* [PATCH net-next] net/packet: Fix a comment about hard_header_len and add a warning for it
@ 2020-09-11 5:03 Xie He
2020-09-11 14:31 ` Willem de Bruijn
0 siblings, 1 reply; 5+ messages in thread
From: Xie He @ 2020-09-11 5:03 UTC (permalink / raw)
To: David S. Miller, Jakub Kicinski, netdev, linux-kernel
Cc: Xie He, Willem de Bruijn, Eric Dumazet, Brian Norris, Cong Wang
This patch tries to clarify the difference between hard_header_len and
needed_headroom by fixing an outdated comment and adding a WARN_ON_ONCE
warning for hard_header_len.
The difference between hard_header_len and needed_headroom as understood
by this patch is based on the following reasons:
1.
In af_packet.c, the function packet_snd first reserves a headroom of
length (dev->hard_header_len + dev->needed_headroom).
Then if the socket is a SOCK_DGRAM socket, it calls dev_hard_header,
which calls dev->header_ops->create, to create the link layer header.
If the socket is a SOCK_RAW socket, it "un-reserves" a headroom of
length (dev->hard_header_len), and checks if the user has provided a
header sized between (dev->min_header_len) and (dev->hard_header_len)
(in dev_validate_header).
This shows the developers of af_packet.c expect hard_header_len to
be consistent with header_ops.
2.
In af_packet.c, the function packet_sendmsg_spkt has a FIXME comment.
That comment states that prepending an LL header internally in a driver
is considered a bug. I believe this bug can be fixed by setting
hard_header_len to 0, making the internal header completely invisible
to af_packet.c (and requesting the headroom in needed_headroom instead).
3.
There is a commit for a WiFi driver:
commit 9454f7a895b8 ("mwifiex: set needed_headroom, not hard_header_len")
According to the discussion about it at:
https://patchwork.kernel.org/patch/11407493/
The author tried to set the WiFi driver's hard_header_len to the Ethernet
header length, and request additional header space internally needed by
setting needed_headroom.
This means this usage is already adopted by driver developers.
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Brian Norris <briannorris@chromium.org>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
---
net/packet/af_packet.c | 17 +++++++++++------
1 file changed, 11 insertions(+), 6 deletions(-)
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index af6c93ed9fa0..93c89d3b2511 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -93,12 +93,15 @@
/*
Assumptions:
- - if device has no dev->hard_header routine, it adds and removes ll header
- inside itself. In this case ll header is invisible outside of device,
- but higher levels still should reserve dev->hard_header_len.
- Some devices are enough clever to reallocate skb, when header
- will not fit to reserved space (tunnel), another ones are silly
- (PPP).
+ - If the device has no dev->header_ops, there is no LL header visible
+ above the device. In this case, its hard_header_len should be 0.
+ The device may prepend its own header internally. In this case, its
+ needed_headroom should be set to the space needed for it to add its
+ internal header.
+ For example, a WiFi driver pretending to be an Ethernet driver should
+ set its hard_header_len to be the Ethernet header length, and set its
+ needed_headroom to be (the real WiFi header length - the fake Ethernet
+ header length).
- packet socket receives packets with pulled ll header,
so that SOCK_RAW should push it back.
@@ -2936,6 +2939,8 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
skb_reset_network_header(skb);
err = -EINVAL;
+ if (!dev->header_ops)
+ WARN_ON_ONCE(dev->hard_header_len != 0);
if (sock->type == SOCK_DGRAM) {
offset = dev_hard_header(skb, dev, ntohs(proto), addr, NULL, len);
if (unlikely(offset < 0))
--
2.25.1
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH net-next] net/packet: Fix a comment about hard_header_len and add a warning for it
2020-09-11 5:03 [PATCH net-next] net/packet: Fix a comment about hard_header_len and add a warning for it Xie He
@ 2020-09-11 14:31 ` Willem de Bruijn
2020-09-11 23:37 ` Xie He
0 siblings, 1 reply; 5+ messages in thread
From: Willem de Bruijn @ 2020-09-11 14:31 UTC (permalink / raw)
To: Xie He
Cc: David S. Miller, Jakub Kicinski, Network Development,
linux-kernel, Willem de Bruijn, Eric Dumazet, Brian Norris,
Cong Wang
On Fri, Sep 11, 2020 at 7:04 AM Xie He <xie.he.0141@gmail.com> wrote:
>
> This patch tries to clarify the difference between hard_header_len and
> needed_headroom by fixing an outdated comment and adding a WARN_ON_ONCE
> warning for hard_header_len.
>
> The difference between hard_header_len and needed_headroom as understood
> by this patch is based on the following reasons:
>
> 1.
>
> In af_packet.c, the function packet_snd first reserves a headroom of
> length (dev->hard_header_len + dev->needed_headroom).
> Then if the socket is a SOCK_DGRAM socket, it calls dev_hard_header,
> which calls dev->header_ops->create, to create the link layer header.
> If the socket is a SOCK_RAW socket, it "un-reserves" a headroom of
> length (dev->hard_header_len), and checks if the user has provided a
> header sized between (dev->min_header_len) and (dev->hard_header_len)
> (in dev_validate_header).
> This shows the developers of af_packet.c expect hard_header_len to
> be consistent with header_ops.
>
> 2.
>
> In af_packet.c, the function packet_sendmsg_spkt has a FIXME comment.
> That comment states that prepending an LL header internally in a driver
> is considered a bug. I believe this bug can be fixed by setting
> hard_header_len to 0, making the internal header completely invisible
> to af_packet.c (and requesting the headroom in needed_headroom instead).
>
> 3.
>
> There is a commit for a WiFi driver:
> commit 9454f7a895b8 ("mwifiex: set needed_headroom, not hard_header_len")
> According to the discussion about it at:
> https://patchwork.kernel.org/patch/11407493/
> The author tried to set the WiFi driver's hard_header_len to the Ethernet
> header length, and request additional header space internally needed by
> setting needed_headroom.
> This means this usage is already adopted by driver developers.
>
> Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Cc: Eric Dumazet <eric.dumazet@gmail.com>
> Cc: Brian Norris <briannorris@chromium.org>
> Cc: Cong Wang <xiyou.wangcong@gmail.com>
> Signed-off-by: Xie He <xie.he.0141@gmail.com>
> ---
> net/packet/af_packet.c | 17 +++++++++++------
> 1 file changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index af6c93ed9fa0..93c89d3b2511 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -93,12 +93,15 @@
>
> /*
> Assumptions:
> - - if device has no dev->hard_header routine, it adds and removes ll header
> - inside itself. In this case ll header is invisible outside of device,
> - but higher levels still should reserve dev->hard_header_len.
> - Some devices are enough clever to reallocate skb, when header
> - will not fit to reserved space (tunnel), another ones are silly
> - (PPP).
> + - If the device has no dev->header_ops, there is no LL header visible
> + above the device. In this case, its hard_header_len should be 0.
> + The device may prepend its own header internally. In this case, its
> + needed_headroom should be set to the space needed for it to add its
> + internal header.
> + For example, a WiFi driver pretending to be an Ethernet driver should
> + set its hard_header_len to be the Ethernet header length, and set its
> + needed_headroom to be (the real WiFi header length - the fake Ethernet
> + header length).
> - packet socket receives packets with pulled ll header,
> so that SOCK_RAW should push it back.
>
> @@ -2936,6 +2939,8 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
> skb_reset_network_header(skb);
>
> err = -EINVAL;
> + if (!dev->header_ops)
> + WARN_ON_ONCE(dev->hard_header_len != 0);
> if (sock->type == SOCK_DGRAM) {
> offset = dev_hard_header(skb, dev, ntohs(proto), addr, NULL, len);
> if (unlikely(offset < 0))
From a quick scan, a few device types that might trigger this
net/atm/clip.c
drivers/net/wan/hdlc_fr.c
drivers/net/appletalk/ipddp.c
drivers/net/ppp/ppp_generic.c
drivers/net/net_failover.c
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH net-next] net/packet: Fix a comment about hard_header_len and add a warning for it
2020-09-11 14:31 ` Willem de Bruijn
@ 2020-09-11 23:37 ` Xie He
2020-09-13 8:10 ` Willem de Bruijn
0 siblings, 1 reply; 5+ messages in thread
From: Xie He @ 2020-09-11 23:37 UTC (permalink / raw)
To: Willem de Bruijn
Cc: David S. Miller, Jakub Kicinski, Network Development,
linux-kernel, Eric Dumazet, Brian Norris, Cong Wang
On Fri, Sep 11, 2020 at 7:32 AM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> From a quick scan, a few device types that might trigger this
>
> net/atm/clip.c
> drivers/net/wan/hdlc_fr.c
> drivers/net/appletalk/ipddp.c
> drivers/net/ppp/ppp_generic.c
> drivers/net/net_failover.c
I have recently fixed this problem in the "net" tree in hdlc_fr.c.
Glad to see the number of drivers that have this problem is not very big.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH net-next] net/packet: Fix a comment about hard_header_len and add a warning for it
2020-09-11 23:37 ` Xie He
@ 2020-09-13 8:10 ` Willem de Bruijn
2020-09-13 8:43 ` Xie He
0 siblings, 1 reply; 5+ messages in thread
From: Willem de Bruijn @ 2020-09-13 8:10 UTC (permalink / raw)
To: Xie He
Cc: Willem de Bruijn, David S. Miller, Jakub Kicinski,
Network Development, linux-kernel, Eric Dumazet, Brian Norris,
Cong Wang
On Sat, Sep 12, 2020 at 1:37 AM Xie He <xie.he.0141@gmail.com> wrote:
>
> On Fri, Sep 11, 2020 at 7:32 AM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> >
> > From a quick scan, a few device types that might trigger this
> >
> > net/atm/clip.c
> > drivers/net/wan/hdlc_fr.c
> > drivers/net/appletalk/ipddp.c
> > drivers/net/ppp/ppp_generic.c
> > drivers/net/net_failover.c
>
> I have recently fixed this problem in the "net" tree in hdlc_fr.c.
>
> Glad to see the number of drivers that have this problem is not very big.
I am concerned about adding a WARN_ON_ONCE that we already expect to
fire on some platforms.
Probably better to add the documentation without the warning.
I know I suggested the check before, sorry for the churn, but I hadn't
checked the existing state yet.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH net-next] net/packet: Fix a comment about hard_header_len and add a warning for it
2020-09-13 8:10 ` Willem de Bruijn
@ 2020-09-13 8:43 ` Xie He
0 siblings, 0 replies; 5+ messages in thread
From: Xie He @ 2020-09-13 8:43 UTC (permalink / raw)
To: Willem de Bruijn
Cc: David S. Miller, Jakub Kicinski, Network Development,
linux-kernel, Eric Dumazet, Brian Norris, Cong Wang
On Sun, Sep 13, 2020 at 1:11 AM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> I am concerned about adding a WARN_ON_ONCE that we already expect to
> fire on some platforms.
>
> Probably better to add the documentation without the warning.
>
> I know I suggested the check before, sorry for the churn, but I hadn't
> checked the existing state yet.
OK. No problem. It's always good to discuss and find out the best way
before we do the changes.
I'll drop the WARN_ON_ONCE part and resubmit the patch.
^ permalink raw reply [flat|nested] 5+ messages in thread
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2020-09-11 5:03 [PATCH net-next] net/packet: Fix a comment about hard_header_len and add a warning for it Xie He
2020-09-11 14:31 ` Willem de Bruijn
2020-09-11 23:37 ` Xie He
2020-09-13 8:10 ` Willem de Bruijn
2020-09-13 8:43 ` Xie He
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