Messages in this thread | | | Date | Tue, 29 Jun 2010 19:18:46 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] sanitize task->comm to avoid leaking escape codes |
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On 06/29, Paul E. McKenney wrote: > > On Tue, Jun 29, 2010 at 03:31:31PM +0200, Oleg Nesterov wrote: > > > So, afaics, set_task_comm()->wmb() buys nothing and should be removed. > > The last zero char in task_struct->comm[] is always here, at least this > > guarantees that strcpy(char *dest, tsk->comm) is always safe. > > > > (I cc'ed the expert, Paul can correct me) > > First, all of the implementations that I can see do task_lock(tsk), which > should prevent readers from seeing any changes. So I am guessing that > you guys want to allow readers to get at ->comm without having to acquire > this lock.
Ah, sorry for confusion.
No, we are not trying to invent the lockless get_task_comm(). I'd say it is not needed, if we really care about the precise ->comm we can take task->alloc_lock.
The only problem is that I believe that set_task_comm() wrongly pretends wmb() can help the lockless reader, it does:
task_lock(tsk);
/* * Threads may access current->comm without holding * the task lock, so write the string carefully. * Readers without a lock may see incomplete new * names but are safe from non-terminating string reads. */ memset(tsk->comm, 0, TASK_COMM_LEN); wmb(); strlcpy(tsk->comm, buf, sizeof(tsk->comm)); task_unlock(tsk);
but afaics this wmb() buys absolutely nothing if we race with the reader doing, say,
printk("my name is %s\n", current->comm);
Afaics, this wmb()
- can't prevent from printing the mixture of the old/new data
- is not needed to make strcpy(somewhere, task->comm) safe, the final char is always '0', we never change it.
- adds the unnecessary confusion
> [... snip a lot of good ideas ...]
Thanks a lot, Paul ;)
Oleg.
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