The kernel's CFI implementation uses the KCFI ABI specifically, and is
not strictly tied to a particular compiler. In preparation for GCC
supporting KCFI, rename CONFIG_CFI_CLANG to CONFIG_CFI (along with
associated options).

Use new "transitional" Kconfig option for old CONFIG_CFI_CLANG that will
enable CONFIG_CFI during olddefconfig.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20250923213422.1105654-3-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

Currently x86 and riscv open-code 4 instances of the same logic to
define a u32 variable with the KCFI typeid of a given function.

Replace the duplicate logic with a common macro.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Co-developed-by: Maxwell Bland <mbland@motorola.com>
Signed-off-by: Maxwell Bland <mbland@motorola.com>
Co-developed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Dao Huang <huangdao1@oppo.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250801001004.1859976-6-samitolvanen@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

With CONFIG_CFI_CLANG, assembly functions called indirectly
from C code must be annotated with type identifiers to pass CFI
checking.  In order to make this easier, the compiler emits a
__kcfi_typeid_<function> symbol for each address-taken function
declaration in C, which contains the expected type identifier that
we can refer to in assembly code.

Add a typed version of SYM_FUNC_START, which emits the type
identifier before the function. Architectures that support KCFI can
define their own __CFI_TYPE macro to override the default preamble
format.

As an example, for the x86_64 blowfish_dec_blk function, the
compiler emits the following type symbol:

$ readelf -sW vmlinux | grep __kcfi_typeid_blowfish_dec_blk
120204: 00000000ef478db5     0 NOTYPE  WEAK   DEFAULT  ABS
	__kcfi_typeid_blowfish_dec_blk

And SYM_TYPED_FUNC_START will generate the following preamble based
on the __CFI_TYPE definition for the architecture:

$ objdump -dr arch/x86/crypto/blowfish-x86_64-asm_64.o
     ...
0000000000000400 <__cfi_blowfish_dec_blk>:
     ...
     40b:       b8 00 00 00 00          mov    $0x0,%eax
                   40c: R_X86_64_32 __kcfi_typeid_blowfish_dec_blk

0000000000000410 <blowfish_dec_blk>:
     ...

Note that the address of all assembly functions annotated with
SYM_TYPED_FUNC_START must be taken in C code that's linked into the
binary or the missing __kcfi_typeid_ symbol will result in a linker
error with CONFIG_CFI_CLANG. If the code that contains the indirect
call is not always compiled in, __ADDRESSABLE(functionname) can be
used to ensure that the __kcfi_typeid_ symbol is emitted.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220908215504.3686827-7-samitolvanen@google.com