netdev - Re: [PATCH] net: sunrpc: svcauth_gss: use BUG_ON instead of if condition followed by BUG

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Message-ID: <20171024151242.Horde.9xuLnnVxPJHwTDEYl2gMQ60@gator4166.hostgator.com>
Date:   Tue, 24 Oct 2017 15:12:42 -0500
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     Jeff Layton <jlayton@...hat.com>,
        Weston Andros Adamson <dros@...key.org>,
        Trond Myklebust <trond.myklebust@...marydata.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-nfs list <linux-nfs@...r.kernel.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: sunrpc: svcauth_gss: use BUG_ON instead of if
 condition followed by BUG


Quoting "J. Bruce Fields" <bfields@...ldses.org>:

> On Tue, Oct 24, 2017 at 02:18:52PM -0400, Jeff Layton wrote:
>> On Tue, 2017-10-24 at 13:53 -0400, J. Bruce Fields wrote:
>> > On Tue, Oct 24, 2017 at 01:26:49PM -0400, Weston Andros Adamson wrote:
>> > > Is there a reason to BUG() in these places? Couldn't we  
>> WARN_ON_ONCE and return an error?
>> >
>> > I think the BUG() will just kill an nfsd thread that isn't holding any
>> > interesting locks.
>> >
>>
>> Not necessarily. If panic_on_oops is set (and it usually is in
>> "production" setups), it'll crash the box there.
>
> Maybe they're getting what they asked for?
>
>> > The failures look unlikely.  (Except for that read_u32... return, I
>> > wonder if we're missing a check there.)
>>
>> Agreed, looks like you only hit an error if the read attempts to go out
>> of bounds. In principle that shouldn't ever happen (and I haven't seen
>> any reports of it).
>>
>> Still...I agree with Dros that it's better to handle this without
>> oopsing if we can. We can return an error from either of those
>> functions. A sane error and a WARN_ONCE would be better here.
>
> OK, OK, OK.
>
> There are also some more BUGs that could use looking into if anyone
> wants to.
>
> --b.
>
> commit eb754930662f
> Author: J. Bruce Fields <bfields@...hat.com>
> Date:   Tue Oct 24 14:58:11 2017 -0400
>
>     rpc: remove some BUG()s
>
>     It would be kinder to WARN() and recover in several spots here instead
>     of BUG()ing.
>
>     Also, it looks like the read_u32_from_xdr_buf() call could actually
>     fail, though it might require a broken (or malicious) client, so convert
>     that to just an error return.
>
>     Reported-by: Weston Andros Adamson <dros@...key.org>
>     Signed-off-by: J. Bruce Fields <bfields@...hat.com>
>
> diff --git a/net/sunrpc/auth_gss/svcauth_gss.c  
> b/net/sunrpc/auth_gss/svcauth_gss.c
> index 7b1ee5a0b03c..73165e9ca5bf 100644
> --- a/net/sunrpc/auth_gss/svcauth_gss.c
> +++ b/net/sunrpc/auth_gss/svcauth_gss.c
> @@ -855,11 +855,13 @@ unwrap_integ_data(struct svc_rqst *rqstp,  
> struct xdr_buf *buf, u32 seq, struct g
>  		return stat;
>  	if (integ_len > buf->len)
>  		return stat;
> -	if (xdr_buf_subsegment(buf, &integ_buf, 0, integ_len))
> -		BUG();
> +	if (xdr_buf_subsegment(buf, &integ_buf, 0, integ_len)) {
> +		WARN_ON_ONCE(1);
> +		return stat;
> +	}
>  	/* copy out mic... */
>  	if (read_u32_from_xdr_buf(buf, integ_len, &mic.len))
> -		BUG();
> +		return stat;
>  	if (mic.len > RPC_MAX_AUTH_SIZE)
>  		return stat;
>  	mic.data = kmalloc(mic.len, GFP_KERNEL);
> @@ -1611,8 +1613,10 @@ svcauth_gss_wrap_resp_integ(struct svc_rqst *rqstp)
>  	BUG_ON(integ_len % 4);
>  	*p++ = htonl(integ_len);
>  	*p++ = htonl(gc->gc_seq);
> -	if (xdr_buf_subsegment(resbuf, &integ_buf, integ_offset, integ_len))
> -		BUG();
> +	if (xdr_buf_subsegment(resbuf, &integ_buf, integ_offset, integ_len)) {
> +		WARN_ON_ONCE(1);
> +		goto out_err;
> +	}
>  	if (resbuf->tail[0].iov_base == NULL) {
>  		if (resbuf->head[0].iov_len + RPC_MAX_AUTH_SIZE > PAGE_SIZE)
>  			goto out_err;

What about the following BUG() at net/sunrpc/svc_xprt.c:1058:

/*
  * Remove a dead transport
  */
static void svc_delete_xprt(struct svc_xprt *xprt)
{
         struct svc_serv *serv = xprt->xpt_server;
         struct svc_deferred_req *dr;

         /* Only do this once */
         if (test_and_set_bit(XPT_DEAD, &xprt->xpt_flags))
                 BUG();

         dprintk("svc: svc_delete_xprt(%p)\n", xprt);
         xprt->xpt_ops->xpo_detach(xprt);

         spin_lock_bh(&serv->sv_lock);
         list_del_init(&xprt->xpt_list);
         WARN_ON_ONCE(!list_empty(&xprt->xpt_ready));
         if (test_bit(XPT_TEMP, &xprt->xpt_flags))
                 serv->sv_tmpcnt--;
         spin_unlock_bh(&serv->sv_lock);

         while ((dr = svc_deferred_dequeue(xprt)) != NULL)
                 kfree(dr);

         call_xpt_users(xprt);
         svc_xprt_put(xprt);
}

I'm suspicious about that comment above the _if_ condition: /* Only do  
this once */

Would it help to replace that BUG with a WARN_ON_ONCE?

Thanks
--
Gustavo A. R. Silva