[PATCH v1 3/4] arm/syscalls: Specific usage of verify_pre_usermode_state

Nicolas Pitre
2017-03-08T22:05:46Z

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