Differences in binary file sizes between RHEL and CentOS

CentOS maintains binary compatibility with Red Hat Enterprise Linux, so applications which run on certain versions of RHEL should be able to run without changes on analogous versions of CentOS. Recently, however, a client asked me why executable binaries from the initscripts package (which contains /bin/ipcalc, /bin/usleep, etc) on RHEL 6.X have slightly different file sizes with those from the CentOS 6.X initscripts package.

First, I needed to verify that the source code in the initscripts srpm's for RHEL and CentOS were identical.

I downloaded initscripts-9.03.46-1.el6.src.rpm for RHEL 6.6 from the Redhat partner site and I downloaded initscripts-9.03.46-1.el6.centos.src.rpm from CentOS vault at the following url:

http://vault.centos.org/6.6/os/Source/SPackages/initscripts-9.03.46-1.el6.centos.src.rpm

I then unpacked the RHEL 6.6 initscripts source rpm's as follows:

[root@localhost srpm]# rpm2cpio initscripts-9.03.46-1.el6.src.rpm | cpio -idmv
initscripts-9.03.46.tar.bz2
initscripts.spec
3146 blocks
[root@localhost srpm]# ls
initscripts-9.03.46-1.el6.src.rpm  initscripts-9.03.46.tar.bz2  initscripts.spec
[root@localhost srpm]# tar -xvf initscripts-9.03.46.tar.bz2
initscripts-9.03.46/
initscripts-9.03.46/.gitignore
initscripts-9.03.46/.tx/
initscripts-9.03.46/.tx/config
initscripts-9.03.46/COPYING
initscripts-9.03.46/Makefile
...

I also did the same for the CentOS 6.6 initscripts package. I then renamed the directories for the extracted srpm's and then used the meld GUI diff tool to compare the .../src as well as the entire extracted initscripts srpm directories for RHEL 6.6 and CentOS 6.6.

As you can see below, the contents of the srpm's are identical:

Compiler options are contained within .../src/Makefile and the options are identical, as you can see from the diff results above. So the binary size differences are not due to differences in the source code, compiler options, or rpm Specfile between RHEL and CentOS.

Next, I did a simple C program compilation test of my own using gcc on a stock installation of RHEL 6.6 and CentOS 6.6.

Here is a simple hello world one-liner I have named hello.c:

#include

int main(void)
{
  printf("hello world!\n");
}

If I compile it with gcc with the following options

gcc hello.c -O0 -std=c99 -Wall -Werror -o hello

I still get slightly different file sizes on RHEL and CentOS:

RHEL 6.6
[root@localhost pset1]# ls -al hello
-rwxr-xr-x. 1 root root 6473 Aug  9 08:31 hello

CentOS 6.6
[root@localhost pset1]# ls -al hello
-rwxr-xr-x. 1 root root 6425 Aug 11 06:15 hello

This is a difference of 48 bytes.

I then used objdump from bintools to examine the assembly code in the compile hello object files. I renamed each object file as hello_rhel66 and hello_cent66, respectively. I am using the -s option with objdump so I can see full contents that also converts hex strings to ASCII.

[fedjun@u36jcFedora Downloads]$ objdump -s hello_rhel66 > hello_rhel66.dump
[fedjun@u36jcFedora Downloads]$ objdump -s hello_cent66 > hello_cent66.dump
[fedjun@u36jcFedora Downloads]$ diff -u hello_rhel66.dump hello_cent66
hello_cent66       hello_cent66.dump  
[fedjun@u36jcFedora Downloads]$ diff -u hello_rhel66.dump hello_cent66.dump
--- hello_rhel66.dump 2016-08-13 10:07:48.893239117 +0900
+++ hello_cent66.dump 2016-08-13 10:08:02.078435160 +0900
@@ -1,5 +1,5 @@

-hello_rhel66:     file format elf64-x86-64
+hello_cent66:     file format elf64-x86-64

 Contents of section .interp:
  400200 2f6c6962 36342f6c 642d6c69 6e75782d  /lib64/ld-linux-
@@ -9,8 +9,8 @@
  40022c 00000000 02000000 06000000 12000000  ................
 Contents of section .note.gnu.build-id:
  40023c 04000000 14000000 03000000 474e5500  ............GNU.
- 40024c 4cc6b3fd d6ec9bb6 e4540da0 aba4807f  L........T......
- 40025c 0f84997f                             ....            
+ 40024c 69320cbb e7408021 2c646e86 8344b173  i2...@.!,dn..D.s
+ 40025c 5e478671                             ^G.q            
 Contents of section .gnu.hash:
  400260 01000000 01000000 01000000 00000000  ................
  400270 00000000 00000000 00000000           ............    
@@ -137,7 +137,4 @@
 Contents of section .comment:
  0000 4743433a 2028474e 55292034 2e342e37  GCC: (GNU) 4.4.7
  0010 20323031 32303331 33202852 65642048   20120313 (Red H
- 0020 61742034 2e342e37 2d313029 00474343  at 4.4.7-10).GCC
- 0030 3a202847 4e552920 342e342e 37203230  : (GNU) 4.4.7 20
- 0040 31323033 31332028 52656420 48617420  120313 (Red Hat 
- 0050 342e342e 372d3131 2900               4.4.7-11).      
+ 0020 61742034 2e342e37 2d313129 00        at 4.4.7-11).

Apparently the contents of the .interp and .comments section differ between the two binaries. I believe the same holds true for each of the individual binaries from the initscripts package on RHEL 6.6 and CentOS 6.6. Each of the object files may contain different comments and time stamps which will lead to different binary file sizes.