Why is my simple `main` program's ELF header say it's a `DYN (Shared object file)` instead of an executable? [duplicate]

Question

Here's a very simple C++ program:

// main.cpp
int main() {}

My Makefile generates the following command to compile the program.

❯ make
g++ -O0 -fverbose-asm  -o main main.cpp

I check with the command file to see it's an ELF executable:

❯ file main
main: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=921d352e49a0e4262aece7e72418290189520782, for GNU/Linux 3.2.0, not stripped

Everything seems fine until I try to inspect the ELF header:

❯ readelf -e main
ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 
...
  Type:                              DYN (Shared object file)

From wikipedia here, it appears that there are different types of files such as EXEC. Why does it say my simple main program is a shared object and not an executable on the ELF header?

From the limited scope of knowledge I have on .so's, I thought they were libraries that is linked but not loaded until runtime. How does that make sense in this context?

Extra information:

❯ g++ --version
g++ (Arch Linux 9.3.0-1) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.

❯ readelf --version
GNU readelf (GNU Binutils) 2.34
Copyright (C) 2020 Free Software Foundation, Inc.

❯ lsb_release -a
LSB Version:    1.4
Distributor ID: Arch
Description:    Arch Linux
Release:    rolling
Codename:   n/a

Henriettahenriette

I get Type EXEC (Executable file) with g++ 9.3.1 and readelf 2.32-31.fc31 – Ovary

Answer 1

Executables that are as compiled as "position independent executables" (with -pie/-fPIE) should be relocated to a random address at runtime. To achieve this, they use the DYN type.

Your version of g++ was configured with --enable-default-pie, so it sets -pie and -fPIE by default. You can disable this, and generate a normal executable, by linking with -no-pie.

Jubal

Thank you! I just checked: Configured with: /build/gcc/src/gcc/configure ... --enable-default-pie .... You were right. Still strange in my opinion that all executables are by default DYN type. Wouldn't there be some performance penalties by enabling pie always? – Henriettahenriette

@Henriettahenriette Yes, there is some performance impact. However, most applications spend quite a bit of their runtime in shared libraries anyway which are already position independent. There are security benefits to randomizing the load address of the main program, so many distros take the small perf impact and just build everything position independent. – Jubal

The perf impact is actually quite negligible for modern architectures (x86-64), while the security impact of ASLR for binaries has much more value, which is why most distros moved in that direction. – Diggings