Add -fzero-init-padding-bits to list of recommended options

Signed-off-by: Thomas Nyman <[email protected]>

Author: thomasnyman

docs/Compiler-Hardening-Guides/Compiler-Options-Hardening-Guide-for-C-and-C++.md

@@ -235,6 +235,7 @@ Table 2: Recommended compiler options that enable run-time protection mechanisms
 | [`-fexceptions`](#-fexceptions)                                                           | GCC 2.95.3<br/>Clang 2.6.0           | Enable exception propagation to harden multi-threaded C code                                 |
 | [`-fhardened`](#-fhardened)                                                               | GCC 14.0.0                           | Enable pre-determined set of hardening options in GCC                                        |
 | [`-Wl,--as-needed`](#-Wl,--as-needed)<br/>[`-Wl,--no-copy-dt-needed-entries`](#-Wl,--no-copy-dt-needed-entries) | Binutils 2.20.0 | Allow linker to omit libraries specified on the command line to link against if they are not used |
+| [`-fzero-init-padding-bits=all`](#-fzero-init-padding-bits=all)                           | GCC 15.0.0                            | Guarantee zero initialization of padding bits in all automatic variable initializers |
 
 [^Guelton20]: The implementation of `-D_FORTIFY_SOURCE={1,2,3}` in the GNU libc (glibc) relies heavily on implementation details within GCC. Clang implements its own style of fortified function calls (originally introduced for Android’s bionic libc) but as of Clang / LLVM 14.0.6 incorrectly produces non-fortified calls to some glibc functions with `_FORTIFY_SOURCE` . Code set to be fortified with Clang will still compile, but may not always benefit from the fortified function variants in glibc. For more information see: Guelton, Serge, [Toward _FORTIFY_SOURCE parity between Clang and GCC. Red Hat Developer](https://developers.redhat.com/blog/2020/02/11/toward-_fortify_source-parity-between-clang-and-gcc), Red Hat Developer, 2020-02-11 and Poyarekar, Siddhesh, [D91677 Avoid simplification of library functions when callee has an implementation](https://reviews.llvm.org/D91677), LLVM Phabricator, 2020-11-17.
 
@@ -1154,6 +1155,25 @@ In rare cases applications may link to libraries solely for the purpose of runni
 
 ---
 
+### Guarantee zero initialization of padding bits in automatic variable initializers
+
+| Compiler Flag                                                                       | Supported since | Description                                                                                    |
+|:----------------------------------------------------------------------------------- |:---------------:|:---------------------------------------------------------------------------------------------- |
+| <span id="-fzero-init-padding-bits=all">`-fzero-init-padding-bits=all`</span>       | GCC 15.0.0      | Guarantee zero initialization of padding bits in all automatic variable initializers           |
+| <span id="-fzero-init-padding-bits=union">`-fzero-init-padding-bits=union`</span>   | GCC 15.0.0      | Guarantee zero initialization of padding bits in unions on top of what the standards guarantee |
+
+#### Synopsis
+
+The `-fzero-init-padding-bits=all` option in GCC guarantees zero initialization of padding bits of structures and unions and reduces the risk that an incomplete initialization reveals sensitive information.
+
+We recommend the use of `{}` initializers for unions as this is a standards-compliant way to zero-initialize a whole union, including its paddings bits, in the C23 and C++ standards. However, older code may use a different approach, some compilers may not support it, and developers sometimes make mistakes, so this option reduces the risk of such problems.
+
+Since version 15, GCC adopted a stricter standards-compliant implementation of struct and union initializers.[^gcc-release-notes-15] Until GCC 14, `{0}` initializers would zero-initialize the whole union. With GCC 15 and later `{0}` initializer in C or C++ for unions no longer guarantees clearing of the whole union (except for static storage duration initialization). Only the first union member is initialized to zero. The `-fzero-init-padding-bits=unions` option in GCC 15 restores the old GCC behavior.
+
+[^gcc-release-notes-15]: GCC team, [GCC 15 Release Series Changes, New Features, and Fixes](https://gcc.gnu.org/gcc-15/changes.html), 2025-05-30.
+
+---
+
 ## Discouraged Compiler Options
 
 This section describes discouraged compiler and linker option flags that may lead to potential defects with security implications in produced binaries.