[PATCH v2] ARM: entry: omit FP emulation for UND exceptions taken in kernel mode

[Ard Biesheuvel]

On Tue, 24 Nov 2020 at 23:59, Dmitry Osipenko <digetx at gmail.com> wrote:
>
> 18.11.2020 16:08, Ard Biesheuvel пишет:
> > diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> > index c4220f51fcf3..0ea8529a4872 100644
> > --- a/arch/arm/kernel/entry-armv.S
> > +++ b/arch/arm/kernel/entry-armv.S
> > @@ -252,31 +252,10 @@ __und_svc:
> >  #else
> >       svc_entry
> >  #endif
> > -     @
> > -     @ call emulation code, which returns using r9 if it has emulated
> > -     @ the instruction, or the more conventional lr if we are to treat
> > -     @ this as a real undefined instruction
> > -     @
> > -     @  r0 - instruction
> > -     @
> > -#ifndef CONFIG_THUMB2_KERNEL
> > -     ldr     r0, [r4, #-4]
> > -#else
> > -     mov     r1, #2
> > -     ldrh    r0, [r4, #-2]                   @ Thumb instruction at LR - 2
> > -     cmp     r0, #0xe800                     @ 32-bit instruction if xx >= 0
> > -     blo     __und_svc_fault
> > -     ldrh    r9, [r4]                        @ bottom 16 bits
> > -     add     r4, r4, #2
> > -     str     r4, [sp, #S_PC]
> > -     orr     r0, r9, r0, lsl #16
> > -#endif
> > -     badr    r9, __und_svc_finish
> > -     mov     r2, r4
> > -     bl      call_fpe
> >
> >       mov     r1, #4                          @ PC correction to apply
> > -__und_svc_fault:
> > + THUMB(      tst     r5, #PSR_T_BIT          )       @ exception taken in Thumb mode?
> > + THUMB(      movne   r1, #2                  )       @ if so, fix up PC correction
> >       mov     r0, sp                          @ struct pt_regs *regs
> >       bl      __und_fault
>
> Am I understanding correctly that when call_fpe was invoked previously,
> it was supposed to print extra debug info about the VFP state? But it
> didn't work properly for thumb mode, correct?

call_fpe was originally called to perform emulation of any UNDEF
instruction that matched the same constraints that also apply when the
UNDEF is taken in user mode. If the UNDEF was triggered by a VFP/NEON
instruction, the VFP handler would check for kernel or user mode, and
trigger an error if the exception was taken in kernel mode.

The Thumb mode opcode matching was wrong in this case, so if a Thumb2
NEON exception triggered an UNDEF exception in kernel mode (which only
happens if there are bugs in the kernel) we would fail to identify it
as a NEON instruction.

This code removes the call into the emulation code from kernel mode
entirely, as it no longer has valid users, and the invalid ones can
simply be served by undef hooks.