What does an lea instruction after a ret instruction mean?

Question

I found x86 lea instructions in an executable file made using clang and gcc. The lea instructions are after the ret instruction as shown below.

 0x???????? <func>
   ...
   pop %ebx
   pop %ebp
   ret
   lea 0x0(%esi,%eiz,1),%esi
   lea 0x0(%edi,%eiz,1),%edi

 0x???????? <next_func>
   ...

What are these lea instructions used for? There is no jmp instruction to the lea instructions.

My environment is Ubuntu 12.04 32-bit and gcc 4.6.3.

http://stackoverflow.com/questions/1658294/whats-the-purpose-of-the-lea-instruction — Sammy, Oct 30 '15 at 03:59
I suggest to compile your C source code `foo.c` with `gcc -fverbose-asm -O -S foo.c` then look inside the generated assembler `foo.s`. I guess that the `lea` is actually some data in your code segment — Basile Starynkevitch, Oct 30 '15 at 04:00
BTW, your Ubuntu is quite old, and your GCC compiler is pathetically old. Current GCC is 5.2. — Basile Starynkevitch, Oct 30 '15 at 04:03

Jerry Coffin · Answer 1 · 2015-10-30T04:09:23.800

3

It's probably not anything--it's just padding to let the next function start at an address that's probably a multiple of at least 8 (and quite possibly 16).

Depending on the rest of the code, it's possible that it's actually a table. Some implementations of a switch statement, for example, use a constant table that's often stored in the code segment (even though, strictly speaking, it's more like data than code).

The first is a lot more likely though. As an aside, such space is often filled with 0x03 instead. This is a single-byte debug-break instruction, so if some undefined behavior results in attempting to execute that code, it immediately stops execution and breaks to the debugger (if available).

edited Oct 30 '15 at 04:09

answered Oct 30 '15 at 04:01

Jerry Coffin

476,176
80
629
1,111

Thank you very much for your understandable explanations! by rewriting the lea instruction for alignment, it's likely to be variety of things. – comboy Oct 30 '15 at 04:48
2

This instruction is merely a very long nop. – R.. GitHub STOP HELPING ICE Oct 30 '15 at 05:11

What does an lea instruction after a ret instruction mean?

1 Answers1