%cs at entry to decompressor varies a _lot_ among Linux 2.4.x, 2.6.y kernels.

Decompressor must adapt; no universal constants exist for __BOOT_CS, _BOOT_DS.
	p_vmlinx.cpp stub/l_vmlinz.asm

committer: jreiser <jreiser> 1103605278 +0000

src/p_vmlinx.cpp

@@ -487,39 +487,38 @@ void PackVmlinuxI386::unpack(OutputFile *fo)
 //-----
 //
 //----- arch/i386/boot/compressed/upx-head.S
-//#include <asm/segment.h>
+//		.text
-//#ifndef __BOOT_CS  /* Linux 2.4.x */
-//#define __BOOT_CS __KERNEL_CS
-//#define __BOOT_DS __KERNEL_DS
-//#endif
-//
-//      .text
 //startup_32: .globl startup_32  # In: %esi=0x90000 setup data "real_mode pointer"
-//      cli  # but if it matters, then there is a race!
+//		#cli  # this must be true already
 //
-//      movl $ __BOOT_DS,%eax
+//		/* The only facts about segments here, that are true for all kernels:
-//      movl %eax,%ss; leal 0x9000(%esi),%esp  # 0x99000 typical
+//		 * %cs is a valid "flat" code segment; no other segment reg is valid;
-//          /* Linux Documentation/i386/boot.txt "SAMPLE BOOT CONFIGURATION" says
+//		 * the next segment after %cs is a valid "flat" data segment, but
-//             0x8000-0x8FFF  Stack and heap  [inside the "real mode segment",
+//		 * no segment register designates it yet.
-//             just below the command line at offset 0x9000].
+//		 */
+//		movl %cs,%eax; addl $1<<3,%eax  # the next segment after %cs
+//		movl %eax,%ds
+//		movl %eax,%es
+//		leal 0x9000(%esi),%ecx  # 0x99000 typical
+//		movl %ecx,-8(%ecx)  # 32-bit offset for stack pointer
+//		movl %eax,-4(%ecx)  # segment for stack pointer
+//		lss -8(%ecx),%esp  # %ss:%esp= %ds:0x99000
+//		    /* Linux Documentation/i386/boot.txt "SAMPLE BOOT CONFIGURATION" says
+//		       0x8000-0x8FFF  Stack and heap  [inside the "real mode segment",
+//		       just below the command line at offset 0x9000].
 //
-//             arch/i386/boot/compressed/head.S "Do the decompression ..." says
+//		       arch/i386/boot/compressed/head.S "Do the decompression ..." says
-//             %esi contains the "real mode pointer" [as a 32-bit addr].
+//		       %esi contains the "real mode pointer" [as a 32-bit addr].
 //
-//             In any case, avoid EBDA (Extended BIOS Data Area) below 0xA0000.
+//		       In any case, avoid EBDA (Extended BIOS Data Area) below 0xA0000.
-//             boot.txt says 0x9A000 is the limit.  LILO goes up to 0x9B000.
+//		       boot.txt says 0x9A000 is the limit.  LILO goes up to 0x9B000.
-//          */
+//		    */
 //
-//      pushl $0; popf  # subsumes "cli; cld"; also clears NT for buggy BIOS
+//		pushl $0; popf  # subsumes "cli; cld"; also clears NT for buggy BIOS
 //
-//      movl %eax,%ds  # all non-code segments identical
+//		movl $ 0x100000,%eax  # destination of uncompression (and entry point)
-//      movl %eax,%es
+//		push %cs
-//      movl %eax,%fs
+/* Fall into .text of upx-compressed vmlinux. */
-//      movl %eax,%gs
-//
-//      movl $ 0x100000,%eax  # destination of uncompression (and entry point)
-//      pushl $ __BOOT_CS
-///* Fall into .text of upx-compressed vmlinux. */
 //-----
 
 // Approximate translation for Linux 2.4.x:

src/stub/l_vmlinz.asm

@@ -34,25 +34,38 @@
                 SECTION .text
                 ORG     0
 
-; gdt segment 3 is flat data
-%define __BOOT_DS 3*8
-; gdt segment 2 is flat code
-%define __BOOT_CS 2*8
-
 ; =============
 ; ============= ENTRY POINT
 ; =============
 
 start:
 ;       __LINUZ000__
-                cli
+                ;cli    ;this must be true already
-                xor     eax, eax
+
-                mov     al, __BOOT_DS
+        ; The only facts about segments here, that are true for all kernels:
-                mov     ds, eax
+        ; %cs is a valid "flat" code segment; no other segment reg is valid;
-                mov     es, eax
+        ; the next segment after %cs is a valid "flat" data segment, but
-; fs, gs set by startup_32 in arch/i386/kernel/head.S
+        ; no segment register designates it yet.
-                mov     ss, eax
+                mov eax, cs
-                lea     esp, ['STAK' + esi]  ; (0x9000 + 0x90000) typical
+                add eax, byte 1<<3      ; the next segment after %cs
+                mov ds, eax
+                mov es, eax
+        ; fs, gs set by startup_32 in arch/i386/kernel/head.S
+
+        ; Linux Documentation/i386/boot.txt "SAMPLE BOOT CONFIGURATION" says
+        ; 0x8000-0x8FFF  Stack and heap  [inside the "real mode segment",
+        ; just below the command line at offset 0x9000].
+
+        ; arch/i386/boot/compressed/head.S "Do the decompression ..." says
+        ; %esi contains the "real mode pointer" [as a 32-bit addr].
+
+        ; In any case, avoid EBDA (Extended BIOS Data Area) below 0xA0000.
+        ; boot.txt says 0x9A000 is the limit.  LILO goes up to 0x9B000.
+
+                lea ecx, ['STAK' + esi]  ; (0x9000 + 0x90000) typical
+                mov      [-8 + ecx], ecx  ; 32-bit offset for stack pointer
+                mov      [-4 + ecx], eax  ; segment for stack
+                lss esp, [-8 + ecx]       ; %ss:%esp= %ds:0x99000
 
                 push    byte 0
                 popf            ; BIOS can leave random flags (such as NT)
@@ -62,7 +75,7 @@ start:
 
                 or      ebp, byte -1    ; decompressor assumption
                 mov     eax, 'KEIP'     ; 0x100000 : address of startup_32
-                push    byte __BOOT_CS        ; MATCH00
+                push    cs      ; MATCH00
                 push    eax     ; MATCH00  entry address
                 push    edi     ; MATCH01  save
                 push    esi     ; MATCH02  save