netdev - Re: [PATCH 2/2 net] sctp: fix an error code in sctp_sf_eat

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Date: Fri, 9 Jun 2023 23:26:13 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Xin Long <lucien.xin@...il.com>
Cc: Dan Carpenter <dan.carpenter@...aro.org>, Vlad Yasevich
 <vladislav.yasevich@...com>, Marcelo Ricardo Leitner
 <marcelo.leitner@...il.com>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 linux-sctp@...r.kernel.org, netdev@...r.kernel.org,
 kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 2/2 net] sctp: fix an error code in sctp_sf_eat_auth()

On Fri, 9 Jun 2023 19:04:17 -0400 Xin Long wrote:
> On Fri, Jun 9, 2023 at 12:41 PM Dan Carpenter <dan.carpenter@...aro.org> wrote:
> >
> > On Fri, Jun 09, 2023 at 11:13:03AM -0400, Xin Long wrote:  
> > > This one looks good to me.
> > >
> > > But for the patch 1/2 (somehow it doesn't show up in my mailbox):
> > >
> > >   default:
> > >   pr_err("impossible disposition %d in state %d, event_type %d, event_id %d\n",
> > >         status, state, event_type, subtype.chunk);
> > > - BUG();
> > > + error = status;
> > > + if (error >= 0)
> > > + error = -EINVAL;
> > > + WARN_ON_ONCE(1);
> > >
> > > I think from the sctp_do_sm() perspective, it expects the state_fn
> > > status only from
> > > enum sctp_disposition. It is a BUG to receive any other values and
> > > must be fixed,
> > > as you did in 2/2. It does the same thing as other functions in SCTP code, like
> > > sctp_sf_eat_data_*(), sctp_retransmit() etc.  
> >
> > It is a bug, sure.  And after my patch is applied it will still trigger
> > a stack trace.  But we should only call the actual BUG() function
> > in order to prevent filesystem corruption or a privilege escalation or
> > something along those lines.  
> Hi, Dan,
> 
> Sorry, I'm not sure about this.
> 
> Look at the places where it's using  BUG(), it's not exactly the case, like
> in ping_err() or ping_common_sendmsg(), BUG() are used more for
> unexpected cases, which don't cause any filesystem corruption or a
> privilege escalation.
> 
> You may also check more others under net/*.

Most BUG()s under net/ are historic. The legit BUG() uses I can think
of are at boot, if something fails you can't bring up the system at all.

https://docs.kernel.org/process/deprecated.html?highlight=bug#bug-and-bug-on

> > Calling BUG() makes the system unusable so it makes bugs harder to
> > debug.  This is even mentioned in checkpatch.pl "Do not crash the kernel
> > unless it is absolutely unavoidable--use WARN_ON_ONCE() plus recovery
> > code (if feasible) instead of BUG() or variants".
> >  
> "absolutely unavoidable", I think in a module, if it gets a case that is not
> expected at all, and the consequence (it may cause or has caused) is
> unsure, WARN_ON_ONCE() is not enough.