Hardening flags enabled by default
format Adds the -Wformat -Wformat-security -Werror=format-security compiler options. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf(foo). This may be a security hole if the format string came from untrusted input and contains %n.
stackprotector Adds the -fstack-protector-strong --param ssp-buffer-size=4 compiler options. This adds safety checks against stack overwrites rendering many potential code injection attacks into aborting situations. In the best case this turns code injection vulnerabilities into denial of service or into non-issues (depending on the application).
fortify Adds the -O2 -D_FORTIFY_SOURCE=2 compiler options. During code generation the compiler knows a great deal of information about buffer sizes (where possible), and attempts to replace insecure unlimited length buffer function calls with length-limited ones. This is especially useful for old, crufty code. Additionally, format strings in writable memory that contain ‚%n‘ are blocked. If an application depends on such a format string, it will need to be worked around.
Additionally, some warnings are enabled which might trigger build failures if compiler warnings are treated as errors in the package build.
pic Adds the -fPIC compiler options. This options adds support for position independent code in shared libraries and thus making Address Space Layout Randomization (ASLR) possible.
Most notably, the Linux kernel, kernel modules and other code not running in an operating system environment like boot loaders will not build with PIC enabled. The compiler will is most cases complain that PIC is not supported for a specific build.
strictoverflow Signed integer overflow is undefined behaviour according to the C standard. If it happens, it is an error in the program as it should check for overflow before it can happen, not afterwards. GCC provides built-in functions to perform arithmetic with overflow checking, which are correct and faster than any custom implementation. As a workaround, the option -fno-strict-overflow makes gcc behave as if signed integer overflows were defined.
This flag should not trigger any build or runtime errors.
relro Adds the -z relro linker option. During program load, several ELF memory sections need to be written to by the linker, but can be turned read-only before turning over control to the program. This prevents some GOT (and .dtors) overwrite attacks, but at least the part of the GOT used by the dynamic linker (.got.plt) is still vulnerable. This flag can break dynamic shared object loading.
For instance, the module systems of Xorg and OpenCV are incompatible with this flag. In almost all cases the bindnow flag must also be disabled and incompatible programs typically fail with similar errors at runtime.
bindnow Adds the -z bindnow linker option. During program load, all dynamic symbols are resolved, allowing for the complete GOT to be marked read-only (due to relro). This prevents GOT overwrite attacks. For very large applications, this can incur some performance loss during initial load while symbols are resolved, but this should not be an issue for daemons.This flag can break dynamic shared object loading.
For instance, the module systems of Xorg and PHP are incompatible with this flag. Programs incompatible with this flag often fail at runtime due to missing symbols
Hardening flags disabled by default
pie Adds the -fPIE compiler and -pie linker options. Position Independent Executables (PIE) are needed to take advantage of Address Space Layout Randomization (ASLR), supported by modern kernel versions. While ASLR can already be enforced for data areas in the stack and heap (brk and mmap), the code areas must be compiled as position-independent. Shared libraries already do this with the pic flag, so they gain ASLR automatically, but binary .text regions need to be build with pie to gain ASLR. When this happens, ROP attacks are much harder since there are no static locations to bounce off of during a memory corruption attack.
from https://blog.mayflower.de/5800-Hardening-Compiler-Flags-for-NixOS.html