CODEWITHSUNDEEP
assemblyx86-64disassemblyrelative-addressing
Zhezhong Jiang
Zhezhong Jiang

Reputation: 83

lea assembly instruction syntax

Hi I am new to assembly language and I am getting confused about the syntax of lea instruction I have seen when I study a piece of code ( which is generated by using gdb command : disassemble main ).

lea    0xa8e96(%rip),%rsi        # 0x4aa5df

The syntax I have seen for lea is 

lea src, dest

But it seems like there is an additional immediate value ( # 0x4aa5df ) following the %rsi register, how should I interpret this correctly?

Edit: I have checked the value stored in the %rip register which is 

(gdb) p /x $rip 
$1 = 0x401730

So adding this with the 0xa8e96 gives me 0x4AA5C6 which does not match 0x4aa5df, am I missing something here ?

Upvotes: 2

Views: 3266

Answers (1)

Zhezhong Jiang
Zhezhong Jiang

Reputation: 83

Thanks for the help from Jester, Unn and Erik. The original C code I used is :

#include <stdio.h>

int main(int argc, char** argv)
{    
    int ret = printf("%s\n", argv[argc-1]);
    argv[0] = '\0'; // NOOP to force gcc to generate a callq instead of jmp
    return ret;
}

And the assembler code generated by using gdb is :

(gdb) disassemble main
Dump of assembler code for function main:
=> 0x0000000000401730 <+0>:     endbr64
   0x0000000000401734 <+4>:     push   %rbx
   0x0000000000401735 <+5>:     movslq %edi,%rdi
   0x0000000000401738 <+8>:     mov    %rsi,%rbx
   0x000000000040173b <+11>:    xor    %eax,%eax
   0x000000000040173d <+13>:    mov    -0x8(%rsi,%rdi,8),%rdx
   0x0000000000401742 <+18>:    lea    0xa8e96(%rip),%rsi        # 0x4aa5df
   0x0000000000401749 <+25>:    mov    $0x1,%edi
   0x000000000040174e <+30>:    callq  0x44bbe0 <__printf_chk>
   0x0000000000401753 <+35>:    movq   $0x0,(%rbx)
   0x000000000040175a <+42>:    pop    %rbx
   0x000000000040175b <+43>:    retq
End of assembler dump.

So the rip does point past the lea instructions and the address should be used in the computation is 0x0000000000401749 , adding this to 0xa8e96 gives the address in the comment # 0x4aa5df.

Upvotes: 2

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