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Date: Tue, 8 Mar 2016 14:46:18 +0100
From: Borislav Petkov <bp@...en8.de>
To: Andy Lutomirski <luto@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
Brian Gerst <brgerst@...il.com>,
Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 2/2] x86/entry: Do enter_from_user_mode with IRQs off
On Sun, Mar 06, 2016 at 06:30:07PM -0800, Andy Lutomirski wrote:
> Now that slow-path syscalls always enter C before enabling
> interrupts, it's straightforward to do enter_from_user_mode before
> enabling interrupts rather than doing it as part of entry tracing.
>
> With this change, we should finally be able to retire
> exception_enter.
>
> This will also enable optimizations based on knowing that we never
> change context tracking state with interrupts on.
>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
> arch/x86/entry/common.c | 39 ++++++++++++++------------------------
> arch/x86/include/asm/thread_info.h | 5 ++++-
> 2 files changed, 18 insertions(+), 26 deletions(-)
>
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 9ef3258320ec..8e8ca0eb36be 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -38,14 +38,17 @@ static struct thread_info *pt_regs_to_thread_info(struct pt_regs *regs)
> return (struct thread_info *)(top_of_stack - THREAD_SIZE);
> }
>
> -#ifdef CONFIG_CONTEXT_TRACKING
> +#ifndef CONFIG_CONTEXT_TRACKING
> +static
> +#else
> +__visible
> +#endif
> /* Called on entry from user mode with IRQs off. */
> -__visible void enter_from_user_mode(void)
> +void enter_from_user_mode(void)
> {
> CT_WARN_ON(ct_state() != CONTEXT_USER);
> user_exit();
> }
> -#endif
I'm guessing you're relying on the compiler to optimize out everything
in the !CONFIG_CONTEXT_TRACKING case, and it seems to do that here.
However, I would do the regular thing we do for cases like that. It is
much easier and faster to read because we do use it everywhere:
#ifdef CONFIG_CONTEXT_TRACKING
/* Called on entry from user mode with IRQs off. */
__visible void enter_from_user_mode(void)
{
CT_WARN_ON(ct_state() != CONTEXT_USER);
user_exit();
}
#else
#define enter_from_user_mode() do { } while (0)
#endif
>
> static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
> {
> @@ -85,17 +88,6 @@ unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
>
> work = ACCESS_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY;
>
> -#ifdef CONFIG_CONTEXT_TRACKING
> - /*
> - * If TIF_NOHZ is set, we are required to call user_exit() before
> - * doing anything that could touch RCU.
> - */
> - if (work & _TIF_NOHZ) {
> - enter_from_user_mode();
> - work &= ~_TIF_NOHZ;
> - }
> -#endif
> -
> #ifdef CONFIG_SECCOMP
> /*
> * Do seccomp first -- it should minimize exposure of other
> @@ -354,6 +346,7 @@ __visible void do_syscall_64(struct pt_regs *regs)
> struct thread_info *ti = pt_regs_to_thread_info(regs);
> unsigned long nr = regs->orig_ax;
>
> + enter_from_user_mode();
> local_irq_enable();
>
> if (READ_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY)
> @@ -376,9 +369,9 @@ __visible void do_syscall_64(struct pt_regs *regs)
>
> #if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
> /*
> - * Does a 32-bit syscall. Called with IRQs on and does all entry and
> - * exit work and returns with IRQs off. This function is extremely hot
> - * in workloads that use it, and it's usually called from
> + * Does a 32-bit syscall. Called with IRQs on in CONTEXT_KERNEL. Does
> + * all entry and exit work and returns with IRQs off. This function is
> + * extremely hot in workloads that use it, and it's usually called from
> * do_fast_syscall_32, so forcibly inline it to improve performance.
> */
> static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
> @@ -419,6 +412,7 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
> /* Handles int $0x80 */
> __visible void do_int80_syscall_32(struct pt_regs *regs)
> {
> + enter_from_user_mode();
> local_irq_enable();
> do_syscall_32_irqs_on(regs);
> }
> @@ -441,11 +435,9 @@ __visible long do_fast_syscall_32(struct pt_regs *regs)
> */
> regs->ip = landing_pad;
>
> - /*
> - * Fetch EBP from where the vDSO stashed it.
> - *
> - * WARNING: We are in CONTEXT_USER and RCU isn't paying attention!
> - */
> + enter_from_user_mode();
> +
> + /* Fetch EBP from where the vDSO stashed it. */
> local_irq_enable();
Comment above should be here.
> if (
> #ifdef CONFIG_X86_64
> @@ -464,9 +456,6 @@ __visible long do_fast_syscall_32(struct pt_regs *regs)
> /* User code screwed up. */
> local_irq_disable();
> regs->ax = -EFAULT;
> -#ifdef CONFIG_CONTEXT_TRACKING
> - enter_from_user_mode();
> -#endif
> prepare_exit_to_usermode(regs);
> return 0; /* Keep it simple: use IRET. */
> }
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index c0778fcab06d..fbdb24e61835 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -134,7 +134,10 @@ struct thread_info {
> #define _TIF_ADDR32 (1 << TIF_ADDR32)
> #define _TIF_X32 (1 << TIF_X32)
>
> -/* work to do in syscall_trace_enter() */
> +/*
> + * work to do in syscall_trace_enter(). Also includes TIF_NOHZ for
> + * enter_from_user_mode()
> + */
> #define _TIF_WORK_SYSCALL_ENTRY \
> (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_EMU | _TIF_SYSCALL_AUDIT | \
> _TIF_SECCOMP | _TIF_SINGLESTEP | _TIF_SYSCALL_TRACEPOINT | \
> --
> 2.5.0
>
>
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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