source: https://www.securityfocus.com/bid/29656/info
NASM is prone to an off-by-one buffer-overflow vulnerability because the application fails to perform adequate boundary checks on user-supplied data.
Successfully exploiting this issue will allow attackers to execute arbitrary code within the context of the affected application. Failed exploit attempts will result in a denial-of-service condition.
NASM 2.02 and prior versions are vulnerable.
; -*- fundamental -*- (asm-mode sucks)
; ****************************************************************************
;
; ldlinux.asm
;
; A program to boot Linux kernels off an MS-DOS formatted floppy disk. This
; functionality is good to have for installation floppies, where it may
; be hard to find a functional Linux system to run LILO off.
;
; This program allows manipulation of the disk to take place entirely
; from MS-LOSS, and can be especially useful in conjunction with the
; umsdos filesystem.
;
; Copyright (C) 1994-2005 H. Peter Anvin
;
; This program is free software; you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation, Inc., 53 Temple Place Ste 330,
; Boston MA 02111-1307, USA; either version 2 of the License, or
; (at your option) any later version; incorporated herein by reference.
;
; ****************************************************************************
%ifndef IS_MDSLINUX
%define IS_SYSLINUX 1
%endif
%define WITH_GFX 1
%include "head.inc"
;
; Some semi-configurable constants... change on your own risk.
;
my_id equ syslinux_id
FILENAME_MAX_LG2 equ 4 ; log2(Max filename size Including final null)
FILENAME_MAX equ 11 ; Max mangled filename size
NULLFILE equ ' ' ; First char space == null filename
NULLOFFSET equ 0 ; Position in which to look
retry_count equ 16 ; How patient are we with the disk?
%assign HIGHMEM_SLOP 0 ; Avoid this much memory near the top
LDLINUX_MAGIC equ 0x3eb202fe ; A random number to identify ourselves with
MAX_OPEN_LG2 equ 6 ; log2(Max number of open files)
MAX_OPEN equ (1 << MAX_OPEN_LG2)
SECTOR_SHIFT equ 9
SECTOR_SIZE equ (1 << SECTOR_SHIFT)
;
; This is what we need to do when idle
;
%macro RESET_IDLE 0
; Nothing
%endmacro
%macro DO_IDLE 0
; Nothing
%endmacro
;
; The following structure is used for "virtual kernels"; i.e. LILO-style
; option labels. The options we permit here are `kernel' and `append
; Since there is no room in the bottom 64K for all of these, we
; stick them at vk_seg:0000 and copy them down before we need them.
;
struc vkernel
vk_vname: resb FILENAME_MAX ; Virtual name **MUST BE FIRST!**
vk_rname: resb FILENAME_MAX ; Real name
vk_appendlen: resw 1
alignb 4
vk_append: resb max_cmd_len+1 ; Command line
alignb 4
vk_end: equ $ ; Should be <= vk_size
endstruc
;
; Segment assignments in the bottom 640K
; Stick to the low 512K in case we're using something like M-systems flash
; which load a driver into low RAM (evil!!)
;
; 0000h - main code/data segment (and BIOS segment)
;
real_mode_seg equ 4000h
cache_seg equ 3000h ; 64K area for metadata cache
vk_seg equ 2000h ; Virtual kernels
xfer_buf_seg equ 1000h ; Bounce buffer for I/O to high mem
comboot_seg equ real_mode_seg ; COMBOOT image loading zone
first_free_seg equ 5000h ; end of syslinux used memory
;
; File structure. This holds the information for each currently open file.
;
struc open_file_t
file_sector resd 1 ; Sector pointer (0 = structure free)
file_left resd 1 ; Number of sectors left
endstruc
%ifndef DEPEND
%if (open_file_t_size & (open_file_t_size-1))
%error "open_file_t is not a power of 2"
%endif
%endif
; ---------------------------------------------------------------------------
; BEGIN CODE
; ---------------------------------------------------------------------------
;
; Memory below this point is reserved for the BIOS and the MBR
;
section .earlybss
trackbufsize equ 8192
trackbuf resb trackbufsize ; Track buffer goes here
getcbuf resb trackbufsize
; ends at 4800h
section .bss
alignb 8
; Expanded superblock
SuperInfo equ $
resq 16 ; The first 16 bytes expanded 8 times
FAT resd 1 ; Location of (first) FAT
RootDirArea resd 1 ; Location of root directory area
RootDir resd 1 ; Location of root directory proper
DataArea resd 1 ; Location of data area
RootDirSize resd 1 ; Root dir size in sectors
TotalSectors resd 1 ; Total number of sectors
EndSector resd 1 ; Location of filesystem end
ClustSize resd 1 ; Bytes/cluster
ClustMask resd 1 ; Sectors/cluster - 1
CopySuper resb 1 ; Distinguish .bs versus .bss
DriveNumber resb 1 ; BIOS drive number
ClustShift resb 1 ; Shift count for sectors/cluster
ClustByteShift resb 1 ; Shift count for bytes/cluster
alignb open_file_t_size
Files resb MAX_OPEN*open_file_t_size
;
; Constants for the xfer_buf_seg
;
; The xfer_buf_seg is also used to store message file buffers. We
; need two trackbuffers (text and graphics), plus a work buffer
; for the graphics decompressor.
;
xbs_textbuf equ 0 ; Also hard-coded, do not change
xbs_vgabuf equ trackbufsize
xbs_vgatmpbuf equ 2*trackbufsize
section .text
;
; Some of the things that have to be saved very early are saved
; "close" to the initial stack pointer offset, in order to
; reduce the code size...
;
StackBuf equ $-44-32 ; Start the stack here (grow down - 4K)
PartInfo equ StackBuf ; Saved partition table entry
FloppyTable equ PartInfo+16 ; Floppy info table (must follow PartInfo)
OrigFDCTabPtr equ StackBuf-4 ; The high dword on the stack
;
; Primary entry point. Tempting as though it may be, we can't put the
; initial "cli" here; the jmp opcode in the first byte is part of the
; "magic number" (using the term very loosely) for the DOS superblock.
;
bootsec equ $
jmp short start ; 2 bytes
nop ; 1 byte
;
; "Superblock" follows -- it's in the boot sector, so it's already
; loaded and ready for us
;
bsOemName db 'SYSLINUX' ; The SYS command sets this, so...
;
; These are the fields we actually care about. We end up expanding them
; all to dword size early in the code, so generate labels for both
; the expanded and unexpanded versions.
;
%macro superb 1
bx %+ %1 equ SuperInfo+($-superblock)*8+4
bs %+ %1 equ $
zb 1
%endmacro
%macro superw 1
bx %+ %1 equ SuperInfo+($-superblock)*8
bs %+ %1 equ $
zw 1
%endmacro
%macro superd 1
bx %+ %1 equ $ ; no expansion for dwords
bs %+ %1 equ $
zd 1
%endmacro
superblock equ $
superw BytesPerSec
superb SecPerClust
superw ResSectors
superb FATs
superw RootDirEnts
superw Sectors
superb Media
superw FATsecs
superw SecPerTrack
superw Heads
superinfo_size equ ($-superblock)-1 ; How much to expand
superd Hidden
superd HugeSectors
;
; This is as far as FAT12/16 and FAT32 are consistent
;
zb 54 ; FAT12/16 need 26 more bytes,
; FAT32 need 54 more bytes
superblock_len equ $-superblock
SecPerClust equ bxSecPerClust
;
; Note we don't check the constraints above now; we did that at install
; time (we hope!)
;
start:
cli ; No interrupts yet, please
cld ; Copy upwards
;
; Set up the stack
;
xor ax,ax
mov ss,ax
mov sp,StackBuf ; Just below BSS
mov es,ax
;
; DS:SI may contain a partition table entry. Preserve it for us.
;
mov cx,8 ; Save partition info
mov di,sp
rep movsw
mov ds,ax ; Now we can initialize DS...
;
; Now sautee the BIOS floppy info block to that it will support decent-
; size transfers; the floppy block is 11 bytes and is stored in the
; INT 1Eh vector (brilliant waste of resources, eh?)
;
; Of course, if BIOSes had been properly programmed, we wouldn't have
; had to waste precious space with this code.
;
mov bx,fdctab
lfs si,[bx] ; FS:SI -> original fdctab
push fs ; Save on stack in case we need to bail
push si
; Save the old fdctab even if hard disk so the stack layout
; is the same. The instructions above do not change the flags
mov [DriveNumber],dl ; Save drive number in DL
and dl,dl ; If floppy disk (00-7F), assume no
; partition table
js harddisk
floppy:
mov cl,6 ; 12 bytes (CX == 0)
; es:di -> FloppyTable already
; This should be safe to do now, interrupts are off...
mov [bx],di ; FloppyTable
mov [bx+2],ax ; Segment 0
fs rep movsw ; Faster to move words
mov cl,[bsSecPerTrack] ; Patch the sector count
mov [di-8],cl
; AX == 0 here
int 13h ; Some BIOSes need this
jmp short not_harddisk
;
; The drive number and possibly partition information was passed to us
; by the BIOS or previous boot loader (MBR). Current "best practice" is to
; trust that rather than what the superblock contains.
;
; Would it be better to zero out bsHidden if we don't have a partition table?
;
; Note: di points to beyond the end of PartInfo
;
harddisk:
test byte [di-16],7Fh ; Sanity check: "active flag" should
jnz no_partition ; be 00 or 80
mov eax,[di-8] ; Partition offset (dword)
mov [bsHidden],eax
no_partition:
;
; Get disk drive parameters (don't trust the superblock.) Don't do this for
; floppy drives -- INT 13:08 on floppy drives will (may?) return info about
; what the *drive* supports, not about the *media*. Fortunately floppy disks
; tend to have a fixed, well-defined geometry which is stored in the superblock.
;
; DL == drive # still
mov ah,08h
int 13h
jc no_driveparm
and ah,ah
jnz no_driveparm
shr dx,8
inc dx ; Contains # of heads - 1
mov [bsHeads],dx
and cx,3fh
mov [bsSecPerTrack],cx
no_driveparm:
not_harddisk:
;
; Ready to enable interrupts, captain
;
sti
;
; Do we have EBIOS (EDD)?
;
eddcheck:
mov bx,55AAh
mov ah,41h ; EDD existence query
mov dl,[DriveNumber]
int 13h
jc .noedd
cmp bx,0AA55h
jne .noedd
test cl,1 ; Extended disk access functionality set
jz .noedd
;
; We have EDD support...
;
mov byte [getlinsec.jmp+1],(getlinsec_ebios-(getlinsec.jmp+2))
.noedd:
;
; Load the first sector of LDLINUX.SYS; this used to be all proper
; with parsing the superblock and root directory; it doesn't fit
; together with EBIOS support, unfortunately.
;
mov eax,[FirstSector] ; Sector start
mov bx,ldlinux_sys ; Where to load it
call getonesec
; Some modicum of integrity checking
cmp dword [ldlinux_magic+4],LDLINUX_MAGIC^HEXDATE
jne kaboom
; Go for it...
jmp ldlinux_ent
;
; getonesec: get one disk sector
;
getonesec:
mov bp,1 ; One sector
; Fall through
;
; getlinsec: load a sequence of BP floppy sector given by the linear sector
; number in EAX into the buffer at ES:BX. We try to optimize
; by loading up to a whole track at a time, but the user
; is responsible for not crossing a 64K boundary.
; (Yes, BP is weird for a count, but it was available...)
;
; On return, BX points to the first byte after the transferred
; block.
;
; This routine assumes CS == DS, and trashes most registers.
;
; Stylistic note: use "xchg" instead of "mov" when the source is a register
; that is dead from that point; this saves space. However, please keep
; the order to dst,src to keep things sane.
;
getlinsec:
add eax,[bsHidden] ; Add partition offset
xor edx,edx ; Zero-extend LBA (eventually allow 64 bits)
.patch: jmp strict near .jmp
.jmp: jmp strict short getlinsec_cbios
;
; getlinsec_ebios:
;
; getlinsec implementation for EBIOS (EDD)
;
getlinsec_ebios:
.loop:
push bp ; Sectors left
.retry2:
call maxtrans ; Enforce maximum transfer size
movzx edi,bp ; Sectors we are about to read
mov cx,retry_count
.retry:
; Form DAPA on stack
push edx
push eax
push es
push bx
push di
push word 16
mov si,sp
pushad
mov dl,[DriveNumber]
push ds
push ss
pop ds ; DS <- SS
mov ah,42h ; Extended Read
int 13h
pop ds
popad
lea sp,[si+16] ; Remove DAPA
jc .error
pop bp
add eax,edi ; Advance sector pointer
sub bp,di ; Sectors left
shl di,SECTOR_SHIFT ; 512-byte sectors
add bx,di ; Advance buffer pointer
and bp,bp
jnz .loop
ret
.error:
; Some systems seem to get "stuck" in an error state when
; using EBIOS. Doesn't happen when using CBIOS, which is
; good, since some other systems get timeout failures
; waiting for the floppy disk to spin up.
pushad ; Try resetting the device
xor ax,ax
mov dl,[DriveNumber]
int 13h
popad
loop .retry ; CX-- and jump if not zero
;shr word [MaxTransfer],1 ; Reduce the transfer size
;jnz .retry2
; Total failure. Try falling back to CBIOS.
mov byte [getlinsec.jmp+1],(getlinsec_cbios-(getlinsec.jmp+2))
;mov byte [MaxTransfer],63 ; Max possibe CBIOS transfer
pop bp
; ... fall through ...
;
; getlinsec_cbios:
;
; getlinsec implementation for legacy CBIOS
;
getlinsec_cbios:
.loop:
push edx
push eax
push bp
push bx
movzx esi,word [bsSecPerTrack]
movzx edi,word [bsHeads]
;
; Dividing by sectors to get (track,sector): we may have
; up to 2^18 tracks, so we need to use 32-bit arithmetric.
;
div esi
xor cx,cx
xchg cx,dx ; CX <- sector index (0-based)
; EDX <- 0
; eax = track #
div edi ; Convert track to head/cyl
; We should test this, but it doesn't fit...
; cmp eax,1023
; ja .error
;
; Now we have AX = cyl, DX = head, CX = sector (0-based),
; BP = sectors to transfer, SI = bsSecPerTrack,
; ES:BX = data target
;
call maxtrans ; Enforce maximum transfer size
; Must not cross track boundaries, so BP <= SI-CX
sub si,cx
cmp bp,si
jna .bp_ok
mov bp,si
.bp_ok:
shl ah,6 ; Because IBM was STOOPID
; and thought 8 bits were enough
; then thought 10 bits were enough...
inc cx ; Sector numbers are 1-based, sigh
or cl,ah
mov ch,al
mov dh,dl
mov dl,[DriveNumber]
xchg ax,bp ; Sector to transfer count
mov ah,02h ; Read sectors
mov bp,retry_count
.retry:
pushad
int 13h
popad
jc .error
.resume:
movzx ecx,al ; ECX <- sectors transferred
shl ax,SECTOR_SHIFT ; Convert sectors in AL to bytes in AX
pop bx
add bx,ax
pop bp
pop eax
pop edx
add eax,ecx
sub bp,cx
jnz .loop
ret
.error:
dec bp
jnz .retry
xchg ax,bp ; Sectors transferred <- 0
shr word [MaxTransfer],1
jnz .resume
; Fall through to disk_error
;
; kaboom: write a message and bail out.
;
disk_error:
kaboom:
xor si,si
mov ss,si
mov sp,StackBuf-4 ; Reset stack
mov ds,si ; Reset data segment
pop dword [fdctab] ; Restore FDC table
.patch: ; When we have full code, intercept here
mov si,bailmsg
; Write error message, this assumes screen page 0
.loop: lodsb
and al,al
jz .done
mov ah,0Eh ; Write to screen as TTY
mov bx,0007h ; Attribute
int 10h
jmp short .loop
.done:
cbw ; AH <- 0
int 16h ; Wait for keypress
int 19h ; And try once more to boot...
.norge: jmp short .norge ; If int 19h returned; this is the end
;
; Truncate BP to MaxTransfer
;
maxtrans:
cmp bp,[MaxTransfer]
jna .ok
mov bp,[MaxTransfer]
.ok: ret
;
; Error message on failure
;
bailmsg: db 'Boot error', 0Dh, 0Ah, 0
; This fails if the boot sector overflows
zb 1F8h-($-$$)
FirstSector dd 0xDEADBEEF ; Location of sector 1
MaxTransfer dw 0x007F ; Max transfer size
bootsignature dw 0AA55h
;
; ===========================================================================
; End of boot sector
; ===========================================================================
; Start of LDLINUX.SYS
; ===========================================================================
ldlinux_sys:
syslinux_banner db 0Dh, 0Ah
%if IS_MDSLINUX
db 'MDSLINUX '
%else
db 'SYSLINUX '
%endif
db version_str, ' ', date, ' ', 0
db 0Dh, 0Ah, 1Ah ; EOF if we "type" this in DOS
align 8, db 0
ldlinux_magic dd LDLINUX_MAGIC
dd LDLINUX_MAGIC^HEXDATE
;
; This area is patched by the installer. It is found by looking for
; LDLINUX_MAGIC, plus 8 bytes.
;
patch_area:
LDLDwords dw 0 ; Total dwords starting at ldlinux_sys
LDLSectors dw 0 ; Number of sectors - (bootsec+this sec)
CheckSum dd 0 ; Checksum starting at ldlinux_sys
; value = LDLINUX_MAGIC - [sum of dwords]
; Space for up to 64 sectors, the theoretical maximum
SectorPtrs times 64 dd 0
ldlinux_ent:
;
; Note that some BIOSes are buggy and run the boot sector at 07C0:0000
; instead of 0000:7C00 and the like. We don't want to add anything
; more to the boot sector, so it is written to not assume a fixed
; value in CS, but we don't want to deal with that anymore from now
; on.
;
jmp 0:.next
.next:
;
; Tell the user we got this far
;
mov si,syslinux_banner
call writestr
;
; Tell the user if we're using EBIOS or CBIOS
;
print_bios:
mov si,cbios_name
cmp byte [getlinsec.jmp+1],(getlinsec_ebios-(getlinsec.jmp+2))
jne .cbios
mov si,ebios_name
.cbios:
mov [BIOSName],si
call writestr
section .bss
%define HAVE_BIOSNAME 1
BIOSName resw 1
section .text
;
; Now we read the rest of LDLINUX.SYS. Don't bother loading the first
; sector again, though.
;
load_rest:
mov si,SectorPtrs
mov bx,7C00h+2*SECTOR_SIZE ; Where we start loading
mov cx,[LDLSectors]
.get_chunk:
jcxz .done
xor bp,bp
lodsd ; First sector of this chunk
mov edx,eax
.make_chunk:
inc bp
dec cx
jz .chunk_ready
inc edx ; Next linear sector
cmp [si],edx ; Does it match
jnz .chunk_ready ; If not, this is it
add si,4 ; If so, add sector to chunk
jmp short .make_chunk
.chunk_ready:
call getlinsecsr
shl bp,SECTOR_SHIFT
add bx,bp
jmp .get_chunk
.done:
;
; All loaded up, verify that we got what we needed.
; Note: the checksum field is embedded in the checksum region, so
; by the time we get to the end it should all cancel out.
;
verify_checksum:
mov si,ldlinux_sys
mov cx,[LDLDwords]
mov edx,-LDLINUX_MAGIC
.checksum:
lodsd
add edx,eax
loop .checksum
and edx,edx ; Should be zero
jz all_read ; We're cool, go for it!
;
; Uh-oh, something went bad...
;
mov si,checksumerr_msg
call writestr
jmp kaboom
;
; -----------------------------------------------------------------------------
; Subroutines that have to be in the first sector
; -----------------------------------------------------------------------------
;
;
; writestr: write a null-terminated string to the console
; This assumes we're on page 0. This is only used for early
; messages, so it should be OK.
;
writestr:
.loop: lodsb
and al,al
jz .return
mov ah,0Eh ; Write to screen as TTY
mov bx,0007h ; Attribute
int 10h
jmp short .loop
.return: ret
; getlinsecsr: save registers, call getlinsec, restore registers
;
getlinsecsr: pushad
call getlinsec
popad
ret
;
; Checksum error message
;
checksumerr_msg db ' Load error - ', 0 ; Boot failed appended
;
; BIOS type string
;
cbios_name db 'CBIOS', 0
ebios_name db 'EBIOS', 0
;
; Debug routine
;
%ifdef debug
safedumpregs:
cmp word [Debug_Magic],0D00Dh
jnz nc_return
jmp dumpregs
%endif
rl_checkpt equ $ ; Must be <= 8000h
rl_checkpt_off equ ($-$$)
%ifndef DEPEND
%if rl_checkpt_off > 400h
%error "Sector 1 overflow"
%endif
%endif
; ----------------------------------------------------------------------------
; End of code and data that have to be in the first sector
; ----------------------------------------------------------------------------
all_read:
;
; Let the user (and programmer!) know we got this far. This used to be
; in Sector 1, but makes a lot more sense here.
;
mov si,copyright_str
call writestr
;
; Insane hack to expand the superblock to dwords
;
expand_super:
xor eax,eax
mov si,superblock
mov di,SuperInfo
mov cx,superinfo_size
.loop:
lodsw
dec si
stosd ; Store expanded word
xor ah,ah
stosd ; Store expanded byte
loop .loop
;
; Compute some information about this filesystem.
;
; First, generate the map of regions
genfatinfo:
mov edx,[bxSectors]
and dx,dx
jnz .have_secs
mov edx,[bsHugeSectors]
.have_secs:
mov [TotalSectors],edx
add edx,eax
mov [EndSector],edx
mov eax,[bxResSectors]
mov [FAT],eax ; Beginning of FAT
mov edx,[bxFATsecs]
and dx,dx
jnz .have_fatsecs
mov edx,[bootsec+36] ; FAT32 BPB_FATsz32
.have_fatsecs:
imul edx,[bxFATs]
add eax,edx
mov [RootDirArea],eax ; Beginning of root directory
mov [RootDir],eax ; For FAT12/16 == root dir location
mov edx,[bxRootDirEnts]
add dx,SECTOR_SIZE/32-1
shr dx,SECTOR_SHIFT-5
mov [RootDirSize],edx
add eax,edx
mov [DataArea],eax ; Beginning of data area
; Next, generate a cluster size shift count and mask
mov eax,[bxSecPerClust]
bsr cx,ax
mov [ClustShift],cl
push cx
add cl,9
mov [ClustByteShift],cl
pop cx
dec ax
mov [ClustMask],eax
inc ax
shl eax,9
mov [ClustSize],eax
;
; FAT12, FAT16 or FAT28^H^H32? This computation is fscking ridiculous.
;
getfattype:
mov eax,[EndSector]
sub eax,[DataArea]
shr eax,cl ; cl == ClustShift
mov cl,nextcluster_fat12-(nextcluster+2)
cmp eax,4085 ; FAT12 limit
jb .setsize
mov cl,nextcluster_fat16-(nextcluster+2)
cmp eax,65525 ; FAT16 limit
jb .setsize
;
; FAT32, root directory is a cluster chain
;
mov cl,[ClustShift]
mov eax,[bootsec+44] ; Root directory cluster
sub eax,2
shl eax,cl
add eax,[DataArea]
mov [RootDir],eax
mov cl,nextcluster_fat28-(nextcluster+2)
.setsize:
mov byte [nextcluster+1],cl
;
; Common initialization code
;
%include "cpuinit.inc"
%include "init.inc"
;
; Clear Files structures
;
mov di,Files
mov cx,(MAX_OPEN*open_file_t_size)/4
xor eax,eax
rep stosd
;
; Initialize the metadata cache
;
call initcache
;
; Now, everything is "up and running"... patch kaboom for more
; verbosity and using the full screen system
;
; E9 = JMP NEAR
mov dword [kaboom.patch],0e9h+((kaboom2-(kaboom.patch+3)) << 8)
;
; Now we're all set to start with our *real* business. First load the
; configuration file (if any) and parse it.
;
; In previous versions I avoided using 32-bit registers because of a
; rumour some BIOSes clobbered the upper half of 32-bit registers at
; random. I figure, though, that if there are any of those still left
; they probably won't be trying to install Linux on them...
;
; The code is still ripe with 16-bitisms, though. Not worth the hassle
; to take'm out. In fact, we may want to put them back if we're going
; to boot ELKS at some point.
;
;
; Load configuration file
;
mov di,syslinux_cfg
call open
jz no_config_file
;
; Now we have the config file open. Parse the config file and
; run the user interface.
;
%include "ui.inc"
;
; Linux kernel loading code is common.
;
%include "runkernel.inc"
;
; COMBOOT-loading code
;
%include "comboot.inc"
%include "com32.inc"
%include "cmdline.inc"
;
; Boot sector loading code
;
%include "bootsect.inc"
;
; Boot a specified local disk. AX specifies the BIOS disk number; or
; 0xFFFF in case we should execute INT 18h ("next device.")
;
local_boot:
%ifdef WITH_GFX
call gfx_done
%endif
call vgaclearmode
lss sp,[cs:Stack] ; Restore stack pointer
xor dx,dx
mov ds,dx
mov es,dx
mov fs,dx
mov gs,dx
mov si,localboot_msg
call cwritestr
cmp ax,-1
je .int18
; Load boot sector from the specified BIOS device and jump to it.
mov dl,al
xor dh,dh
push dx
xor ax,ax ; Reset drive
int 13h
mov ax,0201h ; Read one sector
mov cx,0001h ; C/H/S = 0/0/1 (first sector)
mov bx,trackbuf
int 13h
pop dx
cli ; Abandon hope, ye who enter here
mov si,trackbuf
mov di,07C00h
mov cx,512 ; Probably overkill, but should be safe
rep movsd
mov ss,cx
mov sp,7c00h
jmp 0:07C00h ; Jump to new boot sector
.int18:
int 18h ; Hope this does the right thing...
jmp kaboom ; If we returned, oh boy...
;
; Abort loading code
;
%include "abort.inc"
;
; allocate_file: Allocate a file structure
;
; If successful:
; ZF set
; BX = file pointer
; In unsuccessful:
; ZF clear
;
allocate_file:
TRACER 'a'
push cx
mov bx,Files
mov cx,MAX_OPEN
.check: cmp dword [bx], byte 0
je .found
add bx,open_file_t_size ; ZF = 0
loop .check
; ZF = 0 if we fell out of the loop
.found: pop cx
ret
;
; searchdir:
; Search the root directory for a pre-mangled filename in DS:DI.
;
; NOTE: This file considers finding a zero-length file an
; error. This is so we don't have to deal with that special
; case elsewhere in the program (most loops have the test
; at the end).
;
; If successful:
; ZF clear
; SI = file pointer
; DX:AX = file length in bytes
; If unsuccessful
; ZF set
;
searchdir:
push bx
call allocate_file
jnz .alloc_failure
push cx
push gs
push es
push ds
pop es ; ES = DS
mov eax,[RootDir] ; First root directory sector
.scansector:
call getcachesector
; GS:SI now points to this sector
mov cx,SECTOR_SIZE/32 ; 32 == directory entry size
.scanentry:
cmp byte [gs:si],0
jz .failure ; Hit directory high water mark
push cx
push si
push di
mov cx,11
gs repe cmpsb
pop di
pop si
pop cx
jz .found
add si,32
loop .scanentry
call nextsector
jnc .scansector ; CF is set if we're at end
; If we get here, we failed
.failure:
pop es
pop gs
pop cx
.alloc_failure:
pop bx
xor eax,eax ; ZF <- 1
ret
.found:
mov eax,[gs:si+28] ; File size
add eax,SECTOR_SIZE-1
shr eax,SECTOR_SHIFT
jz .failure ; Zero-length file
mov [bx+4],eax
mov cl,[ClustShift]
mov dx,[gs:si+20] ; High cluster word
shl edx,16
mov dx,[gs:si+26] ; Low cluster word
sub edx,2
shl edx,cl
add edx,[DataArea]
mov [bx],edx ; Starting sector
mov eax,[gs:si+28] ; File length again
mov dx,[gs:si+30] ; 16-bitism, sigh
mov si,bx
and eax,eax ; ZF <- 0
pop es
pop gs
pop cx
pop bx
ret
;
;
; kaboom2: once everything is loaded, replace the part of kaboom
; starting with "kaboom.patch" with this part
kaboom2:
cmp byte [gfx_ok],0
jz .nogfx
mov si,err_failed_gfx
xor di,di
mov al,1
call gfx_infobox
call gfx_done
call do_reboot
.nogfx:
mov si,err_bootfailed
call cwritestr
call getchar
call vgaclearmode
int 19h ; And try once more to boot...
.norge: jmp short .norge ; If int 19h returned; this is the end
;
; mangle_name: Mangle a DOS filename pointed to by DS:SI into a buffer pointed
; to by ES:DI; ends on encountering any whitespace
;
mangle_name:
mov cx,11 ; # of bytes to write
mn_loop:
lodsb
cmp al,' ' ; If control or space, end
jna mn_end
cmp al,'.' ; Period -> space-fill
je mn_is_period
cmp al,'a'
jb mn_not_lower
cmp al,'z'
ja mn_not_uslower
sub al,020h
jmp short mn_not_lower
mn_is_period: mov al,' ' ; We need to space-fill
mn_period_loop: cmp cx,3 ; If <= 3 characters left
jbe mn_loop ; Just ignore it
stosb ; Otherwise, write a period
loop mn_period_loop ; Dec CX and (always) jump
mn_not_uslower: cmp al,ucase_low
jb mn_not_lower
cmp al,ucase_high
ja mn_not_lower
mov bx,ucase_tab-ucase_low
cs xlatb
mn_not_lower: stosb
loop mn_loop ; Don't continue if too long
mn_end:
mov al,' ' ; Space-fill name
rep stosb ; Doesn't do anything if CX=0
ret ; Done
;
; Upper-case table for extended characters; this is technically code page 865,
; but code page 437 users will probably not miss not being able to use the
; cent sign in kernel images too much :-)
;
; The table only covers the range 129 to 164; the rest we can deal with.
;
ucase_low equ 129
ucase_high equ 164
ucase_tab db 154, 144, 'A', 142, 'A', 143, 128, 'EEEIII'
db 142, 143, 144, 146, 146, 'O', 153, 'OUUY', 153, 154
db 157, 156, 157, 158, 159, 'AIOU', 165
;
; unmangle_name: Does the opposite of mangle_name; converts a DOS-mangled
; filename to the conventional representation. This is needed
; for the BOOT_IMAGE= parameter for the kernel.
; NOTE: A 13-byte buffer is mandatory, even if the string is
; known to be shorter.
;
; DS:SI -> input mangled file name
; ES:DI -> output buffer
;
; On return, DI points to the first byte after the output name,
; which is set to a null byte.
;
unmangle_name:
push si ; Save pointer to original name
mov cx,8
mov bp,di
un_copy_body: lodsb
call lower_case
stosb
cmp al,' '
jbe un_cb_space
mov bp,di ; Position of last nonblank+1
un_cb_space: loop un_copy_body
mov di,bp
mov al,'.' ; Don't save
stosb
mov cx,3
un_copy_ext: lodsb
call lower_case
stosb
cmp al,' '
jbe un_ce_space
mov bp,di
un_ce_space: loop un_copy_ext
mov di,bp
mov byte [es:di], 0
pop si
ret
;
; lower_case: Lower case a character in AL
;
lower_case:
cmp al,'A'
jb lc_ret
cmp al,'Z'
ja lc_1
or al,20h
ret
lc_1: cmp al,lcase_low
jb lc_ret
cmp al,lcase_high
ja lc_ret
push bx
mov bx,lcase_tab-lcase_low
cs xlatb
pop bx
lc_ret: ret
;
; getfssec_edx: Get multiple sectors from a file
;
; This routine makes sure the subtransfers do not cross a 64K boundary,
; and will correct the situation if it does, UNLESS *sectors* cross
; 64K boundaries.
;
; ES:BX -> Buffer
; EDX -> Current sector number
; CX -> Sector count (0FFFFh = until end of file)
; Must not exceed the ES segment
; Returns EDX=0, CF=1 on EOF (not necessarily error)
; All arguments are advanced to reflect data read.
;
getfssec_edx:
push ebp
push eax
.getfragment:
xor ebp,ebp ; Fragment sector count
push edx ; Starting sector pointer
.getseccnt:
inc bp
dec cx
jz .do_read
xor eax,eax
mov ax,es
shl ax,4
add ax,bx ; Now AX = how far into 64K block we are
not ax ; Bytes left in 64K block
inc eax
shr eax,SECTOR_SHIFT ; Sectors left in 64K block
cmp bp,ax
jnb .do_read ; Unless there is at least 1 more sector room...
mov eax,edx ; Current sector
inc edx ; Predict it's the linearly next sector
call nextsector
jc .do_read
cmp edx,eax ; Did it match?
jz .getseccnt
.do_read:
pop eax ; Starting sector pointer
call getlinsecsr
lea eax,[eax+ebp-1] ; This is the last sector actually read
shl bp,9
add bx,bp ; Adjust buffer pointer
call nextsector
jc .eof
mov edx,eax
and cx,cx
jnz .getfragment
.done:
pop eax
pop ebp
ret
.eof:
xor edx,edx
stc
jmp .done
;
; getfssec: Get multiple sectors from a file
;
; Same as above, except SI is a pointer to a open_file_t
;
; ES:BX -> Buffer
; DS:SI -> Pointer to open_file_t
; CX -> Sector count (0FFFFh = until end of file)
; Must not exceed the ES segment
; Returns CF=1 on EOF (not necessarily error)
; All arguments are advanced to reflect data read.
;
getfssec:
push edx
movzx edx,cx
cmp edx,[si+4]
jbe .sizeok
mov edx,[si+4]
mov cx,dx
.sizeok:
sub [si+4],edx
mov edx,[si]
call getfssec_edx
mov [si],edx
pop edx
ret
;
; nextcluster: Advance a cluster pointer in EDI to the next cluster
; pointed at in the FAT tables. CF=0 on return if end of file.
;
nextcluster:
jmp strict short nextcluster_fat28 ; This gets patched
nextcluster_fat12:
push eax
push edx
push bx
push cx
push si
mov edx,edi
shr edi,1
pushf ; Save the shifted-out LSB (=CF)
add edx,edi
mov eax,edx
shr eax,9
call getfatsector
mov bx,dx
and bx,1FFh
mov cl,[gs:si+bx]
inc edx
mov eax,edx
shr eax,9
call getfatsector
mov bx,dx
and bx,1FFh
mov ch,[gs:si+bx]
popf
jnc .even
shr cx,4
.even: and cx,0FFFh
movzx edi,cx
cmp di,0FF0h
pop si
pop cx
pop bx
pop edx
pop eax
ret
;
; FAT16 decoding routine.
;
nextcluster_fat16:
push eax
push si
push bx
mov eax,edi
shr eax,SECTOR_SHIFT-1
call getfatsector
mov bx,di
add bx,bx
and bx,1FEh
movzx edi,word [gs:si+bx]
cmp di,0FFF0h
pop bx
pop si
pop eax
ret
;
; FAT28 ("FAT32") decoding routine.
;
nextcluster_fat28:
push eax
push si
push bx
mov eax,edi
shr eax,SECTOR_SHIFT-2
call getfatsector
mov bx,di
add bx,bx
add bx,bx
and bx,1FCh
mov edi,dword [gs:si+bx]
and edi,0FFFFFFFh ; 28 bits only
cmp edi,0FFFFFF0h
pop bx
pop si
pop eax
ret
;
; nextsector: Given a sector in EAX on input, return the next sector
; of the same filesystem object, which may be the root
; directory or a cluster chain. Returns EOF.
;
; Assumes CS == DS.
;
nextsector:
push edi
push edx
mov edx,[DataArea]
mov edi,eax
sub edi,edx
jae .isdata
; Root directory
inc eax
cmp eax,edx
cmc
jmp .done
.isdata:
not edi
test edi,[ClustMask]
jz .endcluster
; It's not the final sector in a cluster
inc eax
jmp .done
.endcluster:
push gs ; nextcluster trashes gs
push cx
not edi
mov cl,[ClustShift]
shr edi,cl
add edi,2
; Now EDI contains the cluster number
call nextcluster
cmc
jc .exit ; There isn't anything else...
; New cluster number now in EDI
sub edi,2
shl edi,cl ; CF <- 0, unless something is very wrong
lea eax,[edi+edx]
.exit:
pop cx
pop gs
.done:
pop edx
pop edi
ret
;
; getfatsector: Check for a particular sector (in EAX) in the FAT cache,
; and return a pointer in GS:SI, loading it if needed.
;
; Assumes CS == DS.
;
getfatsector:
add eax,[FAT] ; FAT starting address
jmp getcachesector
; -----------------------------------------------------------------------------
; Common modules
; -----------------------------------------------------------------------------
%include "getc.inc" ; getc et al
%include "conio.inc" ; Console I/O
%include "plaincon.inc" ; writechr
%include "writestr.inc" ; String output
%include "parseconfig.inc" ; High-level config file handling
%include "parsecmd.inc" ; Low-level config file handling
%include "bcopy32.inc" ; 32-bit bcopy
%include "loadhigh.inc" ; Load a file into high memory
%include "font.inc" ; VGA font stuff
%include "graphics.inc" ; VGA graphics
%include "highmem.inc" ; High memory sizing
%include "strcpy.inc" ; strcpy()
%include "cache.inc" ; Metadata disk cache
%include "gfxboot.inc" ; add gfx things
; -----------------------------------------------------------------------------
; Begin data section
; -----------------------------------------------------------------------------
section .data
;
; Lower-case table for codepage 865
;
lcase_low equ 128
lcase_high equ 165
lcase_tab db 135, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138
db 139, 140, 141, 132, 134, 130, 145, 145, 147, 148, 149
db 150, 151, 152, 148, 129, 155, 156, 155, 158, 159, 160
db 161, 162, 163, 164, 164
copyright_str db ' Copyright (C) 1994-', year, ' H. Peter Anvin'
db CR, LF, 0
boot_prompt db 'boot: ', 0
wipe_char db BS, ' ', BS, 0
err_notfound db 'Could not find kernel image: ',0
err_notkernel db CR, LF, 'Invalid or corrupt kernel image.', CR, LF, 0
err_noram db 'It appears your computer has less than '
asciidec dosram_k
db 'K of low ("DOS")'
db CR, LF
db 'RAM. Linux needs at least this amount to boot. If you get'
db CR, LF
db 'this message in error, hold down the Ctrl key while'
db CR, LF
db 'booting, and I will take your word for it.', CR, LF, 0
err_badcfg db 'Unknown keyword in syslinux.cfg.', CR, LF, 0
err_noparm db 'Missing parameter in syslinux.cfg.', CR, LF, 0
err_noinitrd db CR, LF
err_noinitrda db 'Could not find ramdisk image: ', 0
err_nohighmem db 'Not enough memory to load specified kernel.', CR, LF, 0
err_highload db CR, LF, 'Kernel transfer failure.', CR, LF, 0
err_oldkernel db 'Cannot load a ramdisk with an old kernel image.'
db CR, LF, 0
err_notdos db ': attempted DOS system call', CR, LF, 0
err_comlarge db 'COMBOOT image too large.', CR, LF, 0
err_a20 db CR, LF, 'A20 gate not responding!', CR, LF, 0
err_bootfailed db CR, LF, 'Boot failed: please change disks and press '
db 'a key to continue.', CR, LF, 0
err_failed_gfx db 'Error reading from disk.', 0
ready_msg db 'Ready.', CR, LF, 0
localboot_msg db 'Booting from local disk...', CR, LF, 0
crlfloading_msg db CR, LF
loading_msg db 'Loading ', 0
dotdot_msg db '.'
dot_msg db '.', 0
aborted_msg db ' aborted.' ; Fall through to crlf_msg!
crlf_msg db CR, LF
null_msg db 0
crff_msg db CR, FF, 0
syslinux_cfg db 'SYSLINUXCFG' ; Mangled form
ConfigName db 'syslinux.cfg',0 ; Unmangled form
%if IS_MDSLINUX
manifest db 'MANIFEST '
%endif
;
; Command line options we'd like to take a look at
;
; mem= and vga= are handled as normal 32-bit integer values
initrd_cmd db 'initrd='
initrd_cmd_len equ 7
;
; Config file keyword table
;
%include "keywords.inc"
;
; Extensions to search for (in *forward* order).
;
exten_table: db 'CBT',0 ; COMBOOT (specific)
db 'BSS',0 ; Boot Sector (add superblock)
db 'BS ',0 ; Boot Sector
db 'COM',0 ; COMBOOT (same as DOS)
db 'C32',0 ; COM32
exten_table_end:
dd 0, 0 ; Need 8 null bytes here
;
; Misc initialized (data) variables
;
%ifdef debug ; This code for debugging only
debug_magic dw 0D00Dh ; Debug code sentinel
%endif
alignb 4, db 0
BufSafe dw trackbufsize/SECTOR_SIZE ; Clusters we can load into trackbuf
BufSafeSec dw trackbufsize/SECTOR_SIZE ; = how many sectors?
BufSafeBytes dw trackbufsize ; = how many bytes?
EndOfGetCBuf dw getcbuf+trackbufsize ; = getcbuf+BufSafeBytes
%ifndef DEPEND
%if ( trackbufsize % SECTOR_SIZE ) != 0
%error trackbufsize must be a multiple of SECTOR_SIZE
%endif
%endif
