Question

I am writing a simple C program that I’m supposed to attack with a buffer overflow. So, I don’t want any flags used when compiling. How do I eliminate the default flags from being used?

# readelf -p .GCC.command.line stack

String dump of section '.GCC.command.line':
  [     0]  stack.c
  [     8]  -D_FORTIFY_SOURCE=2
  [    1c]  -mtune=generic
  [    2b]  -march=i486
  [    37]  -frecord-gcc-switches
  [    4d]  -fstack-protector

Answer 1

The only two options there that make any sense to override are -D_FORTIFY_SOURCE and -fstack-protector, so just include -U_FORTIFY_SOURCE and -fnostack-protector, and they’re effectively “gone”.

You can’t compile for “no architecture” (march), and getting rid of mtune is similarly meaningless. GCC has to build code for something. You’re already about as generic as you can get there.

-frecord-gcc-switches, well, without that, you wouldn’t know what switches were included in the first place, but if you really want to get rid of it, just use -fnorecord-gcc-switches.

-f* options are mostly boolean flags, so you can almost always turn them off by adding no at the front as I’ve done above.