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1ec1b12864
qpdf/TODO
Jay Berkenbilt 1ec1b12864 Improve idea for corrupted stream handling
2013-03-17 13:16:14 -04:00

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4.2.0
=====
* If possible, support user-pluggable stream filters. This would
enable external code to provide interpretation for filters that are
missing from qpdf.
* If possible, consider adding RLE, CCITT3, CCITT4, or any other easy
filters. (Low priority for 4.1.0.)
* If possible, support the following types of broken files:
- Files that lack %%EOF at the end but otherwise have a valid
startxref near the end
- Files that have no whitespace token after "endobj" such that
endobj collides with the start of the next object
- Files with individual corrupted streams. Just leave the streams
unfiltered after giving a warning, or maybe do something else
like applying as many of the filters as possible, etc.
QPDFWriter can have some kind of retry mechanism on streams
where filtering fails after filterable returns true.
- Files whose PDF header is malformed, perhaps with no version
number (as literally %PDF-a.b). Maybe keep track of features to
try to infer a version based on encryption formats and object
streams.
- For really hard errors like corrupted streams where there is
virtually guaranteed to be loss, maybe require an additional
option to tell qpdf that it's okay to continue and treat those
as warnings. Probably need separate options for each type of
error plus a generic tryReallyHard kind of method that enables
them all. Then the qpdf command-line tool can have a single
flag that enables all supported aggressive recovery techniques.
- See ../misc/broken-files
4.1.0
=====
* Add to documentation, and mention this documentation in
README.maintainer:
Casting policy.
The C++ code in qpdf is free of old-style casts except where
unavoidable (e.g. where the old-style cast is in a macro provided
by a third-party header file). When there is a need for a cast, it
is handled, in order of preference by rewriting the code to avoid
the need for a cast, calling const_cast, calling static_cast,
calling reinterpret_cast, or calling some combination of the above.
The casting policy explicitly prohibits casting between sizes for
no purpose other than to quiet a compiler warning when there is no
reasonable chance of a problem resulting. The reason for this
exclusion is that it takes away enabling additional compiler
warnings as a tool for making future improvements to this aspect of
the code and also damages the readability of the code. As a last
resort, a compiler-specific pragma may be used to suppress a
warning that we don't want to fix. Examples may include
suppressing warnings about the use of old-style casts in code that
is shared between C and C++ code.
There are a few significant areas where casting is common in the qpdf
sources or where casting would be required to quiet higher levels
of compiler warnings but is omitted at present:
* signed vs. unsigned char. For historical reasons, there are a
lot of places in qpdf's internals that deal with unsigned char,
which means that a lot of casting is required to interoperate
with standard library calls and std::string. In retrospect,
qpdf should have probably used signed char everywhere and just
cast to unsigned char when needed. There are reinterpret_cast
calls to go between char* and unsigned char*, and there are
static_cast calls to go between char and unsigned char. These
should always be safe.
* non-const unsigned char* used in Pipeline interface. The
pipeline interface has a write() call that uses unsigned char*
without a const qualifier. The main reason for this is to
support pipelines that make calls to third-party libraries, such
as zlib, that don't include const in their interfaces.
Unfortunately, there are many places in the code where it is
desirable to have const char* with pipelines. None of the
pipeline implementations in qpdf currently modify the data
passed to write, and doing so would be counter to the intent of
Pipeline. There are places in the code where const_cast is used
to remove the const-ness of pointers going into Pipelines. This
could be potentially unsafe, but there is adequate testing to
assert that it is safe in qpdf's code.
* size_t vs. qpdf_offset_t. This is pretty much unavoidable since
offsets are signed types and sizes are unsigned types. Whenever
it is necessary to seek by an amount given by a size_t, it
becomes necessary to mix and match between size_t and
qpdf_offset_t. Additionally, qpdf sometimes treats memory
buffers like files, and those seek interfaces have to be
consistent with file-based input sources. Neither gcc nor MSVC
give warnings for this case by default, but both have warning
flags that can enable this. (MSVC: /W14267 or /W3 (which also
enables some additional warnings that we ignore); gcc:
-Wconversion -Wsign-conversion). This could matter for files
whose sizes are larger than 2^63 bytes, but it is reasonable to
expect that a world where such files are common would also have
larger size_t and qpdf_offset_t types in it. I am not aware of
any cases where 32-bit systems that have size_t smaller than
qpdf_offset_t could run into problems, though I can't
conclusively rule out the possibility. In the event that
someone should produce a file that qpdf can't handle because of
what is suspected to be issues involving the handling of size_t
vs. qpdf_offset_t (such files may behave properly on 64-bit
systems but not on 32-bit systems and may have very large
embedded files or streams, for example), the above mentioned
warning flags could be enabled and all those implicit
conversions could be carefully scrutinized. (I have already
gone through that exercise once in adding support for files >
4GB in size.) I continue to be commited to supporting large
files on 32-bit systems, but I would not go to any lengths to
support corner cases involving large embedded files or large
streams that work on 64-bit systems but not on 32-bit systems
because of size_t being too small. It is reasonable to assume
that anyone working with such files would be using a 64-bit
system anyway.
* size_t vs. int. There are some cases where size_t and int or
size_t and unsigned int are used interchangeably. These cases
occur when working with very small amounts of memory, such as
with the bit readers (where we're working with just a few bytes
at a time), some cases of strlen, and a few other cases. I have
scrutinized all of these cases and determined them to be safe,
but there is no mechanism in the code to ensure that new unsafe
conversions between int and size_t aren't introduced short of
good testing and strong awareness of the issues. Again, if any
such bugs are suspected in the future, enable the additional
warning flags and scrutinizing the warnings would be in order.
* New public interfaces have been added.
General
=======
* Consider providing a Windows installer for qpdf using NSIS.
* Improve the random number seed to make it more secure so that we
have stronger random numbers, particularly when multiple files are
generated in the same second. This code may need to be
OS-specific. Probably we should add a method in QUtil to seed with
a strong random number and call this automatically the first time
QUtil::random() is called.
* Study what's required to support savable forms that can be saved by
Adobe Reader. Does this require actually signing the document with
an Adobe private key? Search for "Digital signatures" in the PDF
spec, and look at ~/Q/pdf-collection/form-with-full-save.pdf, which
came from Adobe's example site.
* Consider the possibility of doing something locale-aware to support
non-ASCII passwords. Update documentation if this is done.
Consider implementing full Unicode password algorithms from newer
encryption formats.
* Consider impact of article threads on page splitting/merging.
Subramanyam provided a test file; see ../misc/article-threads.pdf.
Email Q-Count: 431864 from 2009-11-03. Other things to consider:
outlines, page labels, thumbnails, zones. There are probably
others.
* See if we can avoid preserving unreferenced objects in object
streams even when preserving the object streams.
* For debugging linearization bugs, consider adding an option to save
pass 1 of linearization. This code is sufficient. Change the
interface to allow specification of a pass1 file, which would
change the behavior as in this patch.
------------------------------
Index: QPDFWriter.cc
===================================================================
--- QPDFWriter.cc (revision 932)
+++ QPDFWriter.cc (working copy)
@@ -1965,11 +1965,15 @@
// Write file in two passes. Part numbers refer to PDF spec 1.4.
+ FILE* XXX = 0;
for (int pass = 1; pass <= 2; ++pass)
{
if (pass == 1)
{
- pushDiscardFilter();
+// pushDiscardFilter();
+ XXX = QUtil::safe_fopen("/tmp/pass1.pdf", "w");
+ pushPipeline(new Pl_StdioFile("pass1", XXX));
+ activatePipelineStack();
}
// Part 1: header
@@ -2204,6 +2208,8 @@
// Restore hint offset
this->xref[hint_id] = QPDFXRefEntry(1, hint_offset, 0);
+ fclose(XXX);
+ XXX = 0;
}
}
}
------------------------------
* Provide APIs for embedded files. See *attachments*.pdf in test
suite. The private method findAttachmentStreams finds at least
cases for modern versions of Adobe Reader (>= 1.7, maybe earlier).
PDF Reference 1.7 section 3.10, "File Specifications", discusses
this.
A sourceforge user asks if qpdf can handle extracting and embedded
resources and references these tools, which may be useful as a
reference.
http://multivalent.sourceforge.net/Tools/pdf/Extract.html
http://multivalent.sourceforge.net/Tools/pdf/Embed.html
* The description of Crypt filters is unclear with respect to how to
use them to override /StmF for specific streams. I'm not sure
whether qpdf will do the right thing for any specific individual
streams that might have crypt filters, but I believe it does based
on my testing of a limited subset. The specification seems to imply
that only embedded file streams and metadata streams can have crypt
filters, and there are already special cases in the code to handle
those. Most likely, it won't be a problem, but someday someone may
find a file that qpdf doesn't work on because of crypt filters.
There is an example in the spec of using a crypt filter on a
metadata stream.
For now, we notice /Crypt filters and decode parameters consistent
with the example in the PDF specification, and the right thing
happens for metadata filters that happen to be uncompressed or
otherwise compressed in a way we can filter. This should handle
all normal cases, but it's more or less just a guess since I don't
have any test files that actually use stream-specific crypt filters
in them.
* The second xref stream for linearized files has to be padded only
because we need file_size as computed in pass 1 to be accurate. If
we were not allowing writing to a pipe, we could seek back to the
beginning and fill in the value of /L in the linearization
dictionary as an optimization to alleviate the need for this
padding. Doing so would require us to pad the /L value
individually and also to save the file descriptor and determine
whether it's seekable. This is probably not worth bothering with.
* The whole xref handling code in the QPDF object allows the same
object with more than one generation to coexist, but a lot of logic
assumes this isn't the case. Anything that creates mappings only
with the object number and not the generation is this way,
including most of the interaction between QPDFWriter and QPDF. If
we wanted to allow the same object with more than one generation to
coexist, which I'm not sure is allowed, we could fix this by
changing xref_table. Alternatively, we could detect and disallow
that case. In fact, it appears that Adobe reader and other PDF
viewing software silently ignores objects of this type, so this is
probably not a big deal.
* Pl_PNGFilter is only partially implemented. If we ever decoded
images, we'd have to finish implementing it along with the other
filter decode parameters and types. For just handling xref
streams, there's really no need as it wouldn't make sense to use
any kind of predictor other than 12 (PNG UP filter).
* If we ever want to have check mode check the integrity of the free
list, this can be done by looking at the code from prior to the
object stream support of 4/5/2008. It's in an if (0) block and
there's a comment about it. There's also something about it in
qpdf.test -- search for "free table". On the other hand, the value
of doing this seems very low since no viewer seems to care, so it's
probably not worth it.
* QPDFObjectHandle::getPageImages() doesn't notice images in
inherited resource dictionaries. See comments in that function.
* Based on an idea suggested by user "Atom Smasher", consider
providing some mechanism to recover earlier versions of a file
embedded prior to appended sections.
* From a suggestion in bug 3152169, consider having an option to
re-encode inline images with an ASCII encoding.
* From github issue 2, provide more in-depth output for examining
hint stream contents.
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