How do i write Simple inline asm instruction from C on Linux 64 bit?

Question

i am writing a simple c program and my requirement is to print RIP(Instruction Pointer) from some function of the program. i dont want to use ptrace.

the one thing i tried with inline asm is: asm("movl %%rip, %0;" : "=r"(val) ) this should copy my rip register value to variable val, but i am getting compilation error.

if i use ebp/esp which are base pointer and stack pointers for 32 bit machine, i dont get any compilation error and my val has some hexadecimal number assigned.

i have few questions here:

1) as my machine is 63 bit, how was above instruction able to read 32 bit registers?

2) why i am not able to read any register for 64 bit, is there any issue b'caz of 'r'?

3) when i use eip which is for 32 bit i get compilation error, does it mean IP registers are restricted for reading?

Stephen Canon Stephen Canon · Accepted Answer · 2010-07-27T13:47:16

Your machine is 64 bit, not 63 bit =). You were able to read the the 32-bit registers because you are compiling and running your program as a 32-bit executable[1].
You are unable to read the 64-bit registers because you are compiling and running your program as a 32-bit executable. Yes, you have a 64-bit processor, but it can still run 32-bit executables (if your OS supports it, which yours clearly does), and evidently your compiler tool chain builds 32-bit by default. If you are using gcc, try using the -m64 flag, or read the compiler documentation for more information.
Unlike rip, the eip register cannot be directly accessed. You can get the value of eip in the manner described by Jim in his answer.

[1] you would be able to read the 32-bit registers from a 64-bit executable anyway; the 32-bit registers are still available in 64-bit mode, just like you can access the 16-bit registers in 32-bit mode.

There are a few problems still in your example:

First, although rip is accessible in 64-bit mode, it's accessible as an addressing mode; it's not a normal register. If you want to load its value, you need to use LEA, not MOV.

Second, because rip is a 64-bit register, you need to use the q suffix on your instructions instead of l. Here's a sample program with these two issues addressed:

#include <stdio.h>
#include <inttypes.h>

int main(int argc, char *argv[]) {
    uint64_t ip;
    asm("leaq (%%rip), %0;": "=r"(ip));
    printf("rip is 0x%016" PRIx64 "\n", ip);
    return 0;
}

which seems to work just fine on my machine.

How do i write Simple inline asm instruction from C on Linux 64 bit?

2 Answers