Re: [PATCH bpf-next v5 2/3] bpf, sockmap: Fix FIONREAD for sockmap

["Jiayuan Chen" <jiayuan.chen@linux.dev>]

January 7, 2026 at 22:23, "Jakub Sitnicki" <jakub@cloudflare.com mailto:jakub@cloudflare.com?to=%22Jakub%20Sitnicki%22%20%3Cjakub%40cloudflare.com%3E > wrote:


> 
> On Tue, Jan 06, 2026 at 01:14 PM +08, Jiayuan Chen wrote:
> 
> > 
> > A socket using sockmap has its own independent receive queue: ingress_msg.
> >  This queue may contain data from its own protocol stack or from other
> >  sockets.
> > 
> >  Therefore, for sockmap, relying solely on copied_seq and rcv_nxt to
> >  calculate FIONREAD is not enough.
> > 
> >  This patch adds a new ingress_size field in the psock structure to record
> >  the data length in ingress_msg. Additionally, we implement new ioctl
> >  interfaces for TCP and UDP to intercept FIONREAD operations. While Unix
> >  and VSOCK also support sockmap and have similar FIONREAD calculation
> >  issues, fixing them would require more extensive changes
> >  (please let me know if modifications are needed). I believe it's not
> >  appropriate to include those changes under this fix patch.
> > 
> Nit: These last two lines don't really belong in the commit message.
> Side notes for reviewers can be added after the "---" marker.
> 
> > 
> > Previous work by John Fastabend made some efforts towards FIONREAD support:
> >  commit e5c6de5fa025 ("bpf, sockmap: Incorrectly handling copied_seq")
> >  Although the current patch is based on the previous work by John Fastabend,
> >  it is acceptable for our Fixes tag to point to the same commit.
> > 
> >  FD1:read()
> >  -- FD1->copied_seq++
> >  | [read data]
> >  |
> >  [enqueue data] v
> >  [sockmap] -> ingress to self -> ingress_msg queue
> >  FD1 native stack ------> ^
> >  -- FD1->rcv_nxt++ -> redirect to other | [enqueue data]
> >  | |
> >  | ingress to FD1
> >  v ^
> >  ... | [sockmap]
> >  FD2 native stack
> > 
> >  Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()")
> >  Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> >  ---
> >  include/linux/skmsg.h | 67 +++++++++++++++++++++++++++++++++++++++++--
> >  net/core/skmsg.c | 3 ++
> >  net/ipv4/tcp_bpf.c | 21 ++++++++++++++
> >  net/ipv4/udp_bpf.c | 25 +++++++++++++---
> >  4 files changed, 110 insertions(+), 6 deletions(-)
> > 
> >  diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> >  index 0323a2b6cf5e..1fa03953043f 100644
> >  --- a/include/linux/skmsg.h
> >  +++ b/include/linux/skmsg.h
> >  @@ -97,6 +97,7 @@ struct sk_psock {
> >  struct sk_buff_head ingress_skb;
> >  struct list_head ingress_msg;
> >  spinlock_t ingress_lock;
> >  + ssize_t ingress_size;
> > 
> The name is not great because we also already have `ingress_bytes`.
> I suggest to rename and add a doc string. Also we don't expect the count
> to ever be negative. Why ssize_t when we store all other byte counts
> there as u32?
> 
>  /** @msg_tot_len: Total bytes queued in ingress_msg list. */
>  u32 msg_tot_len;
> 
> > 
> > unsigned long state;
> >  struct list_head link;
> >  spinlock_t link_lock;
> >  @@ -321,6 +322,27 @@ static inline void sock_drop(struct sock *sk, struct sk_buff *skb)
> >  kfree_skb(skb);
> >  }
> >  
> >  +static inline ssize_t sk_psock_get_msg_size_nolock(struct sk_psock *psock)
> >  +{
> >  + /* Used by ioctl to read ingress_size only; lock-free for performance */
> >  + return READ_ONCE(psock->ingress_size);
> >  +}
> >  +
> >  +static inline void sk_psock_inc_msg_size_locked(struct sk_psock *psock, ssize_t diff)
> >  +{
> >  + /* Use WRITE_ONCE to ensure correct read in sk_psock_get_msg_size_nolock().
> >  + * ingress_lock should be held to prevent concurrent updates to ingress_size
> >  + */
> >  + WRITE_ONCE(psock->ingress_size, psock->ingress_size + diff);
> >  +}
> >  +
> >  +static inline void sk_psock_inc_msg_size(struct sk_psock *psock, ssize_t diff)
> > 
> Not sure about this function name. "inc" usually means increment by one.
> Was that modeled after some existing interface?
> 
> If not, I'd switch rename to sk_psock_msg_len_add(..., int delta)
> 
> Following the naming convention from sk_forward_alloc_add(),
> skb_frag_size_add(), skb_len_add(), etc.
> 
> > 
> > +{
> >  + spin_lock_bh(&psock->ingress_lock);
> >  + sk_psock_inc_msg_size_locked(psock, diff);
> >  + spin_unlock_bh(&psock->ingress_lock);
> >  +}
> >  +
> >  static inline bool sk_psock_queue_msg(struct sk_psock *psock,
> >  struct sk_msg *msg)
> >  {
> >  @@ -329,6 +351,7 @@ static inline bool sk_psock_queue_msg(struct sk_psock *psock,
> >  spin_lock_bh(&psock->ingress_lock);
> >  if (sk_psock_test_state(psock, SK_PSOCK_TX_ENABLED)) {
> >  list_add_tail(&msg->list, &psock->ingress_msg);
> >  + sk_psock_inc_msg_size_locked(psock, msg->sg.size);
> >  ret = true;
> >  } else {
> >  sk_msg_free(psock->sk, msg);
> >  @@ -345,18 +368,25 @@ static inline struct sk_msg *sk_psock_dequeue_msg(struct sk_psock *psock)
> >  
> >  spin_lock_bh(&psock->ingress_lock);
> >  msg = list_first_entry_or_null(&psock->ingress_msg, struct sk_msg, list);
> >  - if (msg)
> >  + if (msg) {
> >  list_del(&msg->list);
> >  + sk_psock_inc_msg_size_locked(psock, -msg->sg.size);
> >  + }
> >  spin_unlock_bh(&psock->ingress_lock);
> >  return msg;
> >  }
> >  
> >  +static inline struct sk_msg *sk_psock_peek_msg_locked(struct sk_psock *psock)
> >  +{
> >  + return list_first_entry_or_null(&psock->ingress_msg, struct sk_msg, list);
> >  +}
> >  +
> >  static inline struct sk_msg *sk_psock_peek_msg(struct sk_psock *psock)
> >  {
> >  struct sk_msg *msg;
> >  
> >  spin_lock_bh(&psock->ingress_lock);
> >  - msg = list_first_entry_or_null(&psock->ingress_msg, struct sk_msg, list);
> >  + msg = sk_psock_peek_msg_locked(psock);
> >  spin_unlock_bh(&psock->ingress_lock);
> >  return msg;
> >  }
> >  @@ -523,6 +553,39 @@ static inline bool sk_psock_strp_enabled(struct sk_psock *psock)
> >  return !!psock->saved_data_ready;
> >  }
> >  
> >  +/* for tcp only, sk is locked */
> >  +static inline ssize_t sk_psock_msg_inq(struct sock *sk)
> >  +{
> >  + struct sk_psock *psock;
> >  + ssize_t inq = 0;
> >  +
> >  + psock = sk_psock_get(sk);
> >  + if (likely(psock)) {
> >  + inq = sk_psock_get_msg_size_nolock(psock);
> >  + sk_psock_put(sk, psock);
> >  + }
> >  + return inq;
> >  +}
> >  +
> >  +/* for udp only, sk is not locked */
> >  +static inline ssize_t sk_msg_first_length(struct sock *sk)
> > 
> s/_length/_len/
> 
> > 
> > +{
> >  + struct sk_psock *psock;
> >  + struct sk_msg *msg;
> >  + ssize_t inq = 0;
> >  +
> >  + psock = sk_psock_get(sk);
> >  + if (likely(psock)) {
> >  + spin_lock_bh(&psock->ingress_lock);
> >  + msg = sk_psock_peek_msg_locked(psock);
> >  + if (msg)
> >  + inq = msg->sg.size;
> >  + spin_unlock_bh(&psock->ingress_lock);
> >  + sk_psock_put(sk, psock);
> >  + }
> >  + return inq;
> >  +}
> >  +
> >  #if IS_ENABLED(CONFIG_NET_SOCK_MSG)
> >  
> >  #define BPF_F_STRPARSER (1UL << 1)
> >  diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> >  index d73e03f7713a..c959d52a62b2 100644
> >  --- a/net/core/skmsg.c
> >  +++ b/net/core/skmsg.c
> >  @@ -455,6 +455,7 @@ int __sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg
> >  atomic_sub(copy, &sk->sk_rmem_alloc);
> >  }
> >  msg_rx->sg.size -= copy;
> >  + sk_psock_inc_msg_size(psock, -copy);
> >  
> >  if (!sge->length) {
> >  sk_msg_iter_var_next(i);
> >  @@ -819,9 +820,11 @@ static void __sk_psock_purge_ingress_msg(struct sk_psock *psock)
> >  list_del(&msg->list);
> >  if (!msg->skb)
> >  atomic_sub(msg->sg.size, &psock->sk->sk_rmem_alloc);
> >  + sk_psock_inc_msg_size(psock, -((ssize_t)msg->sg.size));
> > 
> Cast won't be needed after you switch param type to `int`.
> 
> > 
> > sk_msg_free(psock->sk, msg);
> >  kfree(msg);
> >  }
> >  + WARN_ON_ONCE(psock->ingress_size);
> >  }
> >  
> >  static void __sk_psock_zap_ingress(struct sk_psock *psock)
> >  diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
> >  index 6332fc36ffe6..a9c758868f13 100644
> >  --- a/net/ipv4/tcp_bpf.c
> >  +++ b/net/ipv4/tcp_bpf.c
> >  @@ -10,6 +10,7 @@
> >  
> >  #include <net/inet_common.h>
> >  #include <net/tls.h>
> >  +#include <asm/ioctls.h>
> >  
> >  void tcp_eat_skb(struct sock *sk, struct sk_buff *skb)
> >  {
> >  @@ -332,6 +333,25 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk,
> >  return copied;
> >  }
> >  
> >  +static int tcp_bpf_ioctl(struct sock *sk, int cmd, int *karg)
> >  +{
> >  + bool slow;
> >  +
> >  + /* we only care about FIONREAD */
> > 
> Nit: This comment seems redundant. The expression is obvious.
> 
> > 
> > + if (cmd != SIOCINQ)
> >  + return tcp_ioctl(sk, cmd, karg);
> >  +
> >  + /* works similar as tcp_ioctl */
> >  + if (sk->sk_state == TCP_LISTEN)
> >  + return -EINVAL;
> >  +
> >  + slow = lock_sock_fast(sk);
> >  + *karg = sk_psock_msg_inq(sk);
> >  + unlock_sock_fast(sk, slow);
> >  +
> >  + return 0;
> >  +}
> >  +
> >  static int tcp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
> >  int flags, int *addr_len)
> >  {
> >  @@ -610,6 +630,7 @@ static void tcp_bpf_rebuild_protos(struct proto prot[TCP_BPF_NUM_CFGS],
> >  prot[TCP_BPF_BASE].close = sock_map_close;
> >  prot[TCP_BPF_BASE].recvmsg = tcp_bpf_recvmsg;
> >  prot[TCP_BPF_BASE].sock_is_readable = sk_msg_is_readable;
> >  + prot[TCP_BPF_BASE].ioctl = tcp_bpf_ioctl;
> >  
> >  prot[TCP_BPF_TX] = prot[TCP_BPF_BASE];
> >  prot[TCP_BPF_TX].sendmsg = tcp_bpf_sendmsg;
> >  diff --git a/net/ipv4/udp_bpf.c b/net/ipv4/udp_bpf.c
> >  index 0735d820e413..cc1156aef14d 100644
> >  --- a/net/ipv4/udp_bpf.c
> >  +++ b/net/ipv4/udp_bpf.c
> >  @@ -5,6 +5,7 @@
> >  #include <net/sock.h>
> >  #include <net/udp.h>
> >  #include <net/inet_common.h>
> >  +#include <asm/ioctls.h>
> >  
> >  #include "udp_impl.h"
> >  
> >  @@ -111,12 +112,28 @@ enum {
> >  static DEFINE_SPINLOCK(udpv6_prot_lock);
> >  static struct proto udp_bpf_prots[UDP_BPF_NUM_PROTS];
> >  
> >  +static int udp_bpf_ioctl(struct sock *sk, int cmd, int *karg)
> >  +{
> >  + /* we only care about FIONREAD */
> >  + if (cmd != SIOCINQ)
> >  + return udp_ioctl(sk, cmd, karg);
> >  +
> >  + /* works similar as udp_ioctl.
> >  + * man udp(7): "FIONREAD (SIOCINQ): Returns the size of the next
> >  + * pending datagram in the integer in bytes, or 0 when no datagram
> >  + * is pending."
> >  + */
> > 
> Not sure we need to quote man pages here.
> 
> > 
> > + *karg = sk_msg_first_length(sk);
> >  + return 0;
> >  +}
> >  +
> >  static void udp_bpf_rebuild_protos(struct proto *prot, const struct proto *base)
> >  {
> >  - *prot = *base;
> >  - prot->close = sock_map_close;
> >  - prot->recvmsg = udp_bpf_recvmsg;
> >  - prot->sock_is_readable = sk_msg_is_readable;
> >  + *prot = *base;
> >  + prot->close = sock_map_close;
> >  + prot->recvmsg = udp_bpf_recvmsg;
> >  + prot->sock_is_readable = sk_msg_is_readable;
> >  + prot->ioctl = udp_bpf_ioctl;
> >  }
> >  
> >  static void udp_bpf_check_v6_needs_rebuild(struct proto *ops)
> >
>

Thanks Jakub. All good suggestions. I've applied them in v6.