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Subjectpage 0 mapped memory in ELF binaries
Ok, I'm sure this isn't any sort of show stopper for the 2.4.0 series
(or any other series for that matter, and they probably all have it),
but when mapping memory to page 0 in the program header of an ELF,
linux completely ignores the ph_memsz field.. I've attached a program
to demonstrate.. nasm is needed for compilation... (nasm is available
at http://www.cryogen.com/Nasm/)

Just so you know, what I want to do is have x bytes at 0x0, when
only y bytes are in the file, and y will almost always be less than x..

And yes, this is a little odd, but I really want to do it ;)

--
Zinx Verituse (See headers for gpg/pgp key info)
BITS 32

;; Compile with: nasm -f bin zeromap.asm -o zeromap
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; Set this to size you want mapped, and that much will get mapped. This is
;; the ph_filesz field, the ph_memsz is already at 4096*2, but Linux seems to
;; completely ignore that when it's mapping to 0x0
%define ZERO_MAPPED_FILE_SIZE 0

;; You'll want to set this if you set the above, else you'll get bus errors,
;; due to the file not actually being as big as you told the kernel it was.
;; Note that I don't want a bigger file, just a bigger amount of memory.
%define PADDING 0

;; Define this if you want it to just loop over and over, so you can,
;; say, type "cat /proc/`pidof zeromap`/maps" and get a picture of
;; what all is happening ;)
%define INFINITE_LOOP 0

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


;;;;;;;;;;;;;;;;
;; Elf Header
ORG 0x8048000
elf_header:
.e_ident: db 0x7f, "ELF"
.e_ident_class: db 1 ;ELFCLASS32
.e_ident_encoding: db 1 ;ELFDATA2LSB
.e_ident_version: db 1 ;EV_CURRENT
.e_ident_padding: db 0, 0, 0, 0, 0, 0, 0, 0, 0

.e_type: dw 2 ;ET_EXEC
.e_machine: dw 3 ;EM_386
.e_version: dd 1 ;EV_CURRENT
.e_entry: dd main
.e_phoff: dd PROGRAM_HEADER
.e_shoff: dd 0
.e_flags: dd 0
.e_ehsize: dw elf_header_size
.e_phentsize: dw 32
.e_phnum: dw 3
.e_shentsize: dw 0
.e_shnum: dw 0
.e_shstrndx: dw 0
elf_header_size equ $ - elf_header

;;;;;;;;;;;;;;;;;;;;
;; Program Header

PROGRAM_HEADER equ $-$$
program_header_1:
.p_type: dd 1 ;PT_LOAD
.p_offset: dd main - $$
.p_vaddr: dd main
.p_paddr: dd 0
.p_filesz: dd main_size
.p_memsz: dd main_size
.p_flags: dd 1+2+4 ;PF_X + PF_R + PF_W
.p_align: dd 4

; Linux 2.4.0-test10 (at least) doesn't like this one at all.
program_header_2:
.p_type: dd 1 ;PT_LOAD
.p_offset: dd nomem - $$
.p_vaddr: dd 0
.p_paddr: dd 0
.p_filesz: dd ZERO_MAPPED_FILE_SIZE
.p_memsz: dd 4096*2 ;PAGE_SIZE*2
.p_flags: dd 2+4 ;PF_W + PF_R
.p_align: dd 4

; Yet, this one is fine... it's the exact same stuff, just mapped somewhere
; other than 0x0
program_header_3:
.p_type: dd 1 ;PT_LOAD
.p_offset: dd nomem - $$
.p_vaddr: dd nomem
.p_paddr: dd 0
.p_filesz: dd 0
.p_memsz: dd 4096*2 ;PAGE_SIZE*2
.p_flags: dd 2+4 ;PF_W + PF_R
.p_align: dd 4

main:
%if INFINITE_LOOP
jmp $
%endif

mov ebp, nomem
call try_access
mov ebp, 0x0
call try_access

mov eax, 1 ; __NR_exit
xor ebx, ebx
int 0x80 ; exit(0);

try_access:
call write_ebp_msg
xor eax, eax
.loop:
mov ecx, dword [ebp+eax*4]
inc eax
cmp eax, 0x2000
jg .loop

ret

write_ebp_msg:
mov eax, ebp
mov ebx, 16
mov ecx, 8
.loop:
xor edx, edx
div ebx
mov dl, [hex+edx]
mov [msg_try_mapped.addr+ecx-1], dl
loop .loop

mov eax, 4 ; __NR_write
mov ebx, 2
mov ecx, msg_try_mapped
mov edx, msg_try_mapped.len
int 0x80 ; write(2, msg_try_mapped, strlen(msg_try_mapped));
ret

; NOT REACHED
msg_try_mapped: db "Accessing 4096*2 bytes of mapped(?) memory at 0x"
.addr: db "00000000"
db 10
.len equ $-msg_try_mapped
hex: db "0123456789abcdef"

main_size equ $-main

align 4096, db 0 ; align to the page size
nomem:
times PADDING db 0
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