Subject: Re: [PATCH 3/3] bpf: Make sure that ->comm does not change under us.

From: Richard Weinberger <[email-masked]>
Date: Mon, 16 Oct 2017 23:10:37 +0200

Am Montag, 16. Oktober 2017, 23:02:06 CEST schrieb Daniel Borkmann:
> On 10/16/2017 10:55 PM, Richard Weinberger wrote:
> > Am Montag, 16. Oktober 2017, 22:50:43 CEST schrieb Daniel Borkmann:
> >>>    	struct task_struct *task = current;
> >>> 
> >>> +	task_lock(task);
> >>> 
> >>>    	strncpy(buf, task->comm, size);
> >>> 
> >>> +	task_unlock(task);
> >> 
> >> Wouldn't this potentially lead to a deadlock? E.g. you attach yourself
> >> to task_lock() / spin_lock() / etc, and then the BPF prog triggers the
> >> bpf_get_current_comm() taking the lock again ...
> > 
> > Yes, but doesn't the same apply to the use case when I attach to strncpy()
> > and run bpf_get_current_comm()?
> 
> You mean due to recursion? In that case trace_call_bpf() would bail out
> due to the bpf_prog_active counter.

Ah, that's true.
So, when someone wants to use bpf_get_current_comm() while tracing task_lock,
we have a problem. I agree.
On the other hand, without locking the function may return wrong results.

Thanks,
//richard