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Update casting policy in the documentation

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Jay Berkenbilt 2019-06-21 08:53:08 -04:00
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2 changed files with 59 additions and 130 deletions
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README-maintainer
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@ -119,7 +119,7 @@ RELEASE PREPARATION
* If any interfaces were added or changed, check C API to see whether
changes are appropriate there as well. If necessary, review the
casting policy in the manual, and ensure that integer types are
properly handled.
properly handled with QIntC or the appropriate cast.
* Increment shared library version information as needed (`LT_*` in
`configure.ac`)

187
manual/qpdf-manual.xml
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@ -3340,139 +3340,68 @@ outfile.pdf</option>
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.
The <classname>QIntC</classname> namespace, provided by
<filename>include/qpdf/QIntC.hh</filename>, implements safe
functions for converting between integer types. These functions do
range checking and throw a <type>std::range_error</type>, which is
subclass of <type>std::runtime_error</type>, if conversion from one
integer type to another results in loss of information. There are
many cases in which we have to move between different integer
types because of incompatible integer types used in interoperable
interfaces. Some are unavoidable, such as moving between sizes and
offsets, and others are there because of old code that is too in
entrenched to be fixable without breaking source compatibility and
causing pain for users. QPDF is compiled with extra warnings to
detect conversions with potential data loss, and all such cases
should be fixed by either using a function from
<classname>QIntC</classname> or a
<function>static_cast</function>.
</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 committed 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>
When the intention is just to switch the type because of
exchanging data between incompatible interfaces, use
<classname>QIntC</classname>. This is the usual case. However,
there are some cases in which we are explicitly intending to use
the exact same bit pattern with a different type. This is most
common when switching between signed and unsigned characters. A
lot of qpdf's code uses unsigned characters internally, but
<type>std::string</type> and <type>char</type> are signed. Using
<function>QIntC::to_char</function> would be wrong for converting
from unsigned to signed characters because a negative
<type>char</type> value and the corresponding <type>unsigned
char</type> value greater than 127 <emphasis>mean the same
thing</emphasis>. There are also cases in which we use
<function>static_cast</function> when working with bit fields
where we are not representing a numerical value but rather a bunch
of bits packed together in some integer type. Also note that
<type>size_t</type> and <type>long</type> both typically differ
between 32-bit and 64-bit environments, so sometimes an explicit
cast may not be needed to avoid warnings on one platform but may
be needed on another. A conversion with
<classname>QIntC</classname> should always be used when the types
are different even if the underlying size is the same. QPDF's CI
build builds on 32-bit and 64-bit platforms, and the test suite is
very thorough, so it is hard to make any of the potential errors
here without being caught in build or test.
</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.
Non-const <type>unsigned char*</type> is 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>
</sect1>
<sect1 id="ref.encryption">