| Message ID | 20201210155058.14518-2-tparkin@katalix.com |
|---|---|
| State | New |
| Headers | show |
| Series | add ppp_generic ioctl(s) to bridge channels | expand |
Hello, Le 10/12/2020 à 16:50, Tom Parkin a écrit : > This new ioctl pair allows two ppp channels to be bridged together: > frames arriving in one channel are transmitted in the other channel > and vice versa. > > The practical use for this is primarily to support the L2TP Access > Concentrator use-case. The end-user session is presented as a ppp > channel (typically PPPoE, although it could be e.g. PPPoA, or even PPP > over a serial link) and is switched into a PPPoL2TP session for > transmission to the LNS. At the LNS the PPP session is terminated in > the ISP's network. > > When a PPP channel is bridged to another it takes a reference on the > other's struct ppp_file. This reference is dropped when the channels > are unbridged, which can occur either explicitly on userspace calling > the PPPIOCUNBRIDGECHAN ioctl, or implicitly when either channel in the > bridge is unregistered. > > In order to implement the channel bridge, struct channel is extended > with a new field, 'bridge', which points to the other struct channel > making up the bridge. > > This pointer is RCU protected to avoid adding another lock to the data > path. > > To guard against concurrent writes to the pointer, the existing struct > channel lock 'upl' coverage is extended rather than adding a new lock. > > The 'upl' lock is used to protect the existing unit pointer. Since the > bridge effectively replaces the unit (they're mutually exclusive for a > channel) it makes coding easier to use the same lock to cover them > both. > > Signed-off-by: Tom Parkin <tparkin@katalix.com> > --- > drivers/net/ppp/ppp_generic.c | 152 ++++++++++++++++++++++++++++++++- > include/uapi/linux/ppp-ioctl.h | 2 + > 2 files changed, 151 insertions(+), 3 deletions(-) > > diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c > index 7d005896a0f9..09c27f7773f9 100644 > --- a/drivers/net/ppp/ppp_generic.c > +++ b/drivers/net/ppp/ppp_generic.c > @@ -174,7 +174,8 @@ struct channel { > struct ppp *ppp; /* ppp unit we're connected to */ > struct net *chan_net; /* the net channel belongs to */ > struct list_head clist; /* link in list of channels per unit */ > - rwlock_t upl; /* protects `ppp' */ > + rwlock_t upl; /* protects `ppp' and 'bridge' */ > + struct channel __rcu *bridge; /* "bridged" ppp channel */ > #ifdef CONFIG_PPP_MULTILINK > u8 avail; /* flag used in multilink stuff */ > u8 had_frag; /* >= 1 fragments have been sent */ > @@ -606,6 +607,83 @@ static struct bpf_prog *compat_ppp_get_filter(struct sock_fprog32 __user *p) > #endif > #endif > > +/* Bridge one PPP channel to another. > + * When two channels are bridged, ppp_input on one channel is redirected to > + * the other's ops->start_xmit handler. > + * In order to safely bridge channels we must reject channels which are already > + * part of a bridge instance, or which form part of an existing unit. > + * Once successfully bridged, each channel holds a reference on the other > + * to prevent it being freed while the bridge is extant. > + */ > +static int ppp_bridge_channels(struct channel *pch, struct channel *pchb) > +{ > + write_lock_bh(&pch->upl); > + if (pch->ppp || > + rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl))) { > + write_unlock_bh(&pch->upl); > + return -EALREADY; > + } > + rcu_assign_pointer(pch->bridge, pchb); > + write_unlock_bh(&pch->upl); > + This is mostly for my own education: I might be misunderstanding something, but is there anything that would prevent a packet from being forwarded from pch to pchb at this precise moment? If not, then it might be theoretically possible to have any answer to said packet (say, a LCP ACK) to be received before the pchb->bridge is set? > + write_lock_bh(&pchb->upl); > + if (pchb->ppp || > + rcu_dereference_protected(pchb->bridge, lockdep_is_held(&pchb->upl))) { > + write_unlock_bh(&pchb->upl); > + goto err_unset; > + } > + rcu_assign_pointer(pchb->bridge, pch); > + write_unlock_bh(&pchb->upl); > + > + refcount_inc(&pch->file.refcnt); > + refcount_inc(&pchb->file.refcnt); > + > + return 0; > + > +err_unset: > + write_lock_bh(&pch->upl); > + RCU_INIT_POINTER(pch->bridge, NULL); > + write_unlock_bh(&pch->upl); > + synchronize_rcu(); > + return -EALREADY; > +}
On Mon, Jan 11, 2021 at 02:17:13PM +0100, Simon Chopin wrote: > Hello, > > Le 10/12/2020 à 16:50, Tom Parkin a écrit : > > This new ioctl pair allows two ppp channels to be bridged together: > > frames arriving in one channel are transmitted in the other channel > > and vice versa. > > > > The practical use for this is primarily to support the L2TP Access > > Concentrator use-case. The end-user session is presented as a ppp > > channel (typically PPPoE, although it could be e.g. PPPoA, or even PPP > > over a serial link) and is switched into a PPPoL2TP session for > > transmission to the LNS. At the LNS the PPP session is terminated in > > the ISP's network. > > > > When a PPP channel is bridged to another it takes a reference on the > > other's struct ppp_file. This reference is dropped when the channels > > are unbridged, which can occur either explicitly on userspace calling > > the PPPIOCUNBRIDGECHAN ioctl, or implicitly when either channel in the > > bridge is unregistered. > > > > In order to implement the channel bridge, struct channel is extended > > with a new field, 'bridge', which points to the other struct channel > > making up the bridge. > > > > This pointer is RCU protected to avoid adding another lock to the data > > path. > > > > To guard against concurrent writes to the pointer, the existing struct > > channel lock 'upl' coverage is extended rather than adding a new lock. > > > > The 'upl' lock is used to protect the existing unit pointer. Since the > > bridge effectively replaces the unit (they're mutually exclusive for a > > channel) it makes coding easier to use the same lock to cover them > > both. > > > > Signed-off-by: Tom Parkin <tparkin@katalix.com> > > --- > > drivers/net/ppp/ppp_generic.c | 152 ++++++++++++++++++++++++++++++++- > > include/uapi/linux/ppp-ioctl.h | 2 + > > 2 files changed, 151 insertions(+), 3 deletions(-) > > > > diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c > > index 7d005896a0f9..09c27f7773f9 100644 > > --- a/drivers/net/ppp/ppp_generic.c > > +++ b/drivers/net/ppp/ppp_generic.c > > @@ -174,7 +174,8 @@ struct channel { > > struct ppp *ppp; /* ppp unit we're connected to */ > > struct net *chan_net; /* the net channel belongs to */ > > struct list_head clist; /* link in list of channels per unit */ > > - rwlock_t upl; /* protects `ppp' */ > > + rwlock_t upl; /* protects `ppp' and 'bridge' */ > > + struct channel __rcu *bridge; /* "bridged" ppp channel */ > > #ifdef CONFIG_PPP_MULTILINK > > u8 avail; /* flag used in multilink stuff */ > > u8 had_frag; /* >= 1 fragments have been sent */ > > @@ -606,6 +607,83 @@ static struct bpf_prog *compat_ppp_get_filter(struct sock_fprog32 __user *p) > > #endif > > #endif > > > > +/* Bridge one PPP channel to another. > > + * When two channels are bridged, ppp_input on one channel is redirected to > > + * the other's ops->start_xmit handler. > > + * In order to safely bridge channels we must reject channels which are already > > + * part of a bridge instance, or which form part of an existing unit. > > + * Once successfully bridged, each channel holds a reference on the other > > + * to prevent it being freed while the bridge is extant. > > + */ > > +static int ppp_bridge_channels(struct channel *pch, struct channel *pchb) > > +{ > > + write_lock_bh(&pch->upl); > > + if (pch->ppp || > > + rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl))) { > > + write_unlock_bh(&pch->upl); > > + return -EALREADY; > > + } > > + rcu_assign_pointer(pch->bridge, pchb); > > + write_unlock_bh(&pch->upl); > > + > This is mostly for my own education: > > I might be misunderstanding something, but is there anything > that would prevent a packet from being forwarded from pch to pchb at this > precise moment? If not, then it might be theoretically possible to have > any answer to said packet (say, a LCP ACK) to be received before the > pchb->bridge is set? That's possible in theory. But I can't see any problem in practice, because: * It's unlikely the round trip time would be small enough to trigger this situation. * If this situation ever happens, the reply is passed to user space, which is free to forward it to the other channel. If the user space implementation isn't prepared for this situation and just drops the packet, that's fine too. It's just a transient packet drop, which PPP peers are supposed to handle just fine (this can happen anywhere in the network after all). * Any use case I know for channel bridging involves a "live" channel (where LCP and authentication protocols are running) and an "idle" channel (where no protocol is running at all yet). So the problem can be avoided entirely by calling the PPPIOCBRIDGECHAN ioctl on the idle channel file descriptor, rather than on the live channel. Or did you have any other possible problem in mind? > > + write_lock_bh(&pchb->upl); > > + if (pchb->ppp || > > + rcu_dereference_protected(pchb->bridge, lockdep_is_held(&pchb->upl))) { > > + write_unlock_bh(&pchb->upl); > > + goto err_unset; > > + } > > + rcu_assign_pointer(pchb->bridge, pch); > > + write_unlock_bh(&pchb->upl); > > + > > + refcount_inc(&pch->file.refcnt); > > + refcount_inc(&pchb->file.refcnt); > > + > > + return 0; > > + > > +err_unset: > > + write_lock_bh(&pch->upl); > > + RCU_INIT_POINTER(pch->bridge, NULL); > > + write_unlock_bh(&pch->upl); > > + synchronize_rcu(); > > + return -EALREADY; > > +} >
diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c index 7d005896a0f9..09c27f7773f9 100644 --- a/drivers/net/ppp/ppp_generic.c +++ b/drivers/net/ppp/ppp_generic.c @@ -174,7 +174,8 @@ struct channel { struct ppp *ppp; /* ppp unit we're connected to */ struct net *chan_net; /* the net channel belongs to */ struct list_head clist; /* link in list of channels per unit */ - rwlock_t upl; /* protects `ppp' */ + rwlock_t upl; /* protects `ppp' and 'bridge' */ + struct channel __rcu *bridge; /* "bridged" ppp channel */ #ifdef CONFIG_PPP_MULTILINK u8 avail; /* flag used in multilink stuff */ u8 had_frag; /* >= 1 fragments have been sent */ @@ -606,6 +607,83 @@ static struct bpf_prog *compat_ppp_get_filter(struct sock_fprog32 __user *p) #endif #endif +/* Bridge one PPP channel to another. + * When two channels are bridged, ppp_input on one channel is redirected to + * the other's ops->start_xmit handler. + * In order to safely bridge channels we must reject channels which are already + * part of a bridge instance, or which form part of an existing unit. + * Once successfully bridged, each channel holds a reference on the other + * to prevent it being freed while the bridge is extant. + */ +static int ppp_bridge_channels(struct channel *pch, struct channel *pchb) +{ + write_lock_bh(&pch->upl); + if (pch->ppp || + rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl))) { + write_unlock_bh(&pch->upl); + return -EALREADY; + } + rcu_assign_pointer(pch->bridge, pchb); + write_unlock_bh(&pch->upl); + + write_lock_bh(&pchb->upl); + if (pchb->ppp || + rcu_dereference_protected(pchb->bridge, lockdep_is_held(&pchb->upl))) { + write_unlock_bh(&pchb->upl); + goto err_unset; + } + rcu_assign_pointer(pchb->bridge, pch); + write_unlock_bh(&pchb->upl); + + refcount_inc(&pch->file.refcnt); + refcount_inc(&pchb->file.refcnt); + + return 0; + +err_unset: + write_lock_bh(&pch->upl); + RCU_INIT_POINTER(pch->bridge, NULL); + write_unlock_bh(&pch->upl); + synchronize_rcu(); + return -EALREADY; +} + +static int ppp_unbridge_channels(struct channel *pch) +{ + struct channel *pchb, *pchbb; + + write_lock_bh(&pch->upl); + pchb = rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl)); + if (!pchb) { + write_unlock_bh(&pch->upl); + return -EINVAL; + } + RCU_INIT_POINTER(pch->bridge, NULL); + write_unlock_bh(&pch->upl); + + /* Only modify pchb if phcb->bridge points back to pch. + * If not, it implies that there has been a race unbridging (and possibly + * even rebridging) pchb. We should leave pchb alone to avoid either a + * refcount underflow, or breaking another established bridge instance. + */ + write_lock_bh(&pchb->upl); + pchbb = rcu_dereference_protected(pchb->bridge, lockdep_is_held(&pchb->upl)); + if (pchbb == pch) + RCU_INIT_POINTER(pchb->bridge, NULL); + write_unlock_bh(&pchb->upl); + + synchronize_rcu(); + + if (pchbb == pch) + if (refcount_dec_and_test(&pch->file.refcnt)) + ppp_destroy_channel(pch); + + if (refcount_dec_and_test(&pchb->file.refcnt)) + ppp_destroy_channel(pchb); + + return 0; +} + static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg) { struct ppp_file *pf; @@ -641,8 +719,9 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg) } if (pf->kind == CHANNEL) { - struct channel *pch; + struct channel *pch, *pchb; struct ppp_channel *chan; + struct ppp_net *pn; pch = PF_TO_CHANNEL(pf); @@ -657,6 +736,31 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg) err = ppp_disconnect_channel(pch); break; + case PPPIOCBRIDGECHAN: + if (get_user(unit, p)) + break; + err = -ENXIO; + pn = ppp_pernet(current->nsproxy->net_ns); + spin_lock_bh(&pn->all_channels_lock); + pchb = ppp_find_channel(pn, unit); + /* Hold a reference to prevent pchb being freed while + * we establish the bridge. + */ + if (pchb) + refcount_inc(&pchb->file.refcnt); + spin_unlock_bh(&pn->all_channels_lock); + if (!pchb) + break; + err = ppp_bridge_channels(pch, pchb); + /* Drop earlier refcount now bridge establishment is complete */ + if (refcount_dec_and_test(&pchb->file.refcnt)) + ppp_destroy_channel(pchb); + break; + + case PPPIOCUNBRIDGECHAN: + err = ppp_unbridge_channels(pch); + break; + default: down_read(&pch->chan_sem); chan = pch->chan; @@ -2089,6 +2193,40 @@ static bool ppp_decompress_proto(struct sk_buff *skb) return pskb_may_pull(skb, 2); } +/* Attempt to handle a frame via. a bridged channel, if one exists. + * If the channel is bridged, the frame is consumed by the bridge. + * If not, the caller must handle the frame by normal recv mechanisms. + * Returns true if the frame is consumed, false otherwise. + */ +static bool ppp_channel_bridge_input(struct channel *pch, struct sk_buff *skb) +{ + struct channel *pchb; + + rcu_read_lock(); + pchb = rcu_dereference(pch->bridge); + if (!pchb) + goto out_rcu; + + spin_lock(&pchb->downl); + if (!pchb->chan) { + /* channel got unregistered */ + kfree_skb(skb); + goto outl; + } + + skb_scrub_packet(skb, !net_eq(pch->chan_net, pchb->chan_net)); + if (!pchb->chan->ops->start_xmit(pchb->chan, skb)) + kfree_skb(skb); + +outl: + spin_unlock(&pchb->downl); +out_rcu: + rcu_read_unlock(); + + /* If pchb is set then we've consumed the packet */ + return !!pchb; +} + void ppp_input(struct ppp_channel *chan, struct sk_buff *skb) { @@ -2100,6 +2238,10 @@ ppp_input(struct ppp_channel *chan, struct sk_buff *skb) return; } + /* If the channel is bridged, transmit via. bridge */ + if (ppp_channel_bridge_input(pch, skb)) + return; + read_lock_bh(&pch->upl); if (!ppp_decompress_proto(skb)) { kfree_skb(skb); @@ -2796,8 +2938,11 @@ ppp_unregister_channel(struct ppp_channel *chan) list_del(&pch->list); spin_unlock_bh(&pn->all_channels_lock); + ppp_unbridge_channels(pch); + pch->file.dead = 1; wake_up_interruptible(&pch->file.rwait); + if (refcount_dec_and_test(&pch->file.refcnt)) ppp_destroy_channel(pch); } @@ -3270,7 +3415,8 @@ ppp_connect_channel(struct channel *pch, int unit) goto out; write_lock_bh(&pch->upl); ret = -EINVAL; - if (pch->ppp) + if (pch->ppp || + rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl))) goto outl; ppp_lock(ppp); diff --git a/include/uapi/linux/ppp-ioctl.h b/include/uapi/linux/ppp-ioctl.h index 7bd2a5a75348..8dbecb3ad036 100644 --- a/include/uapi/linux/ppp-ioctl.h +++ b/include/uapi/linux/ppp-ioctl.h @@ -115,6 +115,8 @@ struct pppol2tp_ioc_stats { #define PPPIOCATTCHAN _IOW('t', 56, int) /* attach to ppp channel */ #define PPPIOCGCHAN _IOR('t', 55, int) /* get ppp channel number */ #define PPPIOCGL2TPSTATS _IOR('t', 54, struct pppol2tp_ioc_stats) +#define PPPIOCBRIDGECHAN _IOW('t', 53, int) /* bridge one channel to another */ +#define PPPIOCUNBRIDGECHAN _IO('t', 54) /* unbridge channel */ #define SIOCGPPPSTATS (SIOCDEVPRIVATE + 0) #define SIOCGPPPVER (SIOCDEVPRIVATE + 1) /* NEVER change this!! */
This new ioctl pair allows two ppp channels to be bridged together: frames arriving in one channel are transmitted in the other channel and vice versa. The practical use for this is primarily to support the L2TP Access Concentrator use-case. The end-user session is presented as a ppp channel (typically PPPoE, although it could be e.g. PPPoA, or even PPP over a serial link) and is switched into a PPPoL2TP session for transmission to the LNS. At the LNS the PPP session is terminated in the ISP's network. When a PPP channel is bridged to another it takes a reference on the other's struct ppp_file. This reference is dropped when the channels are unbridged, which can occur either explicitly on userspace calling the PPPIOCUNBRIDGECHAN ioctl, or implicitly when either channel in the bridge is unregistered. In order to implement the channel bridge, struct channel is extended with a new field, 'bridge', which points to the other struct channel making up the bridge. This pointer is RCU protected to avoid adding another lock to the data path. To guard against concurrent writes to the pointer, the existing struct channel lock 'upl' coverage is extended rather than adding a new lock. The 'upl' lock is used to protect the existing unit pointer. Since the bridge effectively replaces the unit (they're mutually exclusive for a channel) it makes coding easier to use the same lock to cover them both. Signed-off-by: Tom Parkin <tparkin@katalix.com> --- drivers/net/ppp/ppp_generic.c | 152 ++++++++++++++++++++++++++++++++- include/uapi/linux/ppp-ioctl.h | 2 + 2 files changed, 151 insertions(+), 3 deletions(-)