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443 lines
15 KiB
Text
443 lines
15 KiB
Text
<chapter id="implementation">
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<title>Low-level Implementation</title>
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<para>Details of Wine's Low-level Implementation...</para>
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<sect1 id="undoc-func">
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<title>Undocumented APIs</title>
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<para>
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Some background: On the i386 class of machines, stack entries are
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usually dword (4 bytes) in size, little-endian. The stack grows
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downward in memory. The stack pointer, maintained in the
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<literal>esp</literal> register, points to the last valid entry;
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thus, the operation of pushing a value onto the stack involves
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decrementing <literal>esp</literal> and then moving the value into
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the memory pointed to by <literal>esp</literal>
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(i.e., <literal>push p</literal> in assembly resembles
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<literal>*(--esp) = p;</literal> in C). Removing (popping)
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values off the stack is the reverse (i.e., <literal>pop p</literal>
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corresponds to <literal>p = *(esp++);</literal> in C).
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</para>
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<para>
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In the <literal>stdcall</literal> calling convention, arguments are
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pushed onto the stack right-to-left. For example, the C call
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<function>myfunction(40, 20, 70, 30);</function> is expressed in
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Intel assembly as:
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<screen>
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push 30
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push 70
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push 20
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push 40
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call myfunction
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</screen>
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The called function is responsible for removing the arguments
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off the stack. Thus, before the call to myfunction, the
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stack would look like:
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<screen>
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[local variable or temporary]
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[local variable or temporary]
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30
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70
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20
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esp -> 40
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</screen>
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After the call returns, it should look like:
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<screen>
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[local variable or temporary]
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esp -> [local variable or temporary]
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</screen>
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</para>
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<para>
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To restore the stack to this state, the called function must know how
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many arguments to remove (which is the number of arguments it takes).
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This is a problem if the function is undocumented.
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</para>
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<para>
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One way to attempt to document the number of arguments each function
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takes is to create a wrapper around that function that detects the
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stack offset. Essentially, each wrapper assumes that the function will
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take a large number of arguments. The wrapper copies each of these
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arguments into its stack, calls the actual function, and then calculates
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the number of arguments by checking esp before and after the call.
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</para>
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<para>
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The main problem with this scheme is that the function must actually
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be called from another program. Many of these functions are seldom
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used. An attempt was made to aggressively query each function in a
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given library (<filename>ntdll.dll</filename>) by passing 64 arguments,
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all 0, to each function. Unfortunately, Windows NT quickly goes to a
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blue screen of death, even if the program is run from a
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non-administrator account.
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</para>
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<para>
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Another method that has been much more successful is to attempt to
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figure out how many arguments each function is removing from the
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stack. This instruction, <literal>ret hhll</literal> (where
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<symbol>hhll</symbol> is the number of bytes to remove, i.e. the
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number of arguments times 4), contains the bytes
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<literal>0xc2 ll hh</literal> in memory. It is a reasonable
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assumption that few, if any, functions take more than 16 arguments;
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therefore, simply searching for
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<literal>hh == 0 && ll < 0x40</literal> starting from the
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address of a function yields the correct number of arguments most
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of the time.
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</para>
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<para>
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Of course, this is not without errors. <literal>ret 00ll</literal>
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is not the only instruction that can have the byte sequence
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<literal>0xc2 ll 0x0</literal>; for example,
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<literal>push 0x000040c2</literal> has the byte sequence
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<literal>0x68 0xc2 0x40 0x0 0x0</literal>, which matches
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the above. Properly, the utility should look for this sequence
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only on an instruction boundary; unfortunately, finding
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instruction boundaries on an i386 requires implementing a full
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disassembler -- quite a daunting task. Besides, the probability
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of having such a byte sequence that is not the actual return
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instruction is fairly low.
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</para>
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<para>
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Much more troublesome is the non-linear flow of a function. For
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example, consider the following two functions:
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<screen>
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somefunction1:
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jmp somefunction1_impl
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somefunction2:
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ret 0004
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somefunction1_impl:
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ret 0008
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</screen>
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In this case, we would incorrectly detect both
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<function>somefunction1</function> and
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<function>somefunction2</function> as taking only a single
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argument, whereas <function>somefunction1</function> really
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takes two arguments.
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</para>
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<para>
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With these limitations in mind, it is possible to implement more stubs
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in Wine and, eventually, the functions themselves.
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</para>
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</sect1>
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<sect1 id="accel-impl">
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<title>Accelerators</title>
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<para>
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There are <emphasis>three</emphasis> differently sized
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accelerator structures exposed to the user:
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</para>
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<orderedlist>
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<listitem>
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<para>
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Accelerators in NE resources. This is also the internal
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layout of the global handle <type>HACCEL</type> (16 and
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32) in Windows 95 and Wine. Exposed to the user as Win16
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global handles <type>HACCEL16</type> and
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<type>HACCEL32</type> by the Win16/Win32 API.
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These are 5 bytes long, with no padding:
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<programlisting>
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BYTE fVirt;
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WORD key;
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WORD cmd;
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</programlisting>
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</para>
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</listitem>
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<listitem>
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<para>
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Accelerators in PE resources. They are exposed to the user
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only by direct accessing PE resources.
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These have a size of 8 bytes:
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</para>
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<programlisting>
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BYTE fVirt;
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BYTE pad0;
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WORD key;
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WORD cmd;
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WORD pad1;
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</programlisting>
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</listitem>
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<listitem>
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<para>
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Accelerators in the Win32 API. These are exposed to the
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user by the <function>CopyAcceleratorTable</function>
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and <function>CreateAcceleratorTable</function> functions
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in the Win32 API.
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These have a size of 6 bytes:
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</para>
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<programlisting>
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BYTE fVirt;
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BYTE pad0;
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WORD key;
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WORD cmd;
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</programlisting>
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</listitem>
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</orderedlist>
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<para>
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Why two types of accelerators in the Win32 API? We can only
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guess, but my best bet is that the Win32 resource compiler
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can/does not handle struct packing. Win32 <type>ACCEL</type>
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is defined using <function>#pragma(2)</function> for the
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compiler but without any packing for RC, so it will assume
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<function>#pragma(4)</function>.
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</para>
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</sect1>
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<sect1 id="hardware-trace">
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<title>Doing A Hardware Trace</title>
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<para>
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The primary reason to do this is to reverse engineer a
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hardware device for which you don't have documentation, but
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can get to work under Wine.
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</para>
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<para>
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This lot is aimed at parallel port devices, and in particular
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parallel port scanners which are now so cheap they are
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virtually being given away. The problem is that few
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manufactures will release any programming information which
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prevents drivers being written for Sane, and the traditional
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technique of using DOSemu to produce the traces does not work
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as the scanners invariably only have drivers for Windows.
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</para>
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<para>
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Presuming that you have compiled and installed wine the first
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thing to do is is to enable direct hardware access to your
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parallel port. To do this edit <filename>config</filename>
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(usually in <filename>~/.wine/</filename>) and in the
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ports section add the following two lines
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</para>
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<programlisting>
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read=0x378,0x379,0x37a,0x37c,0x77a
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write=0x378,x379,0x37a,0x37c,0x77a
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</programlisting>
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<para>
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This adds the necessary access required for SPP/PS2/EPP/ECP
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parallel port on LPT1. You will need to adjust these number
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accordingly if your parallel port is on LPT2 or LPT0.
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</para>
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<para>
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When starting wine use the following command line, where
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<literal>XXXX</literal> is the program you need to run in
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order to access your scanner, and <literal>YYYY</literal> is
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the file your trace will be stored in:
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</para>
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<programlisting>
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wine -debugmsg +io XXXX 2> >(sed 's/^[^:]*:io:[^ ]* //' > YYYY)
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</programlisting>
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<para>
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You will need large amounts of hard disk space (read hundreds
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of megabytes if you do a full page scan), and for reasonable
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performance a really fast processor and lots of RAM.
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</para>
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<para>
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You will need to postprocess the output into a more manageable
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format, using the <command>shrink</command> program. First
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you need to compile the source (which is located at the end of
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this section):
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<programlisting>
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cc shrink.c -o shrink
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</programlisting>
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</para>
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<para>
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Use the <command>shrink</command> program to reduce the
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physical size of the raw log as follows:
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</para>
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<programlisting>
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cat log | shrink > log2
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</programlisting>
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<para>
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The trace has the basic form of
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</para>
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<programlisting>
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XXXX > YY @ ZZZZ:ZZZZ
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</programlisting>
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<para>
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where <literal>XXXX</literal> is the port in hexidecimal being
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accessed, <literal>YY</literal> is the data written (or read)
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from the port, and <literal>ZZZZ:ZZZZ</literal> is the address
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in memory of the instruction that accessed the port. The
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direction of the arrow indicates whether the data was written
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or read from the port.
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</para>
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<programlisting>
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> data was written to the port
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< data was read from the port
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</programlisting>
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<para>
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My basic tip for interpreting these logs is to pay close
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attention to the addresses of the IO instructions. Their
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grouping and sometimes proximity should reveal the presence of
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subroutines in the driver. By studying the different versions
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you should be able to work them out. For example consider the
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following section of trace from my UMAX Astra 600P
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</para>
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<programlisting>
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0x378 > 55 @ 0297:01ec
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0x37a > 05 @ 0297:01f5
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0x379 < 8f @ 0297:01fa
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0x37a > 04 @ 0297:0211
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0x378 > aa @ 0297:01ec
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0x37a > 05 @ 0297:01f5
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0x379 < 8f @ 0297:01fa
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0x37a > 04 @ 0297:0211
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0x378 > 00 @ 0297:01ec
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0x37a > 05 @ 0297:01f5
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0x379 < 8f @ 0297:01fa
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0x37a > 04 @ 0297:0211
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0x378 > 00 @ 0297:01ec
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0x37a > 05 @ 0297:01f5
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0x379 < 8f @ 0297:01fa
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0x37a > 04 @ 0297:0211
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0x378 > 00 @ 0297:01ec
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0x37a > 05 @ 0297:01f5
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0x379 < 8f @ 0297:01fa
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0x37a > 04 @ 0297:0211
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0x378 > 00 @ 0297:01ec
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0x37a > 05 @ 0297:01f5
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0x379 < 8f @ 0297:01fa
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0x37a > 04 @ 0297:0211
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</programlisting>
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<para>
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As you can see there is a repeating structure starting at
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address <literal>0297:01ec</literal> that consists of four io
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accesses on the parallel port. Looking at it the first io
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access writes a changing byte to the data port the second
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always writes the byte <literal>0x05</literal> to the control
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port, then a value which always seems to
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<literal>0x8f</literal> is read from the status port at which
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point a byte <literal>0x04</literal> is written to the control
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port. By studying this and other sections of the trace we can
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write a C routine that emulates this, shown below with some
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macros to make reading/writing on the parallel port easier to
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read.
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</para>
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<programlisting>
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#define r_dtr(x) inb(x)
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#define r_str(x) inb(x+1)
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#define r_ctr(x) inb(x+2)
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#define w_dtr(x,y) outb(y, x)
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#define w_str(x,y) outb(y, x+1)
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#define w_ctr(x,y) outb(y, x+2)
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/* Seems to be sending a command byte to the scanner */
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int udpp_put(int udpp_base, unsigned char command)
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{
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int loop, value;
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w_dtr(udpp_base, command);
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w_ctr(udpp_base, 0x05);
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for (loop=0; loop < 10; loop++)
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if ((value = r_str(udpp_base)) & 0x80)
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{
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w_ctr(udpp_base, 0x04);
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return value & 0xf8;
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}
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return (value & 0xf8) | 0x01;
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}
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</programlisting>
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<para>
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For the UMAX Astra 600P only seven such routines exist (well
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14 really, seven for SPP and seven for EPP). Whether you
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choose to disassemble the driver at this point to verify the
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routines is your own choice. If you do, the address from the
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trace should help in locating them in the disassembly.
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</para>
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<para>
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You will probably then find it useful to write a script/perl/C
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program to analyse the logfile and decode them futher as this
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can reveal higher level grouping of the low level routines.
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For example from the logs from my UMAX Astra 600P when decoded
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further reveal (this is a small snippet)
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</para>
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<programlisting>
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start:
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put: 55 8f
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put: aa 8f
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put: 00 8f
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put: 00 8f
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put: 00 8f
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put: c2 8f
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wait: ff
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get: af,87
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wait: ff
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get: af,87
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end: cc
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start:
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put: 55 8f
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put: aa 8f
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put: 00 8f
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put: 03 8f
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put: 05 8f
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put: 84 8f
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wait: ff
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</programlisting>
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<para>
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From this it is easy to see that <varname>put</varname>
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routine is often grouped together in five successive calls
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sending information to the scanner. Once these are understood
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it should be possible to process the logs further to show the
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higher level routines in an easy to see format. Once the
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highest level format that you can derive from this process is
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understood, you then need to produce a series of scans varying
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only one parameter between them, so you can discover how to
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set the various parameters for the scanner.
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</para>
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<para>
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The following is the <filename>shrink.c</filename> program:
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<programlisting>
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/* Copyright David Campbell <campbell@torque.net> */
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#include <stdio.h>
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#include <string.h>
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void
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main (void)
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{
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char buff[256], lastline[256];
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int count;
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count = 0;
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lastline[0] = 0;
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while (!feof (stdin))
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{
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fgets (buff, sizeof (buff), stdin);
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if (strcmp (buff, lastline) == 0)
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{
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count++;
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}
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else
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{
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if (count > 1)
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fprintf (stdout, "# Last line repeated %i times #\n", count);
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fprintf (stdout, "%s", buff);
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strcpy (lastline, buff);
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count = 1;
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}
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}
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}
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</programlisting>
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</para>
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</sect1>
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</chapter>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-parent-document:("wine-devel.sgml" "set" "book" "part" "chapter" "")
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End:
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-->
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