linux-kernel - Re: [PATCH] myri10ge: avoid uninitialized variable use

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Message-ID: <20250621074915.GG9190@horms.kernel.org>
Date: Sat, 21 Jun 2025 08:49:15 +0100
From: Simon Horman <horms@...nel.org>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Arnd Bergmann <arnd@...db.de>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] myri10ge: avoid uninitialized variable use

On Fri, Jun 20, 2025 at 01:26:28PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> While compile testing on less common architectures, I noticed that gcc-10 on
> s390 finds a bug that all other configurations seem to miss:
> 
> drivers/net/ethernet/myricom/myri10ge/myri10ge.c: In function 'myri10ge_set_multicast_list':
> drivers/net/ethernet/myricom/myri10ge/myri10ge.c:391:25: error: 'cmd.data0' is used uninitialized in this function [-Werror=uninitialized]
>   391 |  buf->data0 = htonl(data->data0);
>       |                         ^~
> drivers/net/ethernet/myricom/myri10ge/myri10ge.c:392:25: error: '*((void *)&cmd+4)' is used uninitialized in this function [-Werror=uninitialized]
>   392 |  buf->data1 = htonl(data->data1);
>       |                         ^~
> drivers/net/ethernet/myricom/myri10ge/myri10ge.c: In function 'myri10ge_allocate_rings':
> drivers/net/ethernet/myricom/myri10ge/myri10ge.c:392:13: error: 'cmd.data1' is used uninitialized in this function [-Werror=uninitialized]
>   392 |  buf->data1 = htonl(data->data1);
> drivers/net/ethernet/myricom/myri10ge/myri10ge.c:1939:22: note: 'cmd.data1' was declared here
>  1939 |  struct myri10ge_cmd cmd;
>       |                      ^~~
> drivers/net/ethernet/myricom/myri10ge/myri10ge.c:393:13: error: 'cmd.data2' is used uninitialized in this function [-Werror=uninitialized]
>   393 |  buf->data2 = htonl(data->data2);
> drivers/net/ethernet/myricom/myri10ge/myri10ge.c:1939:22: note: 'cmd.data2' was declared here
>  1939 |  struct myri10ge_cmd cmd;
> 
> It would be nice to understand how to make other compilers catch this as
> well, but for the moment I'll just shut up the warning by fixing the
> undefined behavior in this driver.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Hi Arnd,

That is a lovely mess.

Curiously I was not able to reproduce this on s390 with gcc 10.5.0.
Perhaps I needed to try harder. Or perhaps the detection is specific to a
very narrow set of GCC versions.

Regardless I agree with your analysis, but I wonder if the following is
also needed so that .data0, 1 and 2 are always initialised when used.

diff --git a/drivers/net/ethernet/myricom/myri10ge/myri10ge.c b/drivers/net/ethernet/myricom/myri10ge/myri10ge.c
index f9d6ba381361..4743064bc6d4 100644
--- a/drivers/net/ethernet/myricom/myri10ge/myri10ge.c
+++ b/drivers/net/ethernet/myricom/myri10ge/myri10ge.c
@@ -809,6 +809,7 @@ static int myri10ge_update_mac_address(struct myri10ge_priv *mgp,
 		     | (addr[2] << 8) | addr[3]);
 
 	cmd.data1 = ((addr[4] << 8) | (addr[5]));
+	cmd.data2 = 0;
 
 	status = myri10ge_send_cmd(mgp, MXGEFW_SET_MAC_ADDRESS, &cmd, 0);
 	return status;
@@ -820,6 +821,9 @@ static int myri10ge_change_pause(struct myri10ge_priv *mgp, int pause)
 	int status, ctl;
 
 	ctl = pause ? MXGEFW_ENABLE_FLOW_CONTROL : MXGEFW_DISABLE_FLOW_CONTROL;
+	cmd.data0 = 0,
+	cmd.data1 = 0,
+	cmd.data2 = 0,
 	status = myri10ge_send_cmd(mgp, ctl, &cmd, 0);
 
 	if (status) {

...