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Date:   Sun, 7 Nov 2021 00:18:39 +0100
From:   Marc Kleine-Budde <mkl@pengutronix.de>
To:     Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
Cc:     linux-can <linux-can@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Matt Kline <matt@bitbashing.io>,
        Sean Nyekjaer <sean@geanix.com>,
        Chandrasekar Ramakrishnan <rcsekar@samsung.com>
Subject: Re: [RFC PATCH v1] can: m_can: m_can_read_fifo: fix memory leak in
 error branch
Message-ID: <20211106231839.7zcmtxpidemu4owa@pengutronix.de>
References: <20211026180909.1953355-1-mailhol.vincent@wanadoo.fr>
 <20211029113405.hbqcu6chf5e3olrm@pengutronix.de>
 <CAMZ6RqJ1CtphrUxRDWOKEsJF_uzoPbYD2mPiD56VvJ9qB7oxow@mail.gmail.com>
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On 30.10.2021 01:35:01, Vincent MAILHOL wrote:
> On Fri. 29 Oct 2021 at 20:34, Marc Kleine-Budde <mkl@pengutronix.de> wrot=
e:
> > On 27.10.2021 03:09:09, Vincent Mailhol wrote:
> > > In m_can_read_fifo(), if the second call to m_can_fifo_read() fails,
> > > the function jump to the out_fail label and returns without calling
> > > m_can_receive_skb(). This means that the skb previously allocated by
> > > alloc_can_skb() is not freed. In other terms, this is a memory leak.
> > >
> > > This patch adds a new goto statement: out_receive_skb and do some
> > > small code refactoring to fix the issue.
> >
> > This means we pass a skb to the user space, which contains wrong data.
> > Probably 0x0, but if the CAN frame doesn't contain 0x0, it's wrong. That
> > doesn't look like a good idea. If the CAN frame broke due to a CRC issue
> > on the wire it is not received. IMHO it's best to discard the skb and
> > return the error.
>=20
> Arg... Guess I made the right choice to tag the patch as RFC...
>=20
> Just one question, what is the correct function to discard the
> skb? The driver uses the napi polling system (which I am not
> entirely familiar with). Does it mean that the rx is not done in
> IRQ context and that we can simply use kfree_skb() instead of
> dev_kfree_skb_irq()?

The m_can driver is a bit more complicated. It uses NAPI for mmio
devices, but threaded IRQs for SPI devices. Looking at
dev_kfree_skb_any(), it checks for hard IRQs or IRQs disabled, I think
this is not the case for both threaded IRQs and NAPI.

https://elixir.bootlin.com/linux/v5.15/source/net/core/dev.c#L3108

So I think kfree_skb() should be OK.

regards,
Marc

--=20
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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