Messages in this thread | | | Date | Tue, 9 Feb 2010 14:08:18 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] Update comment on find_task_by_pid_ns |
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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.
> 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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