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Code Issues Releases Wiki Activity

Correctly handle reuse of xref stream (fixes #809)

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This commit is contained in:
Jay Berkenbilt 2022-11-19 16:01:54 -05:00 committed by Jay Berkenbilt
parent d79a823d66
commit e9980efec8
9 changed files with 92 additions and 12 deletions
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6
ChangeLog
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@ -1,3 +1,9 @@
2022-11-19 Jay Berkenbilt <ejb@ql.org>
* Bug fix: handle special case of an earlier xref stream object's
object number being reused by an update made by appending the
file. Fixes #809.
2022-10-08 Jay Berkenbilt <ejb@ql.org>
* Fix major performance bug with the openssl crypto provider when

3
include/qpdf/QPDF.hh
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@ -1209,7 +1209,8 @@ class QPDF
qpdf_offset_t offset,
std::string const& description,
QPDFObjGen const& exp_og,
QPDFObjGen& og);
QPDFObjGen& og,
bool skip_cache_if_in_xref);
void resolve(QPDFObjGen const& og);
void resolveObjectsInStream(int obj_stream_number);
void stopOnError(std::string const& message);

63
libqpdf/QPDF.cc
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@ -999,7 +999,12 @@ QPDF::read_xrefStream(qpdf_offset_t xref_offset)
QPDFObjectHandle xref_obj;
try {
xref_obj = readObjectAtOffset(
false, xref_offset, "xref stream", QPDFObjGen(0, 0), x_og);
false,
xref_offset,
"xref stream",
QPDFObjGen(0, 0),
x_og,
true);
} catch (QPDFExc&) {
// ignore -- report error below
}
@ -1638,7 +1643,8 @@ QPDF::readObjectAtOffset(
qpdf_offset_t offset,
std::string const& description,
QPDFObjGen const& exp_og,
QPDFObjGen& og)
QPDFObjGen& og,
bool skip_cache_if_in_xref)
{
bool check_og = true;
if (exp_og.getObj() == 0) {
@ -1718,7 +1724,7 @@ QPDF::readObjectAtOffset(
qpdf_offset_t new_offset =
this->m->xref_table[exp_og].getOffset();
QPDFObjectHandle result = readObjectAtOffset(
false, new_offset, description, exp_og, og);
false, new_offset, description, exp_og, og, false);
QTC::TC("qpdf", "QPDF recovered in readObjectAtOffset");
return result;
} else {
@ -1770,11 +1776,50 @@ QPDF::readObjectAtOffset(
}
}
qpdf_offset_t end_after_space = this->m->file->tell();
updateCache(
og,
QPDFObjectHandle::ObjAccessor::getObject(oh),
end_before_space,
end_after_space);
if (skip_cache_if_in_xref && this->m->xref_table.count(og)) {
// Ordinarily, an object gets read here when resolved
// through xref table or stream. In the special case of
// the xref stream and linearization hint tables, the
// offset comes from another source. For the specific case
// of xref streams, the xref stream is read and loaded
// into the object cache very early in parsing.
// Ordinarily, when a file is updated by appending, items
// inserted into the xref table in later updates take
// precedence over earlier items. In the special case of
// reusing the object number previously used as the xref
// stream, we have the following order of events:
//
// * reused object gets loaded into the xref table
// * old object is read here while reading xref streams
// * original xref entry is ignored (since already in xref table)
//
// It is the second step that causes a problem. Even
// though the xref table is correct in this case, the old
// object is already in the cache and so effectively
// prevails over the reused object. To work around this
// issue, we have a special case for the xref stream (via
// the skip_cache_if_in_xref): if the object is already in
// the xref stream, don't cache what we read here.
//
// It is likely that the same bug may exist for
// linearization hint tables, but the existing code uses
// end_before_space and end_after_space from the cache, so
// fixing that would require more significant rework. The
// chances of a linearization hint stream being reused
// seems smaller because the xref stream is probably the
// highest object in the file and the linearization hint
// stream would be some random place in the middle, so I'm
// leaving that bug unfixed for now. If the bug were to be
// fixed, we could use !check_og in place of
// skip_cache_if_in_xref.
QTC::TC("qpdf", "QPDF skipping cache for known unchecked object");
} else {
updateCache(
og,
QPDFObjectHandle::ObjAccessor::getObject(oh),
end_before_space,
end_after_space);
}
}
return oh;
@ -1809,7 +1854,7 @@ QPDF::resolve(QPDFObjGen const& og)
// Object stored in cache by readObjectAtOffset
QPDFObjGen a_og;
QPDFObjectHandle oh =
readObjectAtOffset(true, offset, "", og, a_og);
readObjectAtOffset(true, offset, "", og, a_og, false);
}
break;

7
libqpdf/QPDF_linearization.cc
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@ -303,7 +303,12 @@ QPDF::readHintStream(Pipeline& pl, qpdf_offset_t offset, size_t length)
{
QPDFObjGen og;
QPDFObjectHandle H = readObjectAtOffset(
false, offset, "linearization hint stream", QPDFObjGen(0, 0), og);
false,
offset,
"linearization hint stream",
QPDFObjGen(0, 0),
og,
false);
ObjCache& oc = this->m->obj_cache[og];
qpdf_offset_t min_end_offset = oc.end_before_space;
qpdf_offset_t max_end_offset = oc.end_after_space;

1
qpdf/qpdf.testcov
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@ -678,3 +678,4 @@ QPDF_json bad calledgetallpages 0
QPDF_json bad pushedinheritedpageresources 0
QPDFPageObjectHelper copied fallback 0
QPDFPageObjectHelper used fallback without copying 0
QPDF skipping cache for known unchecked object 0

1
qpdf/qtest/qpdf/reuse-xref-stream-obj9.out Normal file
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@ -0,0 +1 @@
<< /BaseFont /Times-Italics /Encoding /WinAnsiEncoding /Subtype /Type1 /Type /Font >>

10
qpdf/qtest/qpdf/reuse-xref-stream-xref.out Normal file
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@ -0,0 +1,10 @@
1/0: uncompressed; offset = 25
2/0: compressed; stream = 1, index = 0
3/0: compressed; stream = 1, index = 1
4/0: compressed; stream = 1, index = 2
5/0: compressed; stream = 1, index = 3
6/0: uncompressed; offset = 1054
7/0: uncompressed; offset = 692
8/0: uncompressed; offset = 791
9/0: uncompressed; offset = 1087
10/0: uncompressed; offset = 1196

BIN
qpdf/qtest/qpdf/reuse-xref-stream.pdf Normal file
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Binary file not shown.

13
qpdf/qtest/xref-streams.test
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@ -14,7 +14,7 @@ cleanup();
my $td = new TestDriver('xref-streams');
my $n_tests = 3;
my $n_tests = 5;
# Handle xref stream with more entries than reported (bug 2872265)
$td->runtest("xref with short size",
@ -32,6 +32,17 @@ $td->runtest("show new xref stream",
{$td->FILE => "xref-with-short-size-new.out",
$td->EXIT_STATUS => 0},
$td->NORMALIZE_NEWLINES);
# Handle appended file that reuses the xref stream object number
$td->runtest("override xref stream - xref",
{$td->COMMAND => "qpdf --show-xref reuse-xref-stream.pdf"},
{$td->FILE => "reuse-xref-stream-xref.out",
$td->EXIT_STATUS => 0},
$td->NORMALIZE_NEWLINES);
$td->runtest("override xref stream - object",
{$td->COMMAND => "qpdf --show-object=9 reuse-xref-stream.pdf"},
{$td->FILE => "reuse-xref-stream-obj9.out",
$td->EXIT_STATUS => 0},
$td->NORMALIZE_NEWLINES);
cleanup();
$td->report($n_tests);