AT&T彙編操作文件

相關的系統調用

系統調用	系統調用值	描述
open	5	打開文件
read	3	讀取文件
write	4	寫入文件
close	6	關閉文件

打開關閉文件

• 打開文件
  • EAX 存儲系統調用值
  • EBX 存儲文件名
  • ECX 存儲訪問文件的權限

c語言訪問權限	對應的數值
O_RDONLY	00
O_WRONLY	01
O_RDWR	02
O_CREATE	0100
O_EXCL	0200
O_TRUNC	01000
O_APPEND	02000
O_NONBLOCK	04000
O_SYNC	010000
O_ASYNC	020000

* EDX 存儲創建新文件是的權限
* 返回信息保存在eax

錯誤名	錯誤值	描述
EPERM	1	操作錯誤
ENOENT	2	文件不存在
EBADF	3	錯誤的文件句柄
EACCES	13	權限錯誤
EFAULT	14	錯誤的文件地址
EBUSY	16	設備忙
EEXIST	17	文件存在的
EISDIR	21	是文件目錄
EMFILE	24	太多的打開文件
EFBIG	27	文件太大
EROFS	30	只讀的文件系統
ENAMERTOOLONG	36	文件名太長

movl $5, %eax
movl $filename, %ebx
movl $0120, %ecx
movl $0644, %edx
int $0x80
test %eax, %eax
js badfile

• 關閉文件

movl filehandle %ebx
movl $6, %eax
int $0x80

向文件寫入內容

#cpuidfile.s - An example of writing data to a file
.section .data

filename：
	.asciz "cpuid.txt"
output:
	.asciz "The processor Vendor ID is `XXXXXXXXXXXX'\n"
.section .bss
	.lcomm filehandle, 4
.section .text
.globl _start
_start:
	movl $0, %eax
	cpuid
	movl $output, %edi
	movl %ebx, 28(%edi)
	movl %edx, 32(%edi)
	movl %ecx, 36(%edi)

	movl $5, %eax
	movl $filename, %ebx
	movl $01101, %ecx
	movl $0644, %edx
	int $0x80
	test %eax, %eax
	js badfile
	movl %eax, filehandle

	movl $4, %eax
	movl filehandle, %ebx
	movl $output, %ecx
	movl $42, %edx
	int $0x80
	test %eax, %eax
	js badfile

	movl $6, %eax
	movl filehandle, %eax
	int $0x80

	badfile:
		movl %eax, %ebx
		movl $1, %eax
		int $0x80

讀取文件

• EAX 讀取文件的系統調用號
• EBX 文件句柄
• ECX 內存地址
• EDX 內存大小

內存映射文件

系統調用	值
mmap	90
munmap	91
msync	144

• mmap 系統調用

	void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset);
	
	start: 將映射文件存放的內存地址
	length：映射到內存的字節長度
	prot：內存的保護設置
	flags：映射對象的類型
	fd：要映射的文件句柄
	offset：映射文件的起始地址

prot:

類型	值	描述
PROT_NONE	0	不能訪問
PROT_READ	1	讀訪問
PROT_WRITE	2	寫訪問
PROT_EXEC	4	執行權限

flag：

類型	值	描述
MAP_SHARE	1	內存映射文件可以和其他進程共享
MAP_PRIVATE	2	當前進程私有

• msync 系統調用
  int msync(const void *start, size_t length, int flags);

  flags:
  MS_ASYNC
  MS_SYNC

• munmap 系統調用
  int munmap(void *start, size_t length);

# sizefunc.s - Find the size of a file 
.section .text
.globl sizefunc
.type sizefunc, @function
sizefunc:
	pushl %ebp
	movl %esp, %ebp
	subl $8, %esp
	pushl %edi
	pushl %esi
	pushl %ebx

	movl $140, %eax
	movl 8(%ebp), %ebx
	movl $0, %ecx
	movl $0, %edx
	leal -8(%ebp), %esi
	movl $2, %edi
	int $0x80
	movl -8(%ebp), %eax

	popl %ebx
	popl %esi
	popl %edi
	movl %ebp, %esp
	popl %ebp
	ret

# convert.s - A function to convert lower case letters to upper case
.section .text
.type convert, @function
.globl convert
convert:
	pushl %ebp
	pushl %esp, %ebp
	pushl %esi
	pushl %edi

	movl 12(%ebp), %esi
	movl %esi, %edi
	movl 8(%ebp), %ecx

convert_loop:
	loadsb
	cmpb $0x61, %al
	jl skip
	cmpb $0x7a, %al
	jg skip
	subb $0x20, %al
skip
	stosb	
	loop convert_loop

	pop %edi
	pop %esi
	movl %ebp, %esp
	popl %ebp
	ret
# fileconvert.s - Memory map a file and convert it
.section .bss
	.lcomm filehandle, 4
	.lcomm size, 4
	.lcomm mappedfile, 4
.section .text
.globl _start
_start:
	# get the file name and open it in read/write
	movl %esp, %ebp
	movl$5, %eax
	movl 8(%ebp), %ebx
	movl $0102, %ecx
	movl $0644, %edx
	int $0x80
	test %eax, %eax
	js badfile
	movl %eax, filehandle

	# find the size of the file
	pushl filehandle
	call sizefunc
	movl %eax, size
	addl $4, %esp

	# map file to memory
	push $0
	pushl filehandle
	pushl $1 #MAP_SHARED
	pushl $3 #PROT_READ | PROT_WRITE
	pushl size # file size
	pushl $0 # null
	movl %esp, %ebx
	movl $90, %eax
	int $0x80
	test %eax, %eax
	js badfile
	movl %eax, mappedfile
	addl $24, %esp

	#convert the memory mapped file to all uppers
	pushl mappedfile
	pushl size
	call convert
	addl $8, %esp

	# use munmap to send the changs to the file
	movl $91, %eax
	movl mappedfile, %ebx
	movl size, %ecx
	test %eax, %eax
	jnz badfile

	# close the open file handle
	movl $6, %eax
	movl filehandle, %ebx
	int $0x80

badfile:
	movl %eax, %ebx
	movl $1, %eax
	int $0x80