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xreftable.go
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package file
import (
"bytes"
"errors"
"fmt"
"io"
"github.com/benoitkugler/pdf/model"
"github.com/benoitkugler/pdf/reader/parser"
)
// XrefTable maps object numbers to objects.
type XrefTable map[int]model.Object
// ResolveObject use the xref table to resolve indirect reference.
// If the reference is invalid, the ObjNull{} is returned.
// As convenience, direct objects may also be passed and
// will be returned as it is.
func (table XrefTable) ResolveObject(o parser.Object) parser.Object {
ref, ok := o.(parser.IndirectRef)
if !ok {
return o // return the direct object as it is
}
if o, has := table[ref.ObjectNumber]; has {
return o
}
// An indirect reference to an undefined object shall not be considered an error by a conforming reader;
// it shall be treated as a reference to the null object.
return model.ObjNull{}
}
// xRefTableContext is the main access to PDF objects.
// it is only used during the processing (see xrefTable for the final object)
type xRefTableContext struct {
// object number -> entry
objects map[parser.IndirectRef]*xrefEntry
// object stream are special cases since we
// don't wan't to process them for each object they contain
objectStreams map[int]objectStream
}
func newXRefTable() xRefTableContext {
return xRefTableContext{objects: make(map[parser.IndirectRef]*xrefEntry), objectStreams: make(map[int]objectStream)}
}
// populate object field of the xrefTable
func (ctx *context) processAllObjects() error {
for on, entry := range ctx.xrefTable.objects {
if entry.free {
continue
}
_, err := ctx.resolveObjectNumber(on)
if err != nil {
return err
}
}
return nil
}
func (ctx *context) resolve(o parser.Object) (parser.Object, error) {
ref, ok := o.(parser.IndirectRef)
if !ok {
return o, nil // return the direct object as it is
}
return ctx.resolveObjectNumber(ref)
}
func (ctx *context) resolveObjectNumber(objRef model.ObjIndirectRef) (parser.Object, error) {
// 7.3.10
// An indirect reference to an undefined object shall not be considered an error by a conforming reader;
// it shall be treated as a reference to the null object.
entry, ok := ctx.xrefTable.objects[objRef]
if !ok {
return model.ObjNull{}, nil
}
if entry.object != nil { // already resolved
return entry.object, nil
}
isCompressedObject := entry.streamObjectNumber != 0
// Actually resolve the object. There are two cases:
// - the object is compressed inside an object stream
// - the object is a regular object
// Before recursing, start by assigning null as object,
// so that malicious loops won't lead to infinite recursion
entry.object = model.ObjNull{}
if isCompressedObject {
ob, err := ctx.processObjectStream(entry.streamObjectNumber)
if err != nil {
return nil, err
}
if entry.streamObjectIndex >= len(ob) {
return nil, fmt.Errorf("invalid object index (%d >= %d)", entry.streamObjectIndex, len(ob))
}
entry.object = ob[entry.streamObjectIndex]
} else {
tk, err := ctx.tokenizerAt(entry.offset)
if err != nil {
return nil, fmt.Errorf("invalid offset in xref table (%d): %s", entry.offset, err)
}
_, _, err = parseObjectDeclaration(tk)
if err != nil {
return nil, fmt.Errorf("invalid object declaration (%v): %s", objRef, err)
}
entry.object, err = parser.NewParserFromTokenizer(tk).ParseObject()
if err != nil {
return nil, fmt.Errorf("invalid object content (%v): %s", objRef, err)
}
// stream object are dict with an additional content : lookup up for them
nt, _ := tk.NextToken()
if streamHeader, ok := entry.object.(model.ObjDict); nt.IsOther("stream") && ok {
// before resolving, we need to save the current tokeniser position,
// since it may be used during resolution
streamPosition := entry.offset + int64(tk.StreamPosition())
filters, err := parser.ParseFilters(streamHeader["Filter"], streamHeader["DecodeParms"], ctx.resolve)
if err != nil {
return nil, fmt.Errorf("invalid stream: %s", err)
}
lengthO, err := ctx.resolve(streamHeader["Length"])
if err != nil {
return nil, fmt.Errorf("invalid stream Length: %s", err)
}
length, ok := lengthO.(parser.Integer)
if !ok {
return nil, fmt.Errorf("invalid stream Length: expected integer, got %T", lengthO)
}
// we want the cryted not decoded content
content, err := ctx.extractStreamContent(filters, streamPosition, int(length))
if err != nil {
return nil, fmt.Errorf("failed to resolve %v: %s", objRef, err)
}
entry.object = model.ObjStream{Args: streamHeader, Content: content}
}
}
var err error
if ctx.enc != nil && !isCompressedObject { // object inside streams object shall not be encrypted
entry.object, err = ctx.enc.decryptObject(entry.object, objRef)
}
return entry.object, err
}
// xrefEntry is an object entry in the xref table
// it is created with reference information,
// and its Object field is populated when resolved.
type xrefEntry struct {
object parser.Object // initialy nil
free bool // if true, won't be resolved
offset int64
// for object in object streams
streamObjectNumber int // The object number of the object stream in which this object is stored.
streamObjectIndex int // The index of this object within the object stream.
}
// return the previous offset (0 if it does not exists)
func (ctx *context) parseXRefStream(offset int64) (int64, error) {
// parse this object
streamHeader, err := ctx.parseStreamDictAt(offset)
if err != nil {
return 0, err
}
streamOffset := streamHeader.contentOffset
sd, decoded, err := ctx.xRefStreamDict(streamHeader.dict, streamOffset)
if err != nil {
return 0, err
}
err = ctx.trailer.parseTrailerInfo(streamHeader.dict)
if err != nil {
return 0, err
}
// Parse xRefStream and create xRefTable entries for embedded objects.
err = ctx.extractXRefTableEntriesFromXRefStream(decoded, sd)
if err != nil {
return 0, err
}
// since xRef streams are not regular objects, we do not save them in the xref table
// in particular, it avoids issue with decryption
return sd.prev, nil
}
func (ctx *context) xRefStreamDict(d parser.Dict, streamOffset int64) (xrefStreamDict, []byte, error) {
// The values of all entries shown in Table 17 shall be direct objects; indirect references shall not be
// permitted. For arrays (the Index and W entries), all of their elements shall be direct objects as well. If the
// stream is encoded, the Filter and DecodeParms entries in Table 5 shall also be direct objects.
details, err := parseXRefStreamDict(d)
if err != nil {
return details, nil, err
}
filters, err := parser.ParseDirectFilters(d["Filter"], d["DecodeParms"])
if err != nil {
return details, nil, err
}
// we do not use decodeStreamContent since :
// 1) The cross-reference stream shall not be encrypted and strings appearing in the cross-reference stream
// dictionary shall not be encrypted. It shall not have a Filter entry that specifies a Crypt filter (see 7.4.10,
// "Crypt Filter").
// 2) there is no object number for xref stream
content, err := ctx.extractStreamContent(filters, streamOffset, details.count()*details.entrySize())
if err != nil {
return details, nil, err
}
// Decode stream content:
r, err := filters.DecodeReader(bytes.NewReader(content))
if err != nil {
return details, nil, err
}
decoded, err := io.ReadAll(r)
if err != nil {
return details, nil, err
}
return details, decoded, nil
}
// bufToInt64 interprets the content of buf as an int64.
func bufToInt64(buf []byte) (i int64) {
for _, b := range buf {
i <<= 8
i |= int64(b)
}
return i
}
// For each object embedded in this xRefStream create the corresponding xRef table entry.
func (ctx *context) extractXRefTableEntriesFromXRefStream(buf []byte, xrefDict xrefStreamDict) error {
// Note:
// A value of zero for an element in the W array indicates that the corresponding field shall not be present in the stream,
// and the default value shall be used, if there is one.
// If the first element is zero, the type field shall not be present, and shall default to type 1.
xrefEntryLen, count := xrefDict.entrySize(), xrefDict.count()
L := count * xrefEntryLen
if len(buf) < L {
return fmt.Errorf("extractXRefTableEntriesFromXRefStream: corrupted xrefstream (%d < %d)", len(buf), L)
}
// Sometimes there is an additional xref entry not accounted for by "Index".
// We ignore such a entries and do not treat this as an error.
buf = buf[:L]
i1 := xrefDict.w[0]
i2 := xrefDict.w[1]
i3 := xrefDict.w[2]
j := 0 // current index of object (0 <= j < count)
for _, subsection := range xrefDict.index {
firstObj, nb := subsection[0], subsection[1]
for i := 0; i < nb; i++ {
objectNumber := firstObj + i
offsetEntry := j * xrefEntryLen
c2 := bufToInt64(buf[offsetEntry+i1 : offsetEntry+i1+i2])
c3 := bufToInt64(buf[offsetEntry+i1+i2 : offsetEntry+i1+i2+i3])
var (
xRefTableEntry xrefEntry
generation int
)
switch buf[offsetEntry] {
case 0x00: // free object, ignore
xRefTableEntry = xrefEntry{
offset: c2,
free: true,
}
generation = int(c3)
case 0x01: // in use object
xRefTableEntry = xrefEntry{
offset: c2,
}
generation = int(c3)
case 0x02: // compressed object; generation always 0.
xRefTableEntry = xrefEntry{
streamObjectNumber: int(c2),
streamObjectIndex: int(c3),
}
}
ref := model.ObjIndirectRef{ObjectNumber: objectNumber, GenerationNumber: generation}
// skip already assigned
if _, has := ctx.xrefTable.objects[ref]; !has {
ctx.xrefTable.objects[ref] = &xRefTableEntry
}
j++
}
}
return nil
}
type xrefStreamDict struct {
index [][2]int
w [3]int
length int
size int
prev int64
}
// returns the number of entries, as described by the 'index'
func (x xrefStreamDict) count() int {
total := 0
for _, subsection := range x.index {
total += subsection[1]
}
return total
}
func (x xrefStreamDict) entrySize() int {
return x.w[0] + x.w[1] + x.w[2]
}
var (
errXrefStreamCorruptIndex = errors.New("parseXRefStreamDict: corrupted Index entry")
errXrefStreamCorruptW = errors.New("parseXRefStreamDict: corrupted entry W: expecting array of 3 int")
)
// parseXRefStreamDict creates a XRefStreamDict out of a StreamDict.
func parseXRefStreamDict(dict parser.Dict) (xrefStreamDict, error) {
var out xrefStreamDict
out.prev, _ = offsetFromObject(dict["Prev"])
length, ok := dict["Length"].(parser.Integer)
if !ok {
return out, errors.New("parseXRefStreamDict: \"Length\" not available")
}
out.length = int(length)
size, ok := dict["Size"].(parser.Integer)
if !ok {
return out, errors.New("parseXRefStreamDict: \"Size\" not available")
}
out.size = int(size)
// Read optional parameter Index
indArr, _ := dict["Index"].(parser.Array)
if len(indArr) != 0 {
if len(indArr)%2 > 1 {
return out, errXrefStreamCorruptIndex
}
out.index = make([][2]int, len(indArr)/2)
for i := range out.index {
startObj, ok := indArr[i*2].(parser.Integer)
if !ok {
return out, errXrefStreamCorruptIndex
}
count, ok := indArr[i*2+1].(parser.Integer)
if !ok {
return out, errXrefStreamCorruptIndex
}
out.index = append(out.index, [2]int{int(startObj), int(count)})
}
} else {
out.index = [][2]int{{0, out.size}}
}
// Read parameter W in order to decode the xref table.
// array of integers representing the size of the fields in a single cross-reference entry.
w, _ := dict["W"].(parser.Array) // validate array with 3 positive integers
if len(w) < 3 {
return out, errXrefStreamCorruptW
}
f := func(ok bool, i parser.Integer) bool {
return !ok || i < 0
}
i1, ok := w[0].(parser.Integer)
if f(ok, i1) {
return out, errXrefStreamCorruptW
}
out.w[0] = int(i1)
i2, ok := w[1].(parser.Integer)
if f(ok, i2) {
return out, errXrefStreamCorruptW
}
out.w[1] = int(i2)
i3, ok := w[2].(parser.Integer)
if f(ok, i3) {
return out, errXrefStreamCorruptW
}
out.w[2] = int(i3)
return out, nil
}