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Move QPDF::reconstruct_xref to QPDF::Xref_table

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Also, when recovering trailer from xref streams, pick the last valid
trailer encountered rather than the first.
This commit is contained in:
m-holger 2024-08-10 00:12:53 +01:00
parent 1e072e223a
commit 3fbff84594
5 changed files with 55 additions and 44 deletions
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1
include/qpdf/QPDF.hh
View File

@ -762,7 +762,6 @@ class QPDF
void setTrailer(QPDFObjectHandle obj);
void read_xref(qpdf_offset_t offset);
bool resolveXRefTable();
void reconstruct_xref(QPDFExc& e);
bool parse_xrefFirst(std::string const& line, int& obj, int& num, int& bytes);
bool read_xrefEntry(qpdf_offset_t& f1, int& f2, char& type);
bool read_bad_xrefEntry(qpdf_offset_t& f1, int& f2, char& type);

79
libqpdf/QPDF.cc
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@ -476,7 +476,7 @@ QPDF::parse(char const* password)
}
} catch (QPDFExc& e) {
if (m->attempt_recovery) {
reconstruct_xref(e);
m->xref_table.reconstruct(e);
QTC::TC("qpdf", "QPDF reconstructed xref table");
} else {
throw;
@ -535,40 +535,42 @@ QPDF::setTrailer(QPDFObjectHandle obj)
}
void
QPDF::reconstruct_xref(QPDFExc& e)
QPDF::Xref_table::reconstruct(QPDFExc& e)
{
if (m->xref_table.reconstructed) {
if (reconstructed) {
// Avoid xref reconstruction infinite loops. This is getting very hard to reproduce because
// qpdf is throwing many fewer exceptions while parsing. Most situations are warnings now.
throw e;
}
auto* m = qpdf.m.get();
// If recovery generates more than 1000 warnings, the file is so severely damaged that there
// probably is no point trying to continue.
const auto max_warnings = m->warnings.size() + 1000U;
auto check_warnings = [this, max_warnings]() {
if (m->warnings.size() > max_warnings) {
throw damagedPDF("", 0, "too many errors while reconstructing cross-reference table");
if (qpdf.m->warnings.size() > max_warnings) {
throw damaged_pdf("too many errors while reconstructing cross-reference table");
}
};
m->xref_table.reconstructed = true;
reconstructed = true;
// We may find more objects, which may contain dangling references.
m->fixed_dangling_refs = false;
warn(damagedPDF("", 0, "file is damaged"));
warn(e);
warn(damagedPDF("", 0, "Attempting to reconstruct cross-reference table"));
warn_damaged("file is damaged");
qpdf.warn(e);
warn_damaged("Attempting to reconstruct cross-reference table");
// Delete all references to type 1 (uncompressed) objects
std::set<QPDFObjGen> to_delete;
for (auto const& iter: m->xref_table) {
for (auto const& iter: *this) {
if (iter.second.getType() == 1) {
to_delete.insert(iter.first);
}
}
for (auto const& iter: to_delete) {
m->xref_table.erase(iter);
erase(iter);
}
m->file->seek(0, SEEK_END);
@ -577,46 +579,45 @@ QPDF::reconstruct_xref(QPDFExc& e)
// Don't allow very long tokens here during recovery. All the interesting tokens are covered.
static size_t const MAX_LEN = 10;
while (m->file->tell() < eof) {
QPDFTokenizer::Token t1 = readToken(*m->file, MAX_LEN);
QPDFTokenizer::Token t1 = qpdf.readToken(*m->file, MAX_LEN);
qpdf_offset_t token_start = m->file->tell() - toO(t1.getValue().length());
if (t1.isInteger()) {
auto pos = m->file->tell();
QPDFTokenizer::Token t2 = readToken(*m->file, MAX_LEN);
if ((t2.isInteger()) && (readToken(*m->file, MAX_LEN).isWord("obj"))) {
QPDFTokenizer::Token t2 = qpdf.readToken(*m->file, MAX_LEN);
if (t2.isInteger() && qpdf.readToken(*m->file, MAX_LEN).isWord("obj")) {
int obj = QUtil::string_to_int(t1.getValue().c_str());
int gen = QUtil::string_to_int(t2.getValue().c_str());
if (obj <= m->xref_table.max_id) {
m->xref_table.insert_reconstructed(obj, token_start, gen);
if (obj <= max_id) {
insert_reconstructed(obj, token_start, gen);
} else {
warn(damagedPDF(
"", 0, "ignoring object with impossibly large id " + std::to_string(obj)));
warn_damaged("ignoring object with impossibly large id " + std::to_string(obj));
}
}
m->file->seek(pos, SEEK_SET);
} else if (!m->xref_table.trailer && t1.isWord("trailer")) {
} else if (!trailer && t1.isWord("trailer")) {
auto pos = m->file->tell();
QPDFObjectHandle t = readTrailer();
QPDFObjectHandle t = qpdf.readTrailer();
if (!t.isDictionary()) {
// Oh well. It was worth a try.
} else {
setTrailer(t);
qpdf.setTrailer(t);
}
m->file->seek(pos, SEEK_SET);
}
check_warnings();
m->file->findAndSkipNextEOL();
}
m->xref_table.deleted_objects.clear();
deleted_objects.clear();
if (!m->xref_table.trailer) {
if (!trailer) {
qpdf_offset_t max_offset{0};
// If there are any xref streams, take the last one to appear.
for (auto const& iter: m->xref_table) {
for (auto const& iter: *this) {
auto entry = iter.second;
if (entry.getType() != 1) {
continue;
}
auto oh = getObjectByObjGen(iter.first);
auto oh = qpdf.getObjectByObjGen(iter.first);
try {
if (!oh.isStreamOfType("/XRef")) {
continue;
@ -627,41 +628,41 @@ QPDF::reconstruct_xref(QPDFExc& e)
auto offset = entry.getOffset();
if (offset > max_offset) {
max_offset = offset;
setTrailer(oh.getDict());
trailer = oh.getDict();
}
check_warnings();
}
if (max_offset > 0) {
try {
read_xref(max_offset);
qpdf.read_xref(max_offset);
} catch (std::exception&) {
throw damagedPDF(
"", 0, "error decoding candidate xref stream while recovering damaged file");
throw damaged_pdf(
"error decoding candidate xref stream while recovering damaged file");
}
QTC::TC("qpdf", "QPDF recover xref stream");
}
}
if (!m->xref_table.trailer) {
if (!trailer) {
// We could check the last encountered object to see if it was an xref stream. If so, we
// could try to get the trailer from there. This may make it possible to recover files with
// bad startxref pointers even when they have object streams.
throw damagedPDF("", 0, "unable to find trailer dictionary while recovering damaged file");
throw damaged_pdf("unable to find trailer dictionary while recovering damaged file");
}
if (m->xref_table.empty()) {
if (empty()) {
// We cannot check for an empty xref table in parse because empty tables are valid when
// creating QPDF objects from JSON.
throw damagedPDF("", 0, "unable to find objects while recovering damaged file");
throw damaged_pdf("unable to find objects while recovering damaged file");
}
check_warnings();
if (!m->xref_table.parsed) {
m->xref_table.parsed = true;
getAllPages();
if (!parsed) {
parsed = true;
qpdf.getAllPages();
check_warnings();
if (m->all_pages.empty()) {
m->xref_table.parsed = false;
throw damagedPDF("", 0, "unable to find any pages while recovering damaged file");
parsed = false;
throw damaged_pdf("unable to find any pages while recovering damaged file");
}
}
// We could iterate through the objects looking for streams and try to find objects inside of
@ -1766,7 +1767,7 @@ QPDF::readObjectAtOffset(
} catch (QPDFExc& e) {
if (try_recovery) {
// Try again after reconstructing xref table
reconstruct_xref(e);
m->xref_table.reconstruct(e);
if (m->xref_table.count(exp_og) && (m->xref_table[exp_og].getType() == 1)) {
qpdf_offset_t new_offset = m->xref_table[exp_og].getOffset();
QPDFObjectHandle result =

13
libqpdf/qpdf/QPDF_private.hh
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@ -16,6 +16,8 @@ class QPDF::Xref_table: public std::map<QPDFObjGen, QPDFXRefEntry>
void insert(int obj, int f0, qpdf_offset_t f1, int f2);
void insert_free(QPDFObjGen);
void reconstruct(QPDFExc& e);
QPDFObjectHandle trailer;
bool reconstructed{false};
// Various tables are indexed by object id, with potential size id + 1
@ -31,6 +33,17 @@ class QPDF::Xref_table: public std::map<QPDFObjGen, QPDFXRefEntry>
qpdf_offset_t first_item_offset{0}; // actual value from file
private:
QPDFExc
damaged_pdf(std::string const& msg)
{
return qpdf.damagedPDF("", 0, msg);
}
void
warn_damaged(std::string const& msg)
{
qpdf.warn(damaged_pdf(msg));
}
QPDF& qpdf;
};

5
qpdf/qtest/object-stream.test
View File

@ -102,11 +102,10 @@ $td->runtest("recover file with xref stream",
{$td->COMMAND => "qpdf --static-id --compress-streams=n" .
" recover-xref-stream.pdf a.pdf"},
{$td->FILE => "recover-xref-stream.out", $td->EXIT_STATUS => 3},
$td->EXPECT_FAILURE);
$td->NORMALIZE_NEWLINES);
$td->runtest("check file",
{$td->FILE => "a.pdf"},
{$td->FILE => "recover-xref-stream-recovered.pdf"},
$td->EXPECT_FAILURE);
{$td->FILE => "recover-xref-stream-recovered.pdf"});
# Self-referential object stream
$td->runtest("self-referential object stream",

1
qpdf/qtest/qpdf/recover-xref-stream.out
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@ -1,5 +1,4 @@
WARNING: recover-xref-stream.pdf: file is damaged
WARNING: recover-xref-stream.pdf: can't find startxref
WARNING: recover-xref-stream.pdf: Attempting to reconstruct cross-reference table
WARNING: recover-xref-stream.pdf: reported number of objects (14) is not one plus the highest object number (15)
qpdf: operation succeeded with warnings; resulting file may have some problems