how to access and change char array in c x86 AT&T inline assembly

Question

I'm having trouble solving a school exercise , I'm supposed to change a char array in c using inline assembly. In this case change "ahoy" to "aXoy", but I'm getting segmentation fault. This is my code:

 #include <stdio.h>

    int main() {
        char string[] = "ahoy";
        __asm__ volatile (
                "mov %0, %%eax;"
                "movb $'X', 1(%%eax);"
                : "=m"(string) : "0"(string) : "memory", "eax");
        printf("%s\n", string);
        return 0
    }

with this: "mov %0, %%eax;" I'm trying to store address of the array in register

then with this: "movb $'X', 1(%%eax);" I want to store byte 'X' in location pointed to by (%%eax) offset by 1 byte(char),

I have string both as output and input, and "memory","eax" in clobber since I'm modifying both. What is wrong with my code?

If you want to use string as input AND output then mark it as "+m" instead of "=m"and then drop the "0"(string) altogether. This line mov %0, %%eax; will move the 32-bit value AT the memory address to EAX since %0 is a memory operand. You want to put the ADDRESS of %0 in EAX it appears. Use lea %0, %%eax; instead of mov %0, %%eax; — Michael Petch
Secondly: Are you compiling this as 64-bit or 32-bit code? The inline assembly as is using a 32-bit register with a stack based address (string) will fault since the stack address can't be represented properly in a 32-bit value. Make sure you compile this with -m32 or modify the code to use a 64-bit register for the address (RAX instead of EAX) — Michael Petch
__asm__ volatile ( "lea %0, %%eax;" "movb $'X', 1(%%eax);" : "+m"(string) :: "memory", "eax"); — Michael Petch
The + modifier has been around for almost forever. They are all documented here: gcc.gnu.org/onlinedocs/gcc/Modifiers.html#Modifiers — Michael Petch
Technically speaking in this case using +m you don't need to use the memory clobber since the compiler will realize all of the string in memory first since you are using an m constraint. Alternatively you could have done __asm__ volatile ( "movb $'X', 1(%0);" :: "r"(string) : "memory"); . Memory clobber would be needed since in this case you are passing the address of string through a register and not a memory operand. — Michael Petch

R.. GitHub STOP HELPING ICE R.. GitHub STOP HELPING ICE · Accepted Answer · 2018-05-19T15:31:05

Use gcc -S instead of -c to look at the compiler's assembly output and you should see what's wrong. The "m" constraint produces a memory reference expression for accessing the object associated with it, not a register containing its address. So it will expand to something like (%ecx), not %ecx, and the first mov will load 4 bytes from string, rather than the address of string, into eax.

One way to fix this would be to use a register constraint "r"(string) instead.

Alternatively you could replace the mov with lea: lea %0, %%eax.

There are other issues with your code too like the useless temporary/clobber register eax but they shouldn't keep it from working.

how to access and change char array in c x86 AT&T inline assembly

1 Answers