Re: [tip:x86/asm] x86/entry: Add enter_from_user_mode() and use it in syscalls

Frederic Weisbecker
2015-07-14T23:28:37Z

On Tue, Jul 14, 2015 at 04:04:47PM -0700, Andy Lutomirski wrote:
> On Tue, Jul 14, 2015 at 4:00 PM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > On Tue, Jul 07, 2015 at 03:51:29AM -0700, tip-bot for Andy Lutomirski wrote:
> >> Commit-ID: feed36cde0a10adb957445a37e48f957f30b2273
> >> Gitweb: http://git.kernel.org/tip/feed36cde0a10adb957445a37e48f957f30b2273
> >> Author: Andy Lutomirski <luto@kernel.org>
> >> AuthorDate: Fri, 3 Jul 2015 12:44:25 -0700
> >> Committer: Ingo Molnar <mingo@kernel.org>
> >> CommitDate: Tue, 7 Jul 2015 10:59:06 +0200
> >>
> >> x86/entry: Add enter_from_user_mode() and use it in syscalls
> >>
> >> Changing the x86 context tracking hooks is dangerous because
> >> there are no good checks that we track our context correctly.
> >> Add a helper to check that we're actually in CONTEXT_USER when
> >> we enter from user mode and wire it up for syscall entries.
> >>
> >> Subsequent patches will wire this up for all non-NMI entries as
> >> well. NMIs are their own special beast and cannot currently
> >> switch overall context tracking state. Instead, they have their
> >> own special RCU hooks.
> >>
> >> This is a tiny speedup if !CONFIG_CONTEXT_TRACKING (removes a
> >> branch) and a tiny slowdown if CONFIG_CONTEXT_TRACING (adds a
> >> layer of indirection). Eventually, we should fix up the core
> >> context tracking code to supply a function that does what we
> >> want (and can be much simpler than user_exit), which will enable
> >> us to get rid of the extra call.
> >>
> >> 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/853b42420066ec3fb856779cdc223a6dcb5d355b.1435952415.git.luto@kernel.org
> >> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> >> ---
> >> arch/x86/entry/common.c | 13 ++++++++++++-
> >> 1 file changed, 12 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> >> index 917d0c3..9a327ee 100644
> >> --- a/arch/x86/entry/common.c
> >> +++ b/arch/x86/entry/common.c
> >> @@ -28,6 +28,15 @@
> >> #define CREATE_TRACE_POINTS
> >> #include <trace/events/syscalls.h>
> >>
> >> +#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();
> >> +}
> >> +#endif
> >> +
> >> static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
> >> {
> >> #ifdef CONFIG_X86_64
> >> @@ -65,14 +74,16 @@ unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
> >> work = ACCESS_ONCE(current_thread_info()->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) {
> >> - user_exit();
> >> + enter_from_user_mode();
> >> work &= ~_TIF_NOHZ;
> >
> > We should move the sanity check to user_exit/enter() and use user_exit/enter()
> > only when we actually enter/exit user.
>
> I agree, but I don't know what other arches to.

Right, I'll need to check that carefully, once I fully understand your patchset.

>
> > Here it's the case but syscall_trace_leave()
> > and do_notify_resume() are special case that should probably use exception_enter/exit()
> > unless your patchset have changed things such that there is only one call to user_exit()
> > once we completed everything before resuming userspace. I need to review the rest of
> > the patchset to discover that :-)
>
> syscall_trace_leave and do_notify_resume may be so screwed up that
> your suggestion wouldn't even work. However, the next set of patches
> (out for review but currently stalled pending Brian Gerst's vm86 work)
> remove those functions entirely.

Ok so I'm probably confused. I need to check the resulting code.

>
> --Andy