Add a shellcode to bypass abl dt overlay failure.

Author: sunflower2333

Config/DualBoot.Sm8250DT.cfg

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+StackBase=0x9FC00000
+StackSize=0x00300000

ShellCode/ShellCode.KernelWrapper.S

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+/**
+* A Wrapper for Linux Kernel to by pass android dtb check
+*
+*   Qualcomm application bootloader (ABL) will check msm-id/board-id
+*   or apply overlay before booting kernel. By replacing android dtb with linux dtb,
+*   abl will failed to check or apply dtbo and then refuse to boot.
+*   This wrapper is used to bypass the check.
+*
+*   Scheme:
+*      Only replace kernel instead of replacing dtb. But inject the dtb in kernel
+*
+*   Step:
+*      Inject this wrapper into kernel header and make a payload with linux kernel + linux dtb.
+*      Then, repack the android boot image with the payload, but not replace dtb.
+*      Linux will boot successfully then.
+*
+*   Note:
+*      You need to add memory region for linux dtb in device tree manually
+*      otherwise linux will NOT boot.
+*
+*   Usage:
+*      ./DualBootKernelPatcher OriginalKernel mainline_dtb output DualBoot.Sm8250DT.cfg ShellCode.KernelWrapper.bin
+*      Then repack android image with the output file.
+*
+*   Inspired by @bigfootACA
+*
+*/
+
+/* Dummy Header for shellcode */
+.include "DummyHead.S"
+
+_ShellCodeStart:
+    // Calculate UEFI FD(dtb addr here) start address and store in X4
+    adr     x4, _KernelHead         // Store kernel head address in x4.
+    ldr     x5, _KernelSize         // Store kernel size in x5.
+    add     x4, x4, x5              // Add kernel base + kernel size, store value in x4.
+
+    // Copy dtb to safe place (StackRegion, you can configure it freely in DualBoot config file)
+    ldr     x5, _StackBase          // Store FD Base in x5.
+    ldr     x6, _StackSize          // Store FD Size in x6.
+    bl      _CopyLoop               // Copy DTB to stack region.
+
+    // Set X0 to StackBase, which is the new DTB address
+    ldr     x0, _StackBase          // Store stack base address in x5.
+    b       _LinuxStart             // Boot linux kernel.
+    b      _Dead                    // We should never get here.
+
+// Copy Sub program, X4 is src, X5 is dst, X6 is size
+_CopyLoop:
+    ldp     x2, x3, [x4], #0x10     // Save value at [x4](pointer) to x2 and x3, then x4 add 16.
+    stp     x2, x3, [x5], #0x10     // Save value in x2 and x3 to [x5](pointer), then x5 add 16
+    subs    x6, x6, #0x10           // x6 - 16, if , set CPSR register to 0.
+    b.ne    _CopyLoop               // Check CPSR, if CPSR != 0, jump back to _CopyLoop.
+    ret                             // Return when finish.
+
+_Dead:
+    b       _Dead                   // We should never get here.
+
+.text
+.align 4
+
+_ShellCodeEnd:
+/* Do not remove the last line  */