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

Make sense, as discussed on patch number one, I will write custom
assembly for each architecture when BUG_ON is not required. I will try
to use macros to make it easier to read.

On Wed, Mar 8, 2017 at 2:05 PM, Nicolas Pitre [off-list ref] wrote:

On Wed, 8 Mar 2017, Thomas Garnier wrote:

quoted

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

-- 
Thomas