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

[Daniel Borkmann]

On 10/17/2017 12:10 AM, Alexei Starovoitov wrote:
> On Mon, Oct 16, 2017 at 2:10 PM, Richard Weinberger <richard@xxxxxx> wrote:
> > 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.
>
> it will surely race with somebody else setting task comm and it's fine.
> all of bpf tracing is read-only, so locks are only allowed inside bpf core
> bits like maps. Taking core locks like task_lock() is quite scary.
> bpf scripts rely on bpf_probe_read() of all sorts of kernel fields
> so reading comm here w/o lock is fine.

Yeah, and perf_event_comm() -> perf_event_comm_event() out of __set_task_comm()
is having same approach wrt comm read-out.