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[75.142.250.213]) by smtp.gmail.com with ESMTPSA id m11sm296973ooe.43.2020.08.30.14.35.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 30 Aug 2020 14:35:43 -0700 (PDT) Subject: Re: [PATCH] usb: gadget: net2272: assert for a valid dma request To: Alan Stern Cc: balbi@kernel.org, gregkh@linuxfoundation.org, natechancellor@gmail.com, ndesaulniers@google.com, christophe.jaillet@wanadoo.fr, peter.chen@nxp.com, b-liu@ti.com, chunfeng.yun@mediatek.com, novikov@ispras.ru, yanaijie@huawei.com, linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org References: <20200830183646.24930-1-trix@redhat.com> <20200830212249.GA528080@rowland.harvard.edu> From: Tom Rix Message-ID: <40bac66a-2c05-6f1b-0e46-e577a54aab6e@redhat.com> Date: Sun, 30 Aug 2020 14:35:41 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200830212249.GA528080@rowland.harvard.edu> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/30/20 2:22 PM, Alan Stern wrote: > On Sun, Aug 30, 2020 at 11:36:46AM -0700, trix@redhat.com wrote: >> From: Tom Rix >> >> clang static analysis flags this representive problem >> >> net2272.c:1541:8: warning: Dereference of null pointer >> if ((req->req.length % ep->ep.maxpacket != 0) || >> ^~~~~~~~~~~~~~~ >> This is mostly not a problem. >> >> In net2272_handle_dma(), even though every path through >> the routine dereferences req, it is set to NULL when the >> ep->queue is empty. If the eq->queue was ever empty this >> would have oops. >> >> So the else statement should not be needed. If it is, >> flag it as a BUG. >> >> Signed-off-by: Tom Rix > This patch really seems to be overkill. In particular, Linus Torvalds > feels very strongly that people should not add BUG or BUG_ON calls > except in extreme situations -- which this isn't. > >> --- >> drivers/usb/gadget/udc/net2272.c | 5 ++--- >> 1 file changed, 2 insertions(+), 3 deletions(-) >> >> diff --git a/drivers/usb/gadget/udc/net2272.c b/drivers/usb/gadget/udc/net2272.c >> index 44d1ea2307bb..8301723a8b1a 100644 >> --- a/drivers/usb/gadget/udc/net2272.c >> +++ b/drivers/usb/gadget/udc/net2272.c >> @@ -1506,17 +1506,16 @@ static int net2272_stop(struct usb_gadget *_gadget) >> static void >> net2272_handle_dma(struct net2272_ep *ep) >> { >> - struct net2272_request *req; >> + struct net2272_request *req = NULL; >> unsigned len; >> int status; >> >> if (!list_empty(&ep->queue)) >> req = list_entry(ep->queue.next, >> struct net2272_request, queue); >> - else >> - req = NULL; >> >> dev_vdbg(ep->dev->dev, "handle_dma %s req %p\n", ep->ep.name, req); >> + BUG_ON(!req); > There's no point in adding this. If the function goes on to dereference > a NULL pointer, you'll get the same effect in any case -- an oops. > > If you want to make the point that req had better not be NULL, just get > rid of the "if" test entirely. You could even change the list_entry to > list_first_entry. Since nothing is really going to change, drop this patch. Tom > Alan Stern >