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Code Issues Releases Wiki Activity

Refactor Xref_table::subsections

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Optimistically read subsection headers without reading individual object
entries, assuming that they are 20 bytes long as per the PDF spec. If
problems are encountered, fall back to calling bad_subsections.
This commit is contained in:
m-holger 2024-08-20 16:37:58 +01:00
parent ad10fa3006
commit 28c13f5492
3 changed files with 38 additions and 8 deletions
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42
libqpdf/QPDF.cc
View File

@ -820,20 +820,24 @@ QPDF::Xref_table::subsection(std::string const& line)
while (QUtil::is_space(*p)) {
++p;
}
return {
QUtil::string_to_int(obj_str.c_str()),
QUtil::string_to_int(num_str.c_str()),
file->getLastOffset() + toI(p - start)};
auto obj = QUtil::string_to_int(obj_str.c_str());
auto count = QUtil::string_to_int(num_str.c_str());
if (obj > max_id() || count > max_id() || (obj + count) > max_id()) {
throw damaged_table("xref table subsection header contains impossibly large entry");
}
return {obj, count, file->getLastOffset() + toI(p - start)};
}
std::vector<QPDF::Xref_table::Subsection>
QPDF::Xref_table::bad_subsections(std::string& line)
QPDF::Xref_table::bad_subsections(std::string& line, qpdf_offset_t start)
{
std::vector<QPDF::Xref_table::Subsection> result;
qpdf_offset_t f1 = 0;
int f2 = 0;
char type = '\0';
file->seek(start, SEEK_SET);
while (true) {
line.assign(50, '\0');
file->read(line.data(), line.size());
@ -854,10 +858,36 @@ QPDF::Xref_table::bad_subsections(std::string& line)
}
}
// Optimistically read and parse all subsection headers. If an error is encountered return the
// result of bad_subsections.
std::vector<QPDF::Xref_table::Subsection>
QPDF::Xref_table::subsections(std::string& line)
{
return bad_subsections(line);
auto recovery_offset = file->tell();
try {
std::vector<QPDF::Xref_table::Subsection> result;
while (true) {
line.assign(50, '\0');
file->read(line.data(), line.size());
auto& sub = result.emplace_back(subsection(line));
auto count = std::get<1>(sub);
auto offset = std::get<2>(sub);
file->seek(offset + 20 * toO(count) - 1, SEEK_SET);
file->read(line.data(), 1);
if (!(line[0] == '\n' || line[0] == '\n')) {
return bad_subsections(line, recovery_offset);
}
qpdf_offset_t pos = file->tell();
if (read_token().isWord("trailer")) {
return result;
} else {
file->seek(pos, SEEK_SET);
}
}
} catch (...) {
return bad_subsections(line, recovery_offset);
}
}
bool

2
libqpdf/qpdf/QPDF_private.hh
View File

@ -231,7 +231,7 @@ class QPDF::Xref_table
// Methods to parse tables
qpdf_offset_t process_section(qpdf_offset_t offset);
std::vector<Subsection> subsections(std::string& line);
std::vector<Subsection> bad_subsections(std::string& line);
std::vector<Subsection> bad_subsections(std::string& line, qpdf_offset_t offset);
Subsection subsection(std::string const& line);
bool read_entry(qpdf_offset_t& f1, int& f2, char& type);
bool read_bad_entry(qpdf_offset_t& f1, int& f2, char& type);

2
qpdf/qtest/qpdf/issue-335b.out
View File

@ -1,5 +1,5 @@
WARNING: issue-335b.pdf: can't find PDF header
WARNING: issue-335b.pdf: file is damaged
WARNING: issue-335b.pdf (xref table, offset 23): invalid xref entry (obj=6)
WARNING: issue-335b.pdf (xref table, offset 11): xref table subsection header contains impossibly large entry
WARNING: issue-335b.pdf: Attempting to reconstruct cross-reference table
qpdf: issue-335b.pdf: unable to find trailer dictionary while recovering damaged file