linux - 自修改代码总是在 Linux 上出现段错误

Question

我找到了一篇关于自修改代码的文章并尝试做一些例子，但我总是遇到段错误。据我所知，存在内存权限违规。代码段是 (r)ead/e(x)ecute，因此写入结果的尝试导致此错误。有没有办法通过在运行时或之前更改内存权限来测试程序？我使用的是 linux，示例是用 GAS 汇编语言编写的。

.extern memcpy
.section .data
string:
        .asciz  "whatever"
string_end:
.section .bss
        .lcomm buf, string_end-string
.section .text
.globl main
main:
        call changer
        mov $string, %edx
label:
        push string_end-string
        push $buf
        push $string
        call memcpy
changer:
        mov $offset_to_write, %esi
        mov $label, %edi
        mov $0xb, %ecx
loop1:
        lodsb
        stosb
        loop loop1
        ret
offset_to_write:
        push 0
        call exit
end:

所以在 osgx 建议的修改之后，这里是一个工作代码。(实际上，如果你组装&链接&运行它会崩溃，但如果你使用 gdb 观看它会修改它的代码!)

.extern memcpy
.section .data
string:
        .asciz  "Giorgos"
string_end:
.section .bss
        .lcomm buf, string_end-string
.section .text
.globl main
main:
        lea (main), %esi                # get the start of memory region to
                                        # change its permissions (smc-enabled)
        andl $0xFFFFF000, %esi          # align to start of a pagesize
        pushl   $7                      # permissions==r|w|x
        pushl   $4096                   # page size
        pushl   %esi                    # computed start address
        call    mprotect

        call    changer                 # function that does smc
        mov     $string, %edx
label:
        push    string_end-string       # this code will be overridden
        push    $buf                    # and never be executed!
        push    $string
        call    memcpy
changer:
        mov     $offset_to_write, %esi  # simple copy bytes algorithm
        mov     $label, %edi
        mov     $0xb, %ecx
loop1:
        lodsb
        stosb
        loop    loop1
        ret
offset_to_write:                        # these instructions will be
        push    $0                      # executed eventually
        call    exit
end:

Answer 1

您应该在运行时更改内存访问权限。

#include <sys/mman.h>

void *addr  = get_address_of_instruction_pointer();
int  length = 4096;   /* size of a page */

if (mprotect(addr, length, PROT_READ | PROT_WRITE | PROT_EXEC) == 0) {
    /* current code page is now writable and code from it is allowed for execution */
}