patterns: Pattern for DOS EXE files (#452)

* Initial DOS file

* Update README.md

* Update README.md

* More README

* More README

* Add DOS EXE

From: https://clasqm.github.io/freedos-repo/Games.html

Name: Champ Galagon

README.md

@@ -63,6 +63,7 @@ Everything will immediately show up in ImHex's Content Store and gets bundled wi
 | DICOM | `application/dicom` | [`patterns/dicom.hexpat`](patterns/dicom.hexpat) | DICOM image format |
 | DMG | | [`patterns/dmg.hexpat`](patterns/dmg.hexpat) | Apple Disk Image Trailer (DMG) |
 | DMP | | [`patterns/dmp64.hexpat`](patterns/dmp64.hexpat) | Windows Kernel Dump(DMP64) |
+| DOS | `application/x-dosexec` |  [`patterns/dos.hexpat`](patterns/dos.hexpat) | 16-bit real mode DOS EXE files |
 | DOTNET_BinaryFormatter | | [`patterns/dotnet_binaryformatter.hexpat`](patterns/dotnet_binaryformatter.hexpat) | .NET BinaryFormatter |
 | DPAPI_Blob | | [`patterns/dpapblob.hexpat`](patterns/dpapiblob.hexpat) | Data protection API Blob File Format |
 | DPAPI_MasterKey | | [`patterns/dpapimasterkey.hexpat`](patterns/dpapimasterkey.hexpat) | Data protection API MasterKey |

patterns/dos.hexpat

@@ -0,0 +1,242 @@
+#pragma author Stephen Hewitt
+#pragma description MSDOS executable file
+
+#pragma MIME application/x-dosexec
+#pragma MIME application/x-msdownload
+#pragma MIME application/x-dosexecapplication/zip
+#pragma MIME application/vnd.microsoft.portable-executable
+
+import type.magic;
+import std.io;
+import std.mem;
+import std.math;
+import std.string;
+
+/*
+ * A DOS EXE file, at a high level, consists of three regions:
+ *
+ * Header
+ *  As it's name suggests. Contains info the loader uses.
+ *
+ * Load module
+ *  Contains the program data that is loaded into memory.
+ *
+ * Extra data
+ *  Data appended to the file that isn't loaded into memory.
+ *
+ * We'll call the combined header and load module the
+ * "program image". It's what the DOS loader cares about.
+ */
+
+ /*
+ * Wikipedia: The New Executable (NE or NewEXE) is a 16-bit executable
+ * file format, a successor to the DOS MZ executable format. It was used
+ * in Windows 1.0–3.x, Windows 9x, multitasking MS-DOS 4.0,[1] OS/2 1.x,
+ * and the OS/2 subset of Windows NT up to version 5.0 (Windows 2000).
+ *
+ * Since it was used in DOS we'll support it.
+ *
+ * We'll make it optional since some programs increased
+ * 'headerSizeInParagraphs' and stashed all kind of stuff there.
+ */
+bool EnableNEHeaderExt in;
+
+/*
+ * DOS file offsets/sizes. DOS uses INT 21h for file I/O. File positions and
+ * lengths are tracked using 32-bit signed integers. DOS INT 21h functions
+ * treat the offset as signed, so the highest positive offset is 0x7FFFFFFF.
+ * Attempting to seek beyond that or read/write beyond that will fail.
+ * We'll use a u32.
+ */
+u32 g_loadModule;
+u32 g_loadModuleSize;
+u32 g_programImageSize;
+
+fn formatNumber(u32 num, str msg="") {
+    if (std::string::length(msg)==0)
+        return std::format("0x{:x} ({})", num, num);
+    else
+        return std::format("{} 0x{:x} ({})", msg, num, num);
+};
+
+fn inLoadModule(u32 off, u32 sz) {
+    return off>=g_loadModule && off+sz<=g_loadModule+g_loadModuleSize;
+};
+
+struct Relocation {
+    u16 offset [[color("9AE630")]];
+    u16 segment [[color("FE9A37")]];
+};
+
+struct RelocationAnnotated : Relocation {
+    u32 fileOffset = g_loadModule+offset+segment*16;
+    if (inLoadModule(fileOffset, 2)) {
+        u16 __goto__target @ fileOffset [[highlight_hidden]];
+     }
+     else {
+        str __goto__target = formatNumber(fileOffset, "Not in load module") [[export, highlight_hidden]];
+     }
+};
+
+struct Relocations {
+    if (parent.dosHeader.relocations>0) {
+        Relocation __goto__firstReloc @ $ [[highlight_hidden]];
+        Relocation __goto__lastReloc  @ $+(parent.dosHeader.relocations-1)*sizeof(Relocation) [[highlight_hidden]];
+    }
+    RelocationAnnotated data[parent.dosHeader.relocations] [[inline]];
+};
+
+struct DOSHeader {
+    type::Magic<"MZ"> signature [[hex::spec_name("e_magic")]];
+    u16 extraPageSize [[hex::spec_name("e_cblp")]];
+    u16 numberOfPages [[hex::spec_name("e_cp")]];
+    g_programImageSize =  (extraPageSize==0) ?
+        (numberOfPages*512) :
+        (numberOfPages-1)*512 + extraPageSize;
+    str __programImageSize = formatNumber(g_programImageSize) [[export, highlight_hidden]];
+    u8 __goto__lastByteInProgramImage @ g_programImageSize-1 [[highlight_hidden]];
+    u16 relocations [[name("stubRelocations"), hex::spec_name("e_crlc")]];
+    u16 headerSizeInParagraphs [[hex::spec_name("e_cparhdr")]];
+    u32 headerSize = headerSizeInParagraphs*16;
+    g_loadModule = headerSizeInParagraphs*16;
+    g_loadModuleSize = g_programImageSize - headerSize;
+    str __headerSize = formatNumber(headerSize) [[export, highlight_hidden]];
+    u8 __goto__lastByteInHeader @ headerSize-1 [[highlight_hidden]];
+    u16 minimumAllocatedParagraphs [[hex::spec_name("e_minalloc")]];
+    u16 maximumAllocatedParagraphs [[hex::spec_name("e_maxalloc")]];
+    u16 initialSSValue [[hex::spec_name("e_ss")]];
+    u16 initialRelativeSPValue [[hex::spec_name("e_sp")]];
+    u16 checksum [[name("stubChecksum"), hex::spec_name("e_csum")]];
+    u16 initialRelativeIPValue [[hex::spec_name("e_ip")]];
+    u16 initialCSValue [[hex::spec_name("e_cs")]];
+
+    u32 csAddrFirst = initialCSValue<<4;
+    u32 csAddrLast = (csAddrFirst+0xffff) & ((1<<20)-1);
+
+    u32 csEndGap = 0;
+    if (csAddrFirst <= csAddrLast) {
+        u32 csOffsetFirst = headerSize+csAddrFirst;
+        u32 csOffsetLast  = csOffsetFirst+std::math::min(0x10000, g_loadModuleSize)-1;
+    }
+    else {
+        u32 csOffsetFirst = headerSize;
+        csEndGap = (1<<20)-csAddrFirst;
+        u32 csOffsetLast  = headerSize+(0x10000-csEndGap-1);
+
+        std::warning("EXE has 'initialCSValue' set such that 20-bit address wraps.");
+        std::warning(" My guess would be to get the PSP into the CS.");
+    }
+
+
+    /*
+     * Adding `csEndGap` to the `initialIP` calculation below is required because the
+     * program is started by transferring execution to CS:IP. If `csEndGap` is non-zero
+     * CS and the start of the load-module value do not align; there’s some extra data
+     * the CPU can see before the data in the EXE. What confused me for a bit was why
+     * it’s not required in the relocation target locations I make. The reason, I think,
+     * is that when the loader loads the load-module into memory and then proceeds to
+     * apply the relocations, the offsets are relative to the segment the code is loaded
+     * in and not the execution environment (the CS register from `initialCSValue`).
+     */
+    u32 initialIP = csOffsetFirst+initialRelativeIPValue-csEndGap;
+
+    if (inLoadModule(initialIP, 1))
+        u8 __goto__initiaIP @ initialIP [[highlight_hidden]];
+    else
+        str __goto__initiaIP = formatNumber(initialIP, "Not in load module!") [[export, highlight_hidden]];
+
+    u32 csSize = csOffsetLast-csOffsetFirst+1;
+    if (inLoadModule(csOffsetFirst, csSize)) {
+        std::mem::Bytes<csSize> __select__InitialCS @ csOffsetFirst [[highlight_hidden]];
+        u8 __goto__InitialCS_first @ csOffsetFirst [[highlight_hidden]];
+        u8 __goto__InitialCS_last  @ csOffsetFirst+csSize-1 [[highlight_hidden]];
+    }
+    else {
+        str __select__CS = formatNumber(csOffsetFirst, "Not in image!") [[export, highlight_hidden]];
+    }
+
+    u16 relocationsTablePointer [[hex::spec_name("e_lfarlc")]];
+    u32 sizeofRelocations = relocations*sizeof(Relocation);
+    if (relocations>0 && relocationsTablePointer+sizeofRelocations<g_programImageSize) {
+        std::mem::Bytes<sizeofRelocations> __select__relocationsTable
+            @ relocationsTablePointer [[highlight_hidden]];
+    }
+     else {
+        str __select__relocationsTable =
+            "Not in image or zero length" [[export, highlight_hidden]];
+     }
+    u16 overlayNumber [[hex::spec_name("e_ovno")]];
+};
+
+struct NEDOSHeaderExt {
+    u16 reservedWords[4] [[hex::spec_name("e_res")]];
+    u16 oemIdentifier [[hex::spec_name("e_oemid")]];
+    u16 oemInformation [[hex::spec_name("e_oeminfo")]];
+    u16 otherReservedWords[10] [[hex::spec_name("e_res2")]];
+    u32 newHeaderPointer [[hex::spec_name("e_lfanew")]];
+};
+
+struct NEDOSHeaderExtAnnotated : NEDOSHeaderExt {
+    if (newHeaderPointer < std::mem::size())
+        u8 __goto__newHeader @ newHeaderPointer [[highlight_hidden]];
+    else
+        str __goto__newHeader
+            = formatNumber(newHeaderPointer, "Not in image!") [[export, highlight_hidden]];
+};
+
+/*
+ * The header of a DOS EXE file consists of three regions.
+ *
+ * DOSHeader
+ *  Present in all DOS EXEs. Used by the loader.
+ *
+ * NEDOSHeaderExt
+ *  An extension to the header. Optional.
+ *
+ * Relocations
+ *  An array of segment relocations to the apply to the load module. Optional.
+ *
+ * The header is followed by the load module. There can be gaps between
+ * DOSHeader (or NEDOSHeaderExt if present) and Relocations, and between the
+ * Relocations and the load module. It is not uncommon for EXEs to stash candy
+ * in these gaps.
+ */
+
+struct Header {
+    DOSHeader dosHeader;
+
+    if (EnableNEHeaderExt) {
+        if (dosHeader.relocationsTablePointer < $+sizeof(NEDOSHeaderExt)) {
+            std::warning("NEHeaderExt and Relocations overlap. Disabling NEHeaderExt.");
+        }
+        else {
+            NEDOSHeaderExtAnnotated extHeader;
+        }
+    }
+
+    if (dosHeader.relocations > 0) {
+        if (dosHeader.relocationsTablePointer < $) {
+            std::warning("Relocation table overlaps previous header members");
+        }
+        if (dosHeader.relocationsTablePointer+dosHeader.relocations*sizeof(Relocation) > g_loadModule) {
+            std::warning("Relocation table ends past header.");
+        }
+    }
+
+    if (dosHeader.relocationsTablePointer > $) {
+        u8 header_reloc_gap[dosHeader.relocationsTablePointer-$] [[highlight_hidden]];
+    }
+    Relocations relocations;
+    if (g_loadModule > $) {
+        u8 reloc_loadModule_gap[g_loadModule-$] [[highlight_hidden]];
+    }
+};
+
+struct LoadModule {
+    u8 __goto__first @ $ [[highlight_hidden]];
+    u8 __goto__last  @ $+g_loadModuleSize-1 [[highlight_hidden]];
+    u8 data[g_loadModuleSize];
+} [[color("7393B3")]];
+
+Header header @0;
+LoadModule loadModule @g_loadModule;;