linux-kernel - Re: [PATCH v2 2/5] treewide: Replace open-coded flex arrays in unions

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Message-ID: <202108270915.B4DD070AF@keescook>
Date:   Fri, 27 Aug 2021 09:17:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
Cc:     linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Ayush Sawal <ayush.sawal@...lsio.com>,
        Vinay Kumar Yadav <vinay.yadav@...lsio.com>,
        Rohit Maheshwari <rohitm@...lsio.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Kalle Valo <kvalo@...eaurora.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Stanislaw Gruszka <stf_xl@...pl>,
        Luca Coelho <luciano.coelho@...el.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Johannes Berg <johannes.berg@...el.com>,
        Mordechay Goodstein <mordechay.goodstein@...el.com>,
        Lee Jones <lee.jones@...aro.org>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        Arunachalam Santhanam <arunachalam.santhanam@...bosch.com>,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>,
        linux-crypto@...r.kernel.org, ath10k@...ts.infradead.org,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-can@...r.kernel.org,
        bpf@...r.kernel.org, Rasmus Villemoes <linux@...musvillemoes.dk>,
        Keith Packard <keithp@...thp.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        clang-built-linux@...glegroups.com, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 2/5] treewide: Replace open-coded flex arrays in unions

On Thu, Aug 26, 2021 at 08:24:52AM +0200, Marc Kleine-Budde wrote:
> [...]
> BTW: Is there opportunity for conversion, too?
> 
> | drivers/net/can/peak_canfd/peak_pciefd_main.c:146:32: warning: array of flexible structures

Untested potential solution:

diff --git a/drivers/net/can/peak_canfd/peak_pciefd_main.c b/drivers/net/can/peak_canfd/peak_pciefd_main.c
index 1df3c4b54f03..efa2b5a52bd7 100644
--- a/drivers/net/can/peak_canfd/peak_pciefd_main.c
+++ b/drivers/net/can/peak_canfd/peak_pciefd_main.c
@@ -143,7 +143,11 @@ struct pciefd_rx_dma {
 	__le32 irq_status;
 	__le32 sys_time_low;
 	__le32 sys_time_high;
-	struct pucan_rx_msg msg[];
+	/*
+	 * with "msg" being pciefd_irq_rx_cnt(priv->irq_status)-many
+	 * variable-sized struct pucan_rx_msg following.
+	 */
+	__le32 msg[];
 } __packed __aligned(4);
 
 /* Tx Link record */
@@ -327,7 +331,7 @@ static irqreturn_t pciefd_irq_handler(int irq, void *arg)
 
 	/* handle rx messages (if any) */
 	peak_canfd_handle_msgs_list(&priv->ucan,
-				    rx_dma->msg,
+				    (struct pucan_rx_msg *)rx_dma->msg,
 				    pciefd_irq_rx_cnt(priv->irq_status));
 
 	/* handle tx link interrupt (if any) */


It's not great, but it's also not strictly a flex array, in the sense
that since struct pucan_rx_msg is a variable size, the compiler cannot
reason about the size of struct pciefd_rx_dma based only on the
irq_status encoding...

-- 
Kees Cook