[2.6.37-rc1] sys_ioprio_set and RCU locking...

Daniel J Blueman
2010-11-02T12:15:36Z

With 2.6.37-rc1, I observe sys_ioprio_set not taking the RCU lock [1]
across access to the task credentials.

Inspecting the code in fs/ioprio.c, the tasklist_lock is held for read
across the __task_cred call, which is presumably sufficient to prevent
the task credentials becoming stale.

Thus, is there preference to take the RCU lock for read across the
credential access eg at [2], or annotate the call?

Thanks,
Daniel

--- [1]

===================================================

[ INFO: suspicious rcu_dereference_check() usage. ]

---------------------------------------------------

kernel/pid.c:419 invoked rcu_dereference_check() without protection!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 1

1 lock held by start-stop-daem/2246:

#0: (tasklist_lock){.?.?..}, at: [<ffffffff811a2dfa>]
sys_ioprio_set+0x8a/0x400

stack backtrace:

Pid: 2246, comm: start-stop-daem Not tainted 2.6.37-rc1-330cd+ #2

Call Trace:

[<ffffffff8109f5f4>] lockdep_rcu_dereference+0xa4/0xc0

[<ffffffff81085651>] find_task_by_pid_ns+0x81/0x90

[<ffffffff8108567d>] find_task_by_vpid+0x1d/0x20

[<ffffffff811a3160>] sys_ioprio_set+0x3f0/0x400

[<ffffffff816efa79>] ? trace_hardirqs_on_thunk+0x3a/0x3f

[<ffffffff81003482>] system_call_fastpath+0x16/0x1b

--- [2]

Take the RCU lock for read across acquiring the pointer to the task
credentials and dereferencing it.

Signed-off-by: Daniel J Blueman <daniel.blueman@gmail.com>

diff --git a/fs/ioprio.c b/fs/ioprio.c
index 748cfb9..00cc0e5 100644
--- a/fs/ioprio.c
+++ b/fs/ioprio.c
@@ -139,8 +139,10 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who,
int, ioprio)
break;

do_each_thread(g, p) {
+ rcu_read_lock();
if (__task_cred(p)->uid != who)
continue;
+ rcu_read_unlock();
ret = set_task_ioprio(p, ioprio);
if (ret)
goto free_uid;
@@ -232,8 +234,10 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
break;

do_each_thread(g, p) {
+ rcu_read_lock();
if (__task_cred(p)->uid != user->uid)
continue;
+ rcu_read_unlock();
tmpio = get_task_ioprio(p);
if (tmpio < 0)
continue;
--
Daniel J Blueman