Introduce a Kconfig option for enabling the experimental option to
normalize integer types. This ensures that integer types of the same
size and signedness are considered compatible by the Control Flow
Integrity sanitizer.

The security impact of this flag is minimal. When Sami Tolvanen looked
into it, he found that integer normalization reduced the number of
unique type hashes in the kernel by ~1%, which is acceptable.

This option exists for compatibility with Rust, as C and Rust do not
have the same set of integer types. There are cases where C has two
different integer types of the same size and signedness, but Rust only
has one integer type of that size and signedness. When Rust calls into
C functions using such types in their signature, this results in CFI
failures. One example is 'unsigned long long' and 'unsigned long' which
are both 64-bit on LP64 targets, so on those targets this flag will give
both types the same CFI tag.

This flag changes the ABI heavily. It is not applied automatically when
CONFIG_RUST is turned on to make sure that the CONFIG_RUST option does
not change the ABI of C code. For example, some build may need to make
other changes atomically with toggling this flag. Having it be a
separate option makes it possible to first turn on normalized integer
tags, and then later turn on CONFIG_RUST.

Similarly, when turning on CONFIG_RUST in a build, you may need a few
attempts where the RUST=y commit gets reverted a few times. It is
inconvenient if reverting RUST=y also requires reverting the changes you
made to support normalized integer tags.

To avoid having this flag impact builds that don't care about this, the
next patch in this series will make CONFIG_RUST turn on this option
using `select` rather than `depends on`.
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Gatlin Newhouse <gatlin.newhouse@gmail.com>
Acked-by: Kees Cook <kees@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240801-kcfi-v2-1-c93caed3d121@google.comSigned-off-by: Miguel Ojeda <ojeda@kernel.org>