Re: [tip:x86/asm] x86/entry: Add new, comprehensible entry and exit handlers written in C

Frederic Weisbecker
2015-07-14T23:07:33Z

On Tue, Jul 07, 2015 at 03:51:48AM -0700, tip-bot for Andy Lutomirski wrote:
> Commit-ID: c5c46f59e4e7c1ab244b8d38f2b61d317df90bba
> Gitweb: http://git.kernel.org/tip/c5c46f59e4e7c1ab244b8d38f2b61d317df90bba
> Author: Andy Lutomirski <luto@kernel.org>
> AuthorDate: Fri, 3 Jul 2015 12:44:26 -0700
> Committer: Ingo Molnar <mingo@kernel.org>
> CommitDate: Tue, 7 Jul 2015 10:59:06 +0200
>
> x86/entry: Add new, comprehensible entry and exit handlers written in C
>
> The current x86 entry and exit code, written in a mixture of assembly and
> C code, is incomprehensible due to being open-coded in a lot of places
> without coherent documentation.
>
> It appears to work primary by luck and duct tape: i.e. obvious runtime
> failures were fixed on-demand, without re-thinking the design.
>
> Due to those reasons our confidence level in that code is low, and it is
> very difficult to incrementally improve.
>
> Add new code written in C, in preparation for simply deleting the old
> entry code.
>
> prepare_exit_to_usermode() is a new function that will handle all
> slow path exits to user mode. It is called with IRQs disabled
> and it leaves us in a state in which it is safe to immediately
> return to user mode. IRQs must not be re-enabled at any point
> after prepare_exit_to_usermode() returns and user mode is actually
> entered. (We can, of course, fail to enter user mode and treat
> that failure as a fresh entry to kernel mode.)
>
> All callers of do_notify_resume() will be migrated to call
> prepare_exit_to_usermode() instead; prepare_exit_to_usermode() needs
> to do everything that do_notify_resume() does today, but it also
> takes care of scheduling and context tracking. Unlike
> do_notify_resume(), it does not need to be called in a loop.
>
> syscall_return_slowpath() is exactly what it sounds like: it will
> be called on any syscall exit slow path. It will replace
> syscall_trace_leave() and it calls prepare_exit_to_usermode() on the
> way out.
>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: Denys Vlasenko <vda.linux@googlemail.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: paulmck@linux.vnet.ibm.com
> Link: http://lkml.kernel.org/r/c57c8b87661a4152801d7d3786eac2d1a2f209dd.1435952415.git.luto@kernel.org
> [ Improved the changelog a bit. ]
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
> arch/x86/entry/common.c | 112 +++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 111 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 9a327ee..febc530 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -207,6 +207,7 @@ long syscall_trace_enter(struct pt_regs *regs)
> return syscall_trace_enter_phase2(regs, arch, phase1_result);
> }
>
> +/* Deprecated. */
> void syscall_trace_leave(struct pt_regs *regs)
> {
> bool step;
> @@ -237,8 +238,117 @@ void syscall_trace_leave(struct pt_regs *regs)
> user_enter();
> }
>
> +static struct thread_info *pt_regs_to_thread_info(struct pt_regs *regs)
> +{
> + unsigned long top_of_stack =
> + (unsigned long)(regs + 1) + TOP_OF_KERNEL_STACK_PADDING;
> + return (struct thread_info *)(top_of_stack - THREAD_SIZE);
> +}
> +
> +/* Called with IRQs disabled. */
> +__visible void prepare_exit_to_usermode(struct pt_regs *regs)
> +{
> + if (WARN_ON(!irqs_disabled()))
> + local_irq_disable();
> +
> + /*
> + * In order to return to user mode, we need to have IRQs off with
> + * none of _TIF_SIGPENDING, _TIF_NOTIFY_RESUME, _TIF_USER_RETURN_NOTIFY,
> + * _TIF_UPROBE, or _TIF_NEED_RESCHED set. Several of these flags
> + * can be set at any time on preemptable kernels if we have IRQs on,
> + * so we need to loop. Disabling preemption wouldn't help: doing the
> + * work to clear some of the flags can sleep.
> + */
> + while (true) {
> + u32 cached_flags =
> + READ_ONCE(pt_regs_to_thread_info(regs)->flags);
> +
> + if (!(cached_flags & (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME |
> + _TIF_UPROBE | _TIF_NEED_RESCHED)))
> + break;
> +
> + /* We have work to do. */
> + local_irq_enable();
> +
> + if (cached_flags & _TIF_NEED_RESCHED)
> + schedule();
> +
> + if (cached_flags & _TIF_UPROBE)
> + uprobe_notify_resume(regs);
> +
> + /* deal with pending signal delivery */
> + if (cached_flags & _TIF_SIGPENDING)
> + do_signal(regs);
> +
> + if (cached_flags & _TIF_NOTIFY_RESUME) {
> + clear_thread_flag(TIF_NOTIFY_RESUME);
> + tracehook_notify_resume(regs);
> + }
> +
> + if (cached_flags & _TIF_USER_RETURN_NOTIFY)
> + fire_user_return_notifiers();
> +
> + /* Disable IRQs and retry */
> + local_irq_disable();
> + }

I dreamed so many times about this loop in C!

> +
> + user_enter();

So now we are sure that we have only one call to user_enter() before
resuming userspace, once we've completed everything, rescheduling, signals,
etc... No more context tracking hacky round on signals and rescheduling?

That's great. I need to check if other archs still need schedule_user().

Thanks a lot!