Re: [PATCH] Update comment on find_task_by_pid_ns

Serge E. Hallyn
2010-02-10T16:30:57Z

Quoting Andrew Morton (akpm@linux-foundation.org):
> On Tue, 9 Feb 2010 06:42:45 +0900
> Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:
>
> > OK. I updated description.
> >
> > As of 2.6.32 , below users are missing rcu_read_lock().
> >
> > Users missing rcu_read_lock() when calling find_task_by_vpid():
> >
> > SYSCALL_DEFINE3(ioprio_set) in fs/ioprio.c
> > SYSCALL_DEFINE2(ioprio_get) in fs/ioprio.c
> > cap_get_target_pid() in kernel/capability.c
>
> Actually, cap_get_target_pid() uses rcu_read_lock() and doesn't take
> tasklist_lock.

Hmm - is that in -mm? In my copy here it takes read_lock(&tasklist_lock)

And I'll admit I'm a bit confused as to the current state of things:
do I understand correctly that we now need to take both the tasklist_lock
and rcu_read_lock? (Presumably only for read_lock()?)

> > audit_prepare_user_tty() in kernel/audit.c
> > audit_receive_msg() in kernel/audit.c
> > check_clock() in kernel/posix-cpu-timers.c
> > posix_cpu_timer_create() in kernel/posix-cpu-timers.c
> > SYSCALL_DEFINE3(setpriority) in kernel/sys.c
> > SYSCALL_DEFINE2(getpriority) in kernel/sys.c
> > SYSCALL_DEFINE2(setpgid) in kernel/sys.c
> > SYSCALL_DEFINE1(sched_getscheduler) in kernel/sched.c
> > SYSCALL_DEFINE2(sched_getparam) in kernel/sched.c
> > sched_setaffinity() in kernel/sched.c
> > sched_getaffinity() in kernel/sched.c
> > SYSCALL_DEFINE2(sched_rr_get_interval) in kernel/sched.c
> > tomoyo_is_select_one() in security/tomoyo/common.c
> > tomoyo_read_pid() in security/tomoyo/common.c
> > SYSCALL_DEFINE6(move_pages) in mm/migrate.c
> > SYSCALL_DEFINE4(migrate_pages) in mm/mempolicy.c
> > find_process_by_pid() in arch/mips/kernel/mips-mt-fpaff.c
> > pfm_get_task() in arch/ia64/kernel/perfmon.c
> > cxn_pin_by_pid() in arch/frv/mm/mmu-context.c
> >
> > Users missing rcu_read_lock() when calling find_task_by_pid_ns():
> >
> > rest_init() in init/main.c
> > getthread() in kernel/kgdb.c
> > mconsole_stack() in arch/um/drivers/mconsole_kern.c
> >
> > What should we do? Adding rcu_read_lock()/rcu_read_unlock() to each
> > callers? Or adding rcu_read_lock()/rcu_read_unlock() inside
> > find_task_by_pid_ns()?
>
> Putting rcu_read_lock() in the callee isn't a complete solution.
> Because the function would still be returning a task_struct* without
> any locking held and without taking a reference against it. So that
> pointer is useless to the caller!
>
> We could add a new function which looks up the task and then takes a
> reference on it, insde suitable locks. The caller would then use the
> task_struct and then remember to call put_task_struct() to unpin it.
> This prevents the task_struct from getting freed while it's being
> manipulated, but it doesn't prevent fields within it from being altered
> - that's up to the caller to sort out.
>
> One fix is to go through all those callsites and add the rcu_read_lock.
> That kinda sucks. Perhaps writing the new function which returns a
> pinned task_struct is better?
>
> --
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