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[FREELDR] Make it possible to change the destination base address by changing DESTINATION_BASE #8455

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[FREELDR] Make it possible to change the destination base address by changing DESTINATION_BASE #8455

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992ef15
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iLauncherDev Nov 11, 2025
0f2034e
security change
iLauncherDev Nov 11, 2025
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24 changes: 16 additions & 8 deletions boot/freeldr/freeldr/arch/i386/multiboot.S
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Original file line number Diff line number Diff line change
Expand Up @@ -33,7 +33,10 @@
* multiboot kernels above 1MB. So we let GRUB load us
* there and then relocate ourself to FREELDR_BASE.
*/
#define INITIAL_BASE HEX(200000)
#define INITIAL_BASE TEMPCODE_BASE

/* The address that we want to execute FreeLoader */
#define DESTINATION_BASE FREELDR_BASE


#ifdef _M_IX86
Expand Down Expand Up @@ -156,16 +159,19 @@ mb2:
inc dh

mb3:
/* Adjust the gdt table address */
add dword ptr [(gdtptr + 2) + INITIAL_BASE - FREELDR_BASE], DESTINATION_BASE

/* Relocate itself to lower address */
mov esi, INITIAL_BASE
mov edi, FREELDR_BASE
mov edi, DESTINATION_BASE
mov ecx, offset __bss_start__ - FREELDR_BASE
rep movsb

/* Load segment registers for real-address mode */
mov edi, offset gdtptr
mov edi, offset gdtptr + DESTINATION_BASE - FREELDR_BASE
lgdt lXdtPrefix [rdi]
mov edi, offset idtptr
mov edi, offset idtptr + DESTINATION_BASE - FREELDR_BASE
lidt lXdtPrefix [rdi]

mov ax, HEX(10)
Expand All @@ -176,7 +182,9 @@ mb3:
mov ss, ax

/* Jump to relocated code */
ljmp HEX(08), mb4
push HEX(08)
push offset mb4 + DESTINATION_BASE - FREELDR_BASE
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@iLauncherDev iLauncherDev Nov 12, 2025

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I will need to pass that indirectly just to compile with msvc.

retf

mbfail:
int 3
Expand All @@ -190,7 +198,7 @@ mb4:
mov cr0, rax

/* Jump to real entry point */
ljmp16 FREELDR_BASE / 16, 0
ljmp16 DESTINATION_BASE / 16, 0


/* Force 8-byte alignment */
Expand All @@ -202,8 +210,8 @@ gdt:

/* GDT table pointer */
gdtptr:
.word HEX(17) /* Limit */
.long gdt /* Base Address */
.word HEX(17) /* Limit */
.long OFF(gdt) - FREELDR_BASE /* Base Address */

/* Force 8-byte alignment */
.align 8
Expand Down
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