Re: [RFC PATCH 1/5] powerpc: Syscall hooks for context tracking subsystem

[Li Zhong <zhong@linux.vnet.ibm.com>]

On Thu, 2013-02-07 at 01:29 +0100, Frederic Weisbecker wrote:
> 2013/2/1 Li Zhong <zhong@linux.vnet.ibm.com>:
> > This is the syscall slow path hooks for context tracking subsystem,
> > corresponding to
> > [PATCH] x86: Syscall hooks for userspace RCU extended QS
> >   commit bf5a3c13b939813d28ce26c01425054c740d6731
> >
> > TIF_MEMDIE is moved to the second 16-bits (with value 17), as it seems there
> > is no asm code using it. TIF_NOHZ is added to _TIF_SYCALL_T_OR_A, so it is
> > better for it to be in the same 16 bits with others in the group, so in the
> > asm code, andi. with this group could work.
> >
> > Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
> > ---
> >  arch/powerpc/include/asm/thread_info.h |    7 +++++--
> >  arch/powerpc/kernel/ptrace.c           |    5 +++++
> >  2 files changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/powerpc/include/asm/thread_info.h b/arch/powerpc/include/asm/thread_info.h
> > index 406b7b9..414a261 100644
> > --- a/arch/powerpc/include/asm/thread_info.h
> > +++ b/arch/powerpc/include/asm/thread_info.h
> > @@ -97,7 +97,7 @@ static inline struct thread_info *current_thread_info(void)
> >  #define TIF_PERFMON_CTXSW      6       /* perfmon needs ctxsw calls */
> >  #define TIF_SYSCALL_AUDIT      7       /* syscall auditing active */
> >  #define TIF_SINGLESTEP         8       /* singlestepping active */
> > -#define TIF_MEMDIE             9       /* is terminating due to OOM killer */
> > +#define TIF_NOHZ               9       /* in adaptive nohz mode */
> >  #define TIF_SECCOMP            10      /* secure computing */
> >  #define TIF_RESTOREALL         11      /* Restore all regs (implies NOERROR) */
> >  #define TIF_NOERROR            12      /* Force successful syscall return */
> > @@ -106,6 +106,7 @@ static inline struct thread_info *current_thread_info(void)
> >  #define TIF_SYSCALL_TRACEPOINT 15      /* syscall tracepoint instrumentation */
> >  #define TIF_EMULATE_STACK_STORE        16      /* Is an instruction emulation
> >                                                 for stack store? */
> > +#define TIF_MEMDIE             17      /* is terminating due to OOM killer */
> >
> >  /* as above, but as bit values */
> >  #define _TIF_SYSCALL_TRACE     (1<<TIF_SYSCALL_TRACE)
> > @@ -124,8 +125,10 @@ static inline struct thread_info *current_thread_info(void)
> >  #define _TIF_UPROBE            (1<<TIF_UPROBE)
> >  #define _TIF_SYSCALL_TRACEPOINT        (1<<TIF_SYSCALL_TRACEPOINT)
> >  #define _TIF_EMULATE_STACK_STORE       (1<<TIF_EMULATE_STACK_STORE)
> > +#define _TIF_NOHZ              (1<<TIF_NOHZ)
> >  #define _TIF_SYSCALL_T_OR_A    (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
> > -                                _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT)
> > +                                _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT | \
> > +                                _TIF_NOHZ)
> >
> >  #define _TIF_USER_WORK_MASK    (_TIF_SIGPENDING | _TIF_NEED_RESCHED | \
> >                                  _TIF_NOTIFY_RESUME | _TIF_UPROBE)
> > diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
> > index c497000..62238dd 100644
> > --- a/arch/powerpc/kernel/ptrace.c
> > +++ b/arch/powerpc/kernel/ptrace.c
> > @@ -32,6 +32,7 @@
> >  #include <trace/syscall.h>
> >  #include <linux/hw_breakpoint.h>
> >  #include <linux/perf_event.h>
> > +#include <linux/context_tracking.h>
> >
> >  #include <asm/uaccess.h>
> >  #include <asm/page.h>
> > @@ -1745,6 +1746,8 @@ long do_syscall_trace_enter(struct pt_regs *regs)
> >  {
> >         long ret = 0;
> >
> > +       user_exit();
> > +
> >         secure_computing_strict(regs->gpr[0]);
> >
> >         if (test_thread_flag(TIF_SYSCALL_TRACE) &&
> > @@ -1789,4 +1792,6 @@ void do_syscall_trace_leave(struct pt_regs *regs)
> >         step = test_thread_flag(TIF_SINGLESTEP);
> >         if (step || test_thread_flag(TIF_SYSCALL_TRACE))
> >                 tracehook_report_syscall_exit(regs, step);
> > +
> > +       user_enter();
> 
> In x86-64, schedule_user() and do_notify_resume() can be called before
> syscall_trace_leave(). As a result we may be entering
> syscall_trace_leave() in user mode (from a context tracking POV). To
> fix this I added a call to user_exit() on the very beginning of that
> function.
> 
> You can find the details in 2c5594df344cd1ff0cc9bf007dea3235582b3acf
> ("rcu: Fix unrecovered RCU user mode in syscall_trace_leave()").

Hi Frederic, 

Thank you very much for the reminding. 

> 
> Could this problem happen in ppc as well?

I checked the code(64 bit) today, it seems to me that it won't happen.
But fortunately, we are in the ppc mailing list, please correct me if my
understanding is wrong.

By the way, I enabled CONTEXT_TRACKING_FORCE and PROVE_RCU, so if it
could happen, I think there should be some illegal RCU usage complaints
reported.  

Thanks, Zhong

> Thanks.
>