[PATCH] ARM: entry: omit FP emulation for UND exceptions taken in kernel mode

[Dmitry Osipenko]

26.10.2020 17:35, Ard Biesheuvel пишет:
> There are a couple of problems with the exception entry code that deals
> with FP exceptions (which are reported as UND exceptions) when building
> the kernel in Thumb2 mode:
> - the conditional branch to vfp_kmode_exception in vfp_support_entry()
>   may be out of range for its target, depending on how the linker decides
>   to arrange the sections;
> - when the UND exception is taken in kernel mode, the emulation handling
>   logic is entered via the 'call_fpe' label, which means we end up using
>   the wrong value/mask pairs to match and detect the NEON opcodes.
> 
> Since UND exceptions in kernel mode are unlikely to occur on a hot path
> (as opposed to the user mode version which is invoked for VFP support
> code and lazy restore), we can use the existing undef hook machinery for
> any kernel mode instruction emulation that is needed, including calling
> the existing vfp_kmode_exception() routine for unexpected cases. So drop
> the call to call_fpe, and instead, install an undef hook that will get
> called for NEON and VFP instructions that trigger an UND exception in
> kernel mode.
> 
> Cc: Russell King - ARM Linux admin <linux at armlinux.org.uk>
> Cc: Linus Walleij <linus.walleij at linaro.org>
> Cc: Dmitry Osipenko <digetx at gmail.com>
> Cc: Kees Cook <keescook at chromium.org>
> Cc: Nick Desaulniers <ndesaulniers at google.com>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
> Related discussion here:
> https://lore.kernel.org/linux-arm-kernel/20201021225737.739-1-digetx@gmail.com/

I think yours original patch with the fixes tag is still needed, hence
it should be two patches: 1) fixes the original problem 2) makes the
improvement.