On Wed, 8 Mar 2017, Thomas Garnier wrote:

> Implement specific usage of verify_pre_usermode_state for user-mode
> returns for arm.
> ---
> Based on next-20170308
> ---
>  arch/arm/Kconfig               | 1 +
>  arch/arm/kernel/entry-common.S | 5 +++++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 0d4e71b42c77..704fd8f197fa 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -12,6 +12,7 @@ config ARM
>       select ARCH_HAVE_CUSTOM_GPIO_H
>       select ARCH_HAS_GCOV_PROFILE_ALL
>       select ARCH_MIGHT_HAVE_PC_PARPORT
> +     select ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
>       select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
>       select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
>       select ARCH_SUPPORTS_ATOMIC_RMW
> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> index eb5cd77bf1d8..80cfdc7fabde 100644
> --- a/arch/arm/kernel/entry-common.S
> +++ b/arch/arm/kernel/entry-common.S
> @@ -39,6 +39,9 @@
>  ret_fast_syscall:
>   UNWIND(.fnstart     )
>   UNWIND(.cantunwind  )
> +     push    {r0}                            @ save returned r0
> +     bl      verify_pre_usermode_state
> +     pop     {r0}                            @ restore r0
>       disable_irq_notrace                     @ disable interrupts
>       ldr     r1, [tsk, #TI_FLAGS]            @ re-check for syscall tracing
>       tst     r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK

This feature is configurable, right?

Here the branch overhead is imposed even if the feature is configured 
out. You should consider conditionally defining a macro like some other 
features do.

Furthermore I think we still support old toolchains that don't know what 
push and pop mean. You should use the legacy syntax instead.


Nicolas

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