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Document casting policy

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Jay Berkenbilt 2013-03-25 14:37:25 -04:00
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ChangeLog
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@ -1,3 +1,8 @@
2013-03-25 Jay Berkenbilt <ejb@ql.org>
* manual/qpdf-manual.xml: Document the casting policy that is
followed in qpdf's implementation.
2013-03-11 Jay Berkenbilt <ejb@ql.org>
* When creating Windows binary distributions, make sure to only

4
README.maintainer
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@ -31,7 +31,9 @@ Release Reminders
* Check all open issues in the sourceforge trackers and on github.
* If any interfaces were added or changed, check C API to see whether
changes are appropriate there as well.
changes are appropriate there as well. If necessary, review the
casting policy in the manual, and ensure that integer types are
properly handled.
* Increment shared library version information as needed
(libqpdf/build.mk)

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TODO
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@ -1,3 +1,9 @@
4.1.0
=====
* New public interfaces have been added.
4.2.0
=====
@ -38,107 +44,6 @@
- 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
=======

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manual/qpdf-manual.xml
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@ -1623,6 +1623,166 @@ outfile.pdf</option>
</itemizedlist>
</para>
</sect1>
<sect1 id="ref.casting">
<title>Casting Policy</title>
<para>
This section describes the casting policy followed by qpdf's
implementation. This is no concern to qpdf's end users and
largely of no concern to people writing code that uses qpdf, but
it could be of interest to people who are porting qpdf to a new
platform or who are making modifications to the code.
</para>
<para>
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
<function>const_cast</function>, calling
<function>static_cast</function>, calling
<function>reinterpret_cast</function>, or calling some combination
of the above. As a last resort, a compiler-specific
<literal>#pragma</literal> 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.
</para>
<para>
The casting policy explicitly prohibits casting between integer
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 the practice of adding these additional
casts precludes future use of additional compiler warnings as a
tool for making future improvements to this aspect of the code,
and it also damages the readability of the code.
</para>
<para>
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:
<itemizedlist>
<listitem>
<para>
<type>char</type> vs. <type>unsigned char</type>. For
historical reasons, there are a lot of places in qpdf's
internals that deal with <type>unsigned char</type>, which
means that a lot of casting is required to interoperate with
standard library calls and <type>std::string</type>. In
retrospect, qpdf should have probably used regular (signed)
<type>char</type> and <type>char*</type> everywhere and just
cast to <type>unsigned char</type> when needed, but it's too
late to make that change now. There are
<function>reinterpret_cast</function> calls to go between
<type>char*</type> and <type>unsigned char*</type>, and there
are <function>static_cast</function> calls to go between
<type>char</type> and <type>unsigned char</type>. These should
always be safe.
</para>
</listitem>
<listitem>
<para>
Non-const <type>unsigned char*</type> used in the
<type>Pipeline</type> interface. The pipeline interface has a
<function>write</function> call that uses <type>unsigned
char*</type> without a <type>const</type> qualifier. The main
reason for this is to support pipelines that make calls to
third-party libraries, such as zlib, that don't include
<type>const</type> in their interfaces. Unfortunately, there
are many places in the code where it is desirable to have
<type>const char*</type> 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
<type>Pipeline</type>, but there is nothing in the code to
prevent this from being done. There are places in the code
where <function>const_cast</function> is used to remove the
const-ness of pointers going into <type>Pipeline</type>s. This
could theoretically be unsafe, but there is adequate testing to
assert that it is safe and will remain safe in qpdf's code.
</para>
</listitem>
<listitem>
<para>
<type>size_t</type> vs. <type>qpdf_offset_t</type>. This is
pretty much unavoidable since sizes are unsigned types and
offsets are signed types. Whenever it is necessary to seek by
an amount given by a <type>size_t</type>, it becomes necessary
to mix and match between <type>size_t</type> and
<type>qpdf_offset_t</type>. Additionally, qpdf sometimes
treats memory buffers like files (as with
<type>BufferInputSource</type>, 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:
<option>/W14267</option> or <option>/W3</option>, which also
enables some additional warnings that we ignore; gcc:
<option>-Wconversion -Wsign-conversion</option>). This could
matter for files whose sizes are larger than
2<superscript>63</superscript> bytes, but it is reasonable to
expect that a world where such files are common would also have
larger <type>size_t</type> and <type>qpdf_offset_t</type> types
in it. On most 64-bit systems at the time of this writing (the
release of version 4.1.0 of qpdf), both <type>size_t</type> and
<type>qpdf_offset_t</type> are 64-bit integer types, while on
many current 32-bit systems, <type>size_t</type> is a 32-bit
type while <type>qpdf_offset_t</type> is a 64-bit type. I am
not aware of any cases where 32-bit systems that have
<type>size_t</type> smaller than <type>qpdf_offset_t</type>
could run into problems. Although I can't conclusively rule
out the possibility of such problems existing, I suspect any
cases would be pretty contrived. 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
<type>size_t</type> vs. <type>qpdf_offset_t</type> (such files
may behave properly on 64-bit systems but not on 32-bit systems
because they 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 larger than 4&nbsp;GB 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
<type>size_t</type> being too small. It is reasonable to
assume that anyone working with such files would be using a
64-bit system anyway since many 32-bit applications would have
similar difficulties.
</para>
</listitem>
<listitem>
<para>
<type>size_t</type> vs. <type>int</type> or <type>long</type>.
There are some cases where <type>size_t</type> and
<type>int</type> or <type>long</type> or <type>size_t</type>
and <type>unsigned int</type> or <type>unsigned long</type> 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
<function>strlen</function>, 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 <type>int</type> and <type>size_t</type>
aren't introduced short of good testing and strong awareness of
the issues. Again, if any such bugs are suspected in the
future, enabling the additional warning flags and scrutinizing
the warnings would be in order.
</para>
</listitem>
</itemizedlist>
</para>
<para>
To be clear, I believe qpdf to be well-behaved with respect to
sizes and offsets, and qpdf's test suite includes actual
generation and full processing of files larger than 4&nbsp;GB in
size. The issues raised here are largely academic and should not
in any way be interpreted to mean that qpdf has practical problems
involving sloppiness with integer types. I also believe that
appropriate measures have been taken in the code to avoid problems
with signed vs. unsigned integers from resulting in memory
overwrites or other issues with potential security implications,
though there are never any absolute guarantees.
</para>
</sect1>
<sect1 id="ref.encryption">
<title>Encryption</title>
<para>