Re: [PATCH v3] ARM: traps: Implement KCFI trap handler for ARM32

[Linus Walleij]

On Wed, Jul 15, 2026 at 7:52 PM Kees Cook <kees@xxxxxxxxxx> wrote:

> ARM32 GCC KCFI[1] violations currently show as generic "Oops - undefined
> instruction" errors, making debugging CFI failures difficult. Add a proper
> KCFI trap handler similar to the aarch64 implementation to provide clear
> CFI error messages, including the call target and the expected type.

Reviewed-by: Linus Walleij <linusw@xxxxxxxxxx>

> Clang and GCC trap CFI failures differently on ARM32. Clang lowers
> its checks to a BKPT, handled via the breakpoint/prefetch-abort path
> in hw_breakpoint.c, which cannot recover the target or expected type
> and so must report via report_cfi_failure_noaddr(). GCC instead lowers
> KCFI checks to a UDF (undefined instruction) whose immediate encodes
> the registers involved, so the handler can decode it and call the full
> report_cfi_failure() with the target and expected type.

Aha, that explains a few things.

The UDF is a better approach for ARM I think (Russell will know best).

As noted from earlier conversation the breakpoint usecase overloading
on ARM is a mess, and we would certainly like to avoid it if possible,
albeit I have a patch in Russell's patch tracker to paper it over.

Do you think that long term we could switch Clang to do what
GCC does?

Yours,
Linus Walleij