gcc -fno-stack-protector is not working, how to fix it?

November 21, 2023

I am trying to mock a C stack overflow using strcpy like this:

#include <string.h>
int main() {
    char str[10];
    strcpy(str, "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA");
    return 0;
}

and I try to compile the file using -fno-stack-protector to avoid compile errors:

$ gcc -ggdb -mpreferred-stack-boundary=4 -fno-stack-protector -o overflow overflow.c

but I failed and got this error:

overflow.c: In function ‘main’:
overflow.c:4:2: warning: ‘__builtin_memcpy’ writing 36 bytes into a region of size 10 overflows the destination [-Wstringop-overflow=]
    4 |  strcpy(str, "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA");
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the gcc version output for reference:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 9.4.0-1ubuntu1~20.04.2' --with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,gm2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-9 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-9-9QDOt0/gcc-9-9.4.0/debian/tmp-nvptx/usr,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.2)

Why is the "-fno-stack-protector" option not working?
I want the code compiling without any errors.

I will be so happy if anyone could help me out of here.

Love and respect to you.

>Solution :

That’s a warning from the compiler because it detects the overflow at compile time. Stack protector operates when you run the program. To suppress the warning use the -Wno-stringop-overflow compilation option.