Segfault on movq instruction?

问题

Consider the following short program.

int main(){                      
    asm("movq 0x5F5E100, %rcx;"  
            "startofloop: ; "    
            "sub 0x1, %rcx; "    
            "jne startofloop; ");
}                                

This program compiles fine, but when it is run, it segfaults on the initial movq instruction.

I must be missing something obvious, but I hope someone here can point it out for me.

I am running on Debian 8, with kernel 3.16.0-4-amd64, in case that is relevant.

---

For future reference, this is what the compiler generated.

main:                                                                  
.LFB0:                                                                 
    .cfi_startproc                                                     
    pushq   %rbp                                                       
    .cfi_def_cfa_offset 16                                             
    .cfi_offset 6, -16                                                 
    movq    %rsp, %rbp                                                 
    .cfi_def_cfa_register 6                                            
#APP                                                                   
# 2 "asm_fail.c" 1                                                     
    movq 0x5F5E100, %rcx;startofloop: ; sub 0x1, %rcx; jne startofloop;
# 0 "" 2                                                               
#NO_APP                                                                

回答1:

It turns out that it has been too long since I have written asm, and I had forgotten that one must preface immediate values with $ in AT&T syntax. I found the reminder here when double-checking AT&T syntax.

asm("movq $100000000, %rcx;"
        "startofloop: ; "
        "sub $0x1, %rcx; "
        "jne startofloop; ");

movq 0x5F5E100, %rcx (without a $ on the number) is a load from the absolute address 0x5F5E100

来源：https://stackoverflow.com/questions/37955538/segfault-on-movq-instruction