[PATCH] net: dsa: tag_rtl4_a: Support also egress tags

[PATCH] net: dsa: tag_rtl4_a: Support also egress tags

[Linus Walleij]

Support also transmitting frames using the custom "8899 A"
4 byte tag.

Qingfang came up with the solution: we need to pad the
ethernet frame to 60 bytes using eth_skb_pad(), then the
switch will happily accept frames with custom tags.

Cc: Mauri Sandberg <sandberg@mailfence.com>
Reported-by: DENG Qingfang <dqfext@gmail.com>
Fixes: efd7fe68f0c6 ("net: dsa: tag_rtl4_a: Implement Realtek 4 byte A tag")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 net/dsa/tag_rtl4_a.c | 43 +++++++++++++++++++++++++++++--------------
 1 file changed, 29 insertions(+), 14 deletions(-)

diff --git a/net/dsa/tag_rtl4_a.c b/net/dsa/tag_rtl4_a.c
index 2646abe5a69e..c17d39b4a1a0 100644
--- a/net/dsa/tag_rtl4_a.c
+++ b/net/dsa/tag_rtl4_a.c
@@ -12,9 +12,7 @@
  *
  * The 2 bytes tag form a 16 bit big endian word. The exact
  * meaning has been guessed from packet dumps from ingress
- * frames, as no working egress traffic has been available
- * we do not know the format of the egress tags or if they
- * are even supported.
+ * frames.
  */
 
 #include <linux/etherdevice.h>
@@ -36,17 +34,34 @@
 static struct sk_buff *rtl4a_tag_xmit(struct sk_buff *skb,
 				      struct net_device *dev)
 {
-	/*
-	 * Just let it pass thru, we don't know if it is possible
-	 * to tag a frame with the 0x8899 ethertype and direct it
-	 * to a specific port, all attempts at reverse-engineering have
-	 * ended up with the frames getting dropped.
-	 *
-	 * The VLAN set-up needs to restrict the frames to the right port.
-	 *
-	 * If you have documentation on the tagging format for RTL8366RB
-	 * (tag type A) then please contribute.
-	 */
+	struct dsa_port *dp = dsa_slave_to_port(dev);
+	u8 *tag;
+	u16 *p;
+	u16 out;
+
+	/* Pad out to at least 60 bytes */
+	if (unlikely(eth_skb_pad(skb)))
+		return NULL;
+	if (skb_cow_head(skb, RTL4_A_HDR_LEN) < 0)
+		return NULL;
+
+	netdev_dbg(dev, "add realtek tag to package to port %d\n",
+		   dp->index);
+	skb_push(skb, RTL4_A_HDR_LEN);
+
+	memmove(skb->data, skb->data + RTL4_A_HDR_LEN, 2 * ETH_ALEN);
+	tag = skb->data + 2 * ETH_ALEN;
+
+	/* Set Ethertype */
+	p = (u16 *)tag;
+	*p = htons(RTL4_A_ETHERTYPE);
+
+	out = (RTL4_A_PROTOCOL_RTL8366RB << 12) | (2 << 8);
+	/* The lower bits is the port numer */
+	out |= (u8)dp->index;
+	p = (u16 *)(tag + 2);
+	*p = htons(out);
+
 	return skb;
 }
 
-- 
2.29.2

Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (refs/heads/master):

On Wed, 17 Feb 2021 00:55:42 +0100 you wrote:
> Support also transmitting frames using the custom "8899 A"
> 4 byte tag.
> 
> Qingfang came up with the solution: we need to pad the
> ethernet frame to 60 bytes using eth_skb_pad(), then the
> switch will happily accept frames with custom tags.
> 
> [...]

Here is the summary with links:
  - net: dsa: tag_rtl4_a: Support also egress tags
    https://git.kernel.org/netdev/net-next/c/86dd9868b878

You are awesome, thank you!
--
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html

Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

[Florian Fainelli]

On 2/16/2021 5:00 PM, patchwork-bot+netdevbpf@kernel.org wrote:
> Hello:
> 
> This patch was applied to netdev/net-next.git (refs/heads/master):
> 
> On Wed, 17 Feb 2021 00:55:42 +0100 you wrote:
>> Support also transmitting frames using the custom "8899 A"
>> 4 byte tag.
>>
>> Qingfang came up with the solution: we need to pad the
>> ethernet frame to 60 bytes using eth_skb_pad(), then the
>> switch will happily accept frames with custom tags.
>>
>> [...]
> 
> Here is the summary with links:
>   - net: dsa: tag_rtl4_a: Support also egress tags
>     https://git.kernel.org/netdev/net-next/c/86dd9868b878
> 
> You are awesome, thank you!
> --
> Deet-doot-dot, I am a bot.
> https://korg.docs.kernel.org/patchwork/pwbot.html

Woah not so fast, 1h05 minutes to review a patch, how are we supposed to
keep up?
-- 
Florian

Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

[Andrew Lunn]

On Wed, Feb 17, 2021 at 12:55:42AM +0100, Linus Walleij wrote:
> Support also transmitting frames using the custom "8899 A"
> 4 byte tag.
> 
> Qingfang came up with the solution: we need to pad the
> ethernet frame to 60 bytes using eth_skb_pad(), then the
> switch will happily accept frames with custom tags.
> 
> Cc: Mauri Sandberg <sandberg@mailfence.com>
> Reported-by: DENG Qingfang <dqfext@gmail.com>
> Fixes: efd7fe68f0c6 ("net: dsa: tag_rtl4_a: Implement Realtek 4 byte A tag")
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> ---
>  net/dsa/tag_rtl4_a.c | 43 +++++++++++++++++++++++++++++--------------
>  1 file changed, 29 insertions(+), 14 deletions(-)
> 
> diff --git a/net/dsa/tag_rtl4_a.c b/net/dsa/tag_rtl4_a.c
> index 2646abe5a69e..c17d39b4a1a0 100644
> --- a/net/dsa/tag_rtl4_a.c
> +++ b/net/dsa/tag_rtl4_a.c
> @@ -12,9 +12,7 @@
>   *
>   * The 2 bytes tag form a 16 bit big endian word. The exact
>   * meaning has been guessed from packet dumps from ingress
> - * frames, as no working egress traffic has been available
> - * we do not know the format of the egress tags or if they
> - * are even supported.
> + * frames.
>   */
>  
>  #include <linux/etherdevice.h>
> @@ -36,17 +34,34 @@
>  static struct sk_buff *rtl4a_tag_xmit(struct sk_buff *skb,
>  				      struct net_device *dev)
>  {
> -	/*
> -	 * Just let it pass thru, we don't know if it is possible
> -	 * to tag a frame with the 0x8899 ethertype and direct it
> -	 * to a specific port, all attempts at reverse-engineering have
> -	 * ended up with the frames getting dropped.
> -	 *
> -	 * The VLAN set-up needs to restrict the frames to the right port.
> -	 *
> -	 * If you have documentation on the tagging format for RTL8366RB
> -	 * (tag type A) then please contribute.
> -	 */
> +	struct dsa_port *dp = dsa_slave_to_port(dev);
> +	u8 *tag;
> +	u16 *p;
> +	u16 out;

Reverse Christmass tree.

> +
> +	/* Pad out to at least 60 bytes */
> +	if (unlikely(eth_skb_pad(skb)))
> +		return NULL;

The core will do the padding for you. Turn on .tail_tag in
dsa_device_ops.

> +	if (skb_cow_head(skb, RTL4_A_HDR_LEN) < 0)
> +		return NULL;
> +
> +	netdev_dbg(dev, "add realtek tag to package to port %d\n",
> +		   dp->index);

You can remove this, now that it works. 

> +	skb_push(skb, RTL4_A_HDR_LEN);
> +
> +	memmove(skb->data, skb->data + RTL4_A_HDR_LEN, 2 * ETH_ALEN);
> +	tag = skb->data + 2 * ETH_ALEN;
> +
> +	/* Set Ethertype */
> +	p = (u16 *)tag;
> +	*p = htons(RTL4_A_ETHERTYPE);
> +
> +	out = (RTL4_A_PROTOCOL_RTL8366RB << 12) | (2 << 8);

Do we know what bit 9 means?
   
   Andrew

Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

[DENG Qingfang]

On Wed, Feb 17, 2021 at 11:29 AM Andrew Lunn <andrew@lunn.ch> wrote:
>
> > +     /* Pad out to at least 60 bytes */
> > +     if (unlikely(eth_skb_pad(skb)))
> > +             return NULL;
>
> The core will do the padding for you. Turn on .tail_tag in
> dsa_device_ops.
>

Please don't. .tail_tag does far more than just padding.

Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

[DENG Qingfang]

On Wed, Feb 17, 2021 at 7:55 AM Linus Walleij <linus.walleij@linaro.org> wrote:
>
> +       if (skb_cow_head(skb, RTL4_A_HDR_LEN) < 0)
> +               return NULL;

skb_cow_head is unnecessary here. DSA core will do it for you.

> +
> +       out = (RTL4_A_PROTOCOL_RTL8366RB << 12) | (2 << 8);
> +       /* The lower bits is the port numer */

Typo?

> +       out |= (u8)dp->index;
> +       p = (u16 *)(tag + 2);
> +       *p = htons(out);
> +
>         return skb;
>  }
>
> --
> 2.29.2
>

Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

[Vladimir Oltean]

On Wed, Feb 17, 2021 at 12:55:42AM +0100, Linus Walleij wrote:
> Support also transmitting frames using the custom "8899 A"
> 4 byte tag.
> 
> Qingfang came up with the solution: we need to pad the
> ethernet frame to 60 bytes using eth_skb_pad(), then the
> switch will happily accept frames with custom tags.

I think it's pretty frustrating to realize that 'it didn't werk' because
you only tested with ping.

> Cc: Mauri Sandberg <sandberg@mailfence.com>
> Reported-by: DENG Qingfang <dqfext@gmail.com>

Reported-by seems like a bit of an understatement. Suggested-by maybe?

> Fixes: efd7fe68f0c6 ("net: dsa: tag_rtl4_a: Implement Realtek 4 byte A tag")
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> ---
>  net/dsa/tag_rtl4_a.c | 43 +++++++++++++++++++++++++++++--------------
>  1 file changed, 29 insertions(+), 14 deletions(-)
> 
> diff --git a/net/dsa/tag_rtl4_a.c b/net/dsa/tag_rtl4_a.c
> index 2646abe5a69e..c17d39b4a1a0 100644
> --- a/net/dsa/tag_rtl4_a.c
> +++ b/net/dsa/tag_rtl4_a.c
> @@ -12,9 +12,7 @@
>   *
>   * The 2 bytes tag form a 16 bit big endian word. The exact
>   * meaning has been guessed from packet dumps from ingress
> - * frames, as no working egress traffic has been available
> - * we do not know the format of the egress tags or if they
> - * are even supported.
> + * frames.
>   */
>  
>  #include <linux/etherdevice.h>
> @@ -36,17 +34,34 @@
>  static struct sk_buff *rtl4a_tag_xmit(struct sk_buff *skb,
>  				      struct net_device *dev)
>  {
> -	/*
> -	 * Just let it pass thru, we don't know if it is possible
> -	 * to tag a frame with the 0x8899 ethertype and direct it
> -	 * to a specific port, all attempts at reverse-engineering have
> -	 * ended up with the frames getting dropped.
> -	 *
> -	 * The VLAN set-up needs to restrict the frames to the right port.
> -	 *
> -	 * If you have documentation on the tagging format for RTL8366RB
> -	 * (tag type A) then please contribute.
> -	 */
> +	struct dsa_port *dp = dsa_slave_to_port(dev);
> +	u8 *tag;
> +	u16 *p;
> +	u16 out;
> +
> +	/* Pad out to at least 60 bytes */
> +	if (unlikely(eth_skb_pad(skb)))
> +		return NULL;
> +	if (skb_cow_head(skb, RTL4_A_HDR_LEN) < 0)
> +		return NULL;
> +
> +	netdev_dbg(dev, "add realtek tag to package to port %d\n",
> +		   dp->index);

You should probably remove any sort of printing from the hot path.

> +	skb_push(skb, RTL4_A_HDR_LEN);
> +
> +	memmove(skb->data, skb->data + RTL4_A_HDR_LEN, 2 * ETH_ALEN);
> +	tag = skb->data + 2 * ETH_ALEN;
> +
> +	/* Set Ethertype */
> +	p = (u16 *)tag;
> +	*p = htons(RTL4_A_ETHERTYPE);
> +
> +	out = (RTL4_A_PROTOCOL_RTL8366RB << 12) | (2 << 8);
> +	/* The lower bits is the port numer */
> +	out |= (u8)dp->index;
> +	p = (u16 *)(tag + 2);
> +	*p = htons(out);
> +
>  	return skb;
>  }
>  
> -- 
> 2.29.2
> 

Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

[Vladimir Oltean]

On Wed, Feb 17, 2021 at 01:37:21PM +0800, DENG Qingfang wrote:
> On Wed, Feb 17, 2021 at 11:29 AM Andrew Lunn <andrew@lunn.ch> wrote:
> >
> > > +     /* Pad out to at least 60 bytes */
> > > +     if (unlikely(eth_skb_pad(skb)))
> > > +             return NULL;
> >
> > The core will do the padding for you. Turn on .tail_tag in
> > dsa_device_ops.
> >
> 
> Please don't. .tail_tag does far more than just padding.

Qingfang is right, this is an EtherType tagging protocol not a tail tagger.

Note that there was an interesting discussion with Florian and Kuba
around the Broadcom switches that require padding for short frames too:
https://patchwork.kernel.org/project/netdevbpf/patch/20201101191620.589272-10-vladimir.oltean@nxp.com/

Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

[DENG Qingfang]

On Wed, Feb 17, 2021 at 7:55 AM Linus Walleij <linus.walleij@linaro.org> wrote:
>
> +
> +       /* Pad out to at least 60 bytes */
> +       if (unlikely(eth_skb_pad(skb)))
> +               return NULL;

I just found that this will cause double free (eth_skb_pad will free
the skb if allocation fails, and dsa_slave_xmit will still try to free
it because it returns NULL.
Replace eth_skb_pad(skb) with __skb_put_padto(skb, ETH_ZLEN, false) to
avoid that.

Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

[Vladimir Oltean]

On Wed, Feb 17, 2021 at 08:38:30PM +0800, DENG Qingfang wrote:
> On Wed, Feb 17, 2021 at 7:55 AM Linus Walleij <linus.walleij@linaro.org> wrote:
> > +
> > +       /* Pad out to at least 60 bytes */
> > +       if (unlikely(eth_skb_pad(skb)))
> > +               return NULL;
>
> I just found that this will cause double free (eth_skb_pad will free
> the skb if allocation fails, and dsa_slave_xmit will still try to free
> it because it returns NULL.
> Replace eth_skb_pad(skb) with __skb_put_padto(skb, ETH_ZLEN, false) to
> avoid that.

Good catch, Qingfang.

Re: [PATCH] net: dsa: tag_rtl4_a: Support also egress tags

[Heiner Kallweit]

On 17.02.2021 13:42, Vladimir Oltean wrote:
> On Wed, Feb 17, 2021 at 08:38:30PM +0800, DENG Qingfang wrote:
>> On Wed, Feb 17, 2021 at 7:55 AM Linus Walleij <linus.walleij@linaro.org> wrote:
>>> +
>>> +       /* Pad out to at least 60 bytes */
>>> +       if (unlikely(eth_skb_pad(skb)))
>>> +               return NULL;
>>
>> I just found that this will cause double free (eth_skb_pad will free
>> the skb if allocation fails, and dsa_slave_xmit will still try to free
>> it because it returns NULL.
>> Replace eth_skb_pad(skb) with __skb_put_padto(skb, ETH_ZLEN, false) to
>> avoid that.
> 

That's really a pitfall. I had to do the same in r8169:
cc6528bc9a0c ("r8169: fix potential skb double free in an error path")

> Good catch, Qingfang.
>