CODEWITHSUNDEEP
cgccinline-assembly
ivaigult
ivaigult

Reputation: 6667

GCC inline assembly as binary array

Is there any way in GCC to represent inline __asm__ as char[] array? I want to have something like:

void my_func();

char my_code[] = {
    __asm__("callq %0" :: "r" (my_func))
};

Later my_code will be used as run-time patch, i.e.

void another_function();
mprotect(another_function, getpagesize(), PROT_WRITE | PROT_READ | PROT_EXEC);
memcpy(another_function + offset, my_code, sizeof(my_code));

Any ideas?

Upvotes: 0

Views: 172

Answers (1)

KamilCuk
KamilCuk

Reputation: 140970

You can just define a function, compile it, then get it's source machine code?

#include <stdio.h>
#include <stdint.h>
#include <stddef.h>

void my_func(void) {}

extern void my_code(void);
extern void my_code_end(void);

__attribute__((__used__)) static void _my_code(void) {
        asm volatile(
                ".globl my_code\n"
                "my_code:\n"
                "  callq *%0\n"
                "  nop\n"
                "  ret\n"
                ".globl my_code_end\n"
                "my_code_end:\n"
                :: "r" (my_func)
        );
}

int main() {
        size_t my_code_len = (uintptr_t)my_code_end - (uintptr_t)my_code;
        const unsigned char *arr = (const char*)my_code;
        printf("my_code[%zu]=", my_code_len);
        for (size_t i = 0; i < my_code_len; ++i) {
                printf("%02x", arr[i]);
        }
        printf("\n");
        return 0;
}

Example output:

my_code[4]=ffd090c3

We can check it's ok from the assembly output:

$ objdump -D ./a.out
...
0000000000000727 <my_code>:
 727:   ff d0                   callq  *%rax
 729:   90                      nop
 72a:   c3                      retq   
...

Upvotes: 2

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