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 msg_tot_len 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.
Unix and VSOCK sockets have similar issues, but fixing them is outside
the scope of this patch as it would require more intrusive changes.
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 | 68 +++++++++++++++++++++++++++++++++++++++++--
net/core/skmsg.c | 3 ++
net/ipv4/tcp_bpf.c | 32 ++++++++++++++++++++
net/ipv4/udp_bpf.c | 23 ++++++++++++---
4 files changed, 120 insertions(+), 6 deletions(-)
diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
index dfdc158ab88c..829b281d6c9c 100644
--- a/include/linux/skmsg.h
+++ b/include/linux/skmsg.h
@@ -97,6 +97,8 @@ struct sk_psock {
struct sk_buff_head ingress_skb;
struct list_head ingress_msg;
spinlock_t ingress_lock;
+ /** @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 +323,27 @@ static inline void sock_drop(struct sock *sk, struct sk_buff *skb)
kfree_skb(skb);
}
+static inline u32 sk_psock_get_msg_len_nolock(struct sk_psock *psock)
+{
+ /* Used by ioctl to read msg_tot_len only; lock-free for performance */
+ return READ_ONCE(psock->msg_tot_len);
+}
+
+static inline void sk_psock_msg_len_add_locked(struct sk_psock *psock, int diff)
+{
+ /* Use WRITE_ONCE to ensure correct read in sk_psock_get_msg_len_nolock().
+ * ingress_lock should be held to prevent concurrent updates to msg_tot_len
+ */
+ WRITE_ONCE(psock->msg_tot_len, psock->msg_tot_len + diff);
+}
+
+static inline void sk_psock_msg_len_add(struct sk_psock *psock, int diff)
+{
+ spin_lock_bh(&psock->ingress_lock);
+ sk_psock_msg_len_add_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 +352,7 @@ 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_msg_len_add_locked(psock, msg->sg.size);
ret = true;
} else {
sk_msg_free(psock->sk, msg);
@@ -345,18 +369,25 @@ 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_msg_len_add_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 +554,39 @@ 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_len_nolock(psock);
+ sk_psock_put(sk, psock);
+ }
+ return inq;
+}
+
+/* for udp only, sk is not locked */
+static inline ssize_t sk_msg_first_len(struct sock *sk)
+{
+ 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 ca22ecdbf192..dd1965bf8f6a 100644
--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -458,6 +458,7 @@ 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_msg_len_add(psock, -copy);
if (!sge->length) {
sk_msg_iter_var_next(i);
@@ -822,9 +823,11 @@ 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_msg_len_add(psock, -msg->sg.size);
sk_msg_free(psock->sk, msg);
kfree(msg);
}
+ WARN_ON_ONCE(psock->msg_tot_len);
}
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 5c698fd7fbf8..476101533349 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 +333,36 @@ 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;
+
+ 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);
+
+ /* sk_receive_queue being non-empty only occurs if the sk received
+ * data before being accepted, and no new data has arrived to trigger
+ * read_skb. If data comes from the sk's native stack, we will also
+ * consume sk_receive_queue using read_skb.
+ */
+ if (unlikely(!skb_queue_empty(&sk->sk_receive_queue))) {
+ unlock_sock_fast(sk, slow);
+ return tcp_ioctl(sk, cmd, karg);
+ }
+
+ *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 +641,7 @@ 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..91233e37cd97 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 +112,26 @@ 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)
+{
+ if (cmd != SIOCINQ)
+ return udp_ioctl(sk, cmd, karg);
+
+ /* Since we don't hold a lock, sk_receive_queue may contain data.
+ * BPF might only be processing this data at the moment. We only
+ * care about the data in the ingress_msg here.
+ */
+ *karg = sk_msg_first_len(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)
--
2.43.0On Tue, Jan 13, 2026 at 10:50 AM +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 msg_tot_len 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.
>
> Unix and VSOCK sockets have similar issues, but fixing them is outside
> the scope of this patch as it would require more intrusive changes.
>
> 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 | 68 +++++++++++++++++++++++++++++++++++++++++--
> net/core/skmsg.c | 3 ++
> net/ipv4/tcp_bpf.c | 32 ++++++++++++++++++++
> net/ipv4/udp_bpf.c | 23 ++++++++++++---
> 4 files changed, 120 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> index dfdc158ab88c..829b281d6c9c 100644
> --- a/include/linux/skmsg.h
> +++ b/include/linux/skmsg.h
> @@ -97,6 +97,8 @@ struct sk_psock {
> struct sk_buff_head ingress_skb;
> struct list_head ingress_msg;
> spinlock_t ingress_lock;
> + /** @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 +323,27 @@ static inline void sock_drop(struct sock *sk, struct sk_buff *skb)
> kfree_skb(skb);
> }
>
> +static inline u32 sk_psock_get_msg_len_nolock(struct sk_psock *psock)
> +{
> + /* Used by ioctl to read msg_tot_len only; lock-free for performance */
> + return READ_ONCE(psock->msg_tot_len);
> +}
> +
> +static inline void sk_psock_msg_len_add_locked(struct sk_psock *psock, int diff)
> +{
> + /* Use WRITE_ONCE to ensure correct read in sk_psock_get_msg_len_nolock().
> + * ingress_lock should be held to prevent concurrent updates to msg_tot_len
> + */
> + WRITE_ONCE(psock->msg_tot_len, psock->msg_tot_len + diff);
> +}
> +
> +static inline void sk_psock_msg_len_add(struct sk_psock *psock, int diff)
> +{
> + spin_lock_bh(&psock->ingress_lock);
> + sk_psock_msg_len_add_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 +352,7 @@ 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_msg_len_add_locked(psock, msg->sg.size);
> ret = true;
> } else {
> sk_msg_free(psock->sk, msg);
> @@ -345,18 +369,25 @@ 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_msg_len_add_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 +554,39 @@ 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_len_nolock(psock);
> + sk_psock_put(sk, psock);
> + }
> + return inq;
> +}
> +
> +/* for udp only, sk is not locked */
> +static inline ssize_t sk_msg_first_len(struct sock *sk)
> +{
> + 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 ca22ecdbf192..dd1965bf8f6a 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -458,6 +458,7 @@ 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_msg_len_add(psock, -copy);
>
> if (!sge->length) {
> sk_msg_iter_var_next(i);
> @@ -822,9 +823,11 @@ 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_msg_len_add(psock, -msg->sg.size);
> sk_msg_free(psock->sk, msg);
> kfree(msg);
> }
> + WARN_ON_ONCE(psock->msg_tot_len);
> }
>
> 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 5c698fd7fbf8..476101533349 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 +333,36 @@ 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;
> +
> + 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);
> +
> + /* sk_receive_queue being non-empty only occurs if the sk received
> + * data before being accepted, and no new data has arrived to trigger
> + * read_skb. If data comes from the sk's native stack, we will also
> + * consume sk_receive_queue using read_skb.
> + */
You mean "before being added to sockmap"?
> + if (unlikely(!skb_queue_empty(&sk->sk_receive_queue))) {
> + unlock_sock_fast(sk, slow);
> + return tcp_ioctl(sk, cmd, karg);
> + }
> +
> + *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 +641,7 @@ 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..91233e37cd97 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 +112,26 @@ 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)
> +{
> + if (cmd != SIOCINQ)
> + return udp_ioctl(sk, cmd, karg);
> +
> + /* Since we don't hold a lock, sk_receive_queue may contain data.
> + * BPF might only be processing this data at the moment. We only
> + * care about the data in the ingress_msg here.
> + */
I think we should strive for a design where FIONREAD does not go down
after you add your socket to sockmap, if there was no recvmsg call in
between. To show what I mean, I added this test that's currently failing
for udp:
---8<---
diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
index 1f1289f5a8c2..123c96fcaef0 100644
--- a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
+++ b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
@@ -1229,6 +1229,66 @@ static void test_sockmap_copied_seq(bool strp)
test_sockmap_pass_prog__destroy(skel);
}
+/* Test FIONREAD when data exists in sk_receive_queue before sockmap insertion */
+static void test_sockmap_fionread_pre_insert(int sotype)
+{
+ int map, err, sent, recvd, zero = 0, avail = 0;
+ struct test_sockmap_pass_prog *skel = NULL;
+ int c = -1, p = -1;
+ char buf[10] = "0123456789", rcv[11];
+ struct bpf_program *prog;
+
+ skel = test_sockmap_pass_prog__open_and_load();
+ if (!ASSERT_OK_PTR(skel, "open_and_load"))
+ return;
+
+ prog = skel->progs.prog_skb_verdict;
+ map = bpf_map__fd(skel->maps.sock_map_rx);
+
+ err = bpf_prog_attach(bpf_program__fd(prog), map, BPF_SK_SKB_STREAM_VERDICT, 0);
+ if (!ASSERT_OK(err, "bpf_prog_attach verdict"))
+ goto end;
+
+ err = create_pair(AF_INET, sotype, &c, &p);
+ if (!ASSERT_OK(err, "create_pair"))
+ goto end;
+
+ /* Step 1: Send data BEFORE sockmap insertion - lands in sk_receive_queue */
+ sent = xsend(p, buf, sizeof(buf), 0);
+ if (!ASSERT_EQ(sent, sizeof(buf), "xsend pre-insert"))
+ goto end;
+
+ /* Step 2: Verify FIONREAD reports data in sk_receive_queue */
+ err = poll_read(c, IO_TIMEOUT_SEC);
+ if (!ASSERT_OK(err, "poll_read pre-insert"))
+ goto end;
+ err = ioctl(c, FIONREAD, &avail);
+ ASSERT_OK(err, "ioctl(FIONREAD) pre-insert error");
+ ASSERT_EQ(avail, sizeof(buf), "ioctl(FIONREAD) pre-insert");
+
+ /* Step 3: Insert socket into sockmap */
+ err = bpf_map_update_elem(map, &zero, &c, BPF_ANY);
+ if (!ASSERT_OK(err, "bpf_map_update_elem(c)"))
+ goto end;
+
+ /* Step 4: FIONREAD should still report the data in sk_receive_queue */
+ err = ioctl(c, FIONREAD, &avail);
+ ASSERT_OK(err, "ioctl(FIONREAD) post-insert error");
+ ASSERT_EQ(avail, sizeof(buf), "ioctl(FIONREAD) post-insert");
+
+ /* Verify we can still read the data */
+ recvd = recv_timeout(c, rcv, sizeof(rcv), MSG_DONTWAIT, IO_TIMEOUT_SEC);
+ ASSERT_EQ(recvd, sizeof(buf), "recv post-insert");
+ ASSERT_OK(memcmp(buf, rcv, recvd), "data mismatch");
+
+end:
+ if (c >= 0)
+ close(c);
+ if (p >= 0)
+ close(p);
+ test_sockmap_pass_prog__destroy(skel);
+}
+
/* it is used to send data to via native stack and BPF redirecting */
static void test_sockmap_multi_channels(int sotype)
{
@@ -1373,4 +1433,8 @@ void test_sockmap_basic(void)
test_sockmap_multi_channels(SOCK_STREAM);
if (test__start_subtest("sockmap udp multi channels"))
test_sockmap_multi_channels(SOCK_DGRAM);
+ if (test__start_subtest("sockmap tcp fionread pre-insert"))
+ test_sockmap_fionread_pre_insert(SOCK_STREAM);
+ if (test__start_subtest("sockmap udp fionread pre-insert"))
+ test_sockmap_fionread_pre_insert(SOCK_DGRAM);
}
--->8---
> + *karg = sk_msg_first_len(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)
On Tue, Jan 20, 2026 at 04:00 PM +01, Jakub Sitnicki wrote:
> On Tue, Jan 13, 2026 at 10:50 AM +08, Jiayuan Chen wrote:
[...]
>> diff --git a/net/ipv4/udp_bpf.c b/net/ipv4/udp_bpf.c
>> index 0735d820e413..91233e37cd97 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 +112,26 @@ 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)
>> +{
>> + if (cmd != SIOCINQ)
>> + return udp_ioctl(sk, cmd, karg);
>> +
>> + /* Since we don't hold a lock, sk_receive_queue may contain data.
>> + * BPF might only be processing this data at the moment. We only
>> + * care about the data in the ingress_msg here.
>> + */
>
> I think we should strive for a design where FIONREAD does not go down
> after you add your socket to sockmap, if there was no recvmsg call in
> between. To show what I mean, I added this test that's currently failing
> for udp:
>
> ---8<---
> diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> index 1f1289f5a8c2..123c96fcaef0 100644
> --- a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> +++ b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> @@ -1229,6 +1229,66 @@ static void test_sockmap_copied_seq(bool strp)
> test_sockmap_pass_prog__destroy(skel);
> }
>
> +/* Test FIONREAD when data exists in sk_receive_queue before sockmap insertion */
> +static void test_sockmap_fionread_pre_insert(int sotype)
> +{
> + int map, err, sent, recvd, zero = 0, avail = 0;
> + struct test_sockmap_pass_prog *skel = NULL;
> + int c = -1, p = -1;
> + char buf[10] = "0123456789", rcv[11];
> + struct bpf_program *prog;
> +
> + skel = test_sockmap_pass_prog__open_and_load();
> + if (!ASSERT_OK_PTR(skel, "open_and_load"))
> + return;
> +
> + prog = skel->progs.prog_skb_verdict;
> + map = bpf_map__fd(skel->maps.sock_map_rx);
> +
> + err = bpf_prog_attach(bpf_program__fd(prog), map, BPF_SK_SKB_STREAM_VERDICT, 0);
> + if (!ASSERT_OK(err, "bpf_prog_attach verdict"))
> + goto end;
> +
> + err = create_pair(AF_INET, sotype, &c, &p);
> + if (!ASSERT_OK(err, "create_pair"))
> + goto end;
> +
> + /* Step 1: Send data BEFORE sockmap insertion - lands in sk_receive_queue */
> + sent = xsend(p, buf, sizeof(buf), 0);
> + if (!ASSERT_EQ(sent, sizeof(buf), "xsend pre-insert"))
> + goto end;
> +
> + /* Step 2: Verify FIONREAD reports data in sk_receive_queue */
> + err = poll_read(c, IO_TIMEOUT_SEC);
> + if (!ASSERT_OK(err, "poll_read pre-insert"))
> + goto end;
> + err = ioctl(c, FIONREAD, &avail);
> + ASSERT_OK(err, "ioctl(FIONREAD) pre-insert error");
> + ASSERT_EQ(avail, sizeof(buf), "ioctl(FIONREAD) pre-insert");
> +
> + /* Step 3: Insert socket into sockmap */
> + err = bpf_map_update_elem(map, &zero, &c, BPF_ANY);
> + if (!ASSERT_OK(err, "bpf_map_update_elem(c)"))
> + goto end;
> +
> + /* Step 4: FIONREAD should still report the data in sk_receive_queue */
> + err = ioctl(c, FIONREAD, &avail);
> + ASSERT_OK(err, "ioctl(FIONREAD) post-insert error");
> + ASSERT_EQ(avail, sizeof(buf), "ioctl(FIONREAD) post-insert");
> +
> + /* Verify we can still read the data */
> + recvd = recv_timeout(c, rcv, sizeof(rcv), MSG_DONTWAIT, IO_TIMEOUT_SEC);
> + ASSERT_EQ(recvd, sizeof(buf), "recv post-insert");
> + ASSERT_OK(memcmp(buf, rcv, recvd), "data mismatch");
> +
> +end:
> + if (c >= 0)
> + close(c);
> + if (p >= 0)
> + close(p);
> + test_sockmap_pass_prog__destroy(skel);
> +}
> +
> /* it is used to send data to via native stack and BPF redirecting */
> static void test_sockmap_multi_channels(int sotype)
> {
> @@ -1373,4 +1433,8 @@ void test_sockmap_basic(void)
> test_sockmap_multi_channels(SOCK_STREAM);
> if (test__start_subtest("sockmap udp multi channels"))
> test_sockmap_multi_channels(SOCK_DGRAM);
> + if (test__start_subtest("sockmap tcp fionread pre-insert"))
> + test_sockmap_fionread_pre_insert(SOCK_STREAM);
> + if (test__start_subtest("sockmap udp fionread pre-insert"))
> + test_sockmap_fionread_pre_insert(SOCK_DGRAM);
> }
> --->8---
>
>
>> + *karg = sk_msg_first_len(sk);
>> + return 0;
>> +}
>> +
I've been thinking about this some more and came to the conclusion that
this udp_bpf_ioctl implementation is actually what we want, while
tcp_bpf_ioctl *should not* be checking if the sk_receive_queue is
non-empty.
Why? Because the verdict prog might redirect or drop the skbs from
sk_receive_queue once it actually runs. The messages might never appear
on the msg_ingress queue.
What I think we should be doing, in the end, is kicking the
sk_receive_queue processing on bpf_map_update_elem, if there's data
ready.
The API semantics I'm proposing is:
1. ioctl(FIONREAD) -> reports N bytes
2. bpf_map_update_elem(sk) -> socket inserted into sockmap
3. poll() for POLLIN -> wait for socket to be ready to read
5. ioctl(FIONREAD) -> report N bytes if verdict prog didn't
redirect or drop it
We don't have to add the the queue kick on map update in this series.
If you decide to leave it for later, can I ask that you open an issue at
our GH project [1]?
I don't want it to fall through the cracks. And I sometimes have people
asking what they could help with in sockmap.
Thanks,
-jkbs
[1] https://github.com/sockmap-project/sockmap-project/issues
January 21, 2026 at 17:36, "Jakub Sitnicki" <jakub@cloudflare.com mailto:jakub@cloudflare.com?to=%22Jakub%20Sitnicki%22%20%3Cjakub%40cloudflare.com%3E > wrote:
>
> On Tue, Jan 20, 2026 at 04:00 PM +01, Jakub Sitnicki wrote:
>
> >
> > On Tue, Jan 13, 2026 at 10:50 AM +08, Jiayuan Chen wrote:
> >
> [...]
>
> >
> > >
> > > diff --git a/net/ipv4/udp_bpf.c b/net/ipv4/udp_bpf.c
> > > index 0735d820e413..91233e37cd97 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 +112,26 @@ 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)
> > > +{
> > > + if (cmd != SIOCINQ)
> > > + return udp_ioctl(sk, cmd, karg);
> > > +
> > > + /* Since we don't hold a lock, sk_receive_queue may contain data.
> > > + * BPF might only be processing this data at the moment. We only
> > > + * care about the data in the ingress_msg here.
> > > + */
> > >
> > I think we should strive for a design where FIONREAD does not go down
> > after you add your socket to sockmap, if there was no recvmsg call in
> > between. To show what I mean, I added this test that's currently failing
> > for udp:
> >
> > ---8<---
> > diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> > index 1f1289f5a8c2..123c96fcaef0 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> > @@ -1229,6 +1229,66 @@ static void test_sockmap_copied_seq(bool strp)
> > test_sockmap_pass_prog__destroy(skel);
> > }
> >
> > +/* Test FIONREAD when data exists in sk_receive_queue before sockmap insertion */
> > +static void test_sockmap_fionread_pre_insert(int sotype)
> > +{
> > + int map, err, sent, recvd, zero = 0, avail = 0;
> > + struct test_sockmap_pass_prog *skel = NULL;
> > + int c = -1, p = -1;
> > + char buf[10] = "0123456789", rcv[11];
> > + struct bpf_program *prog;
> > +
> > + skel = test_sockmap_pass_prog__open_and_load();
> > + if (!ASSERT_OK_PTR(skel, "open_and_load"))
> > + return;
> > +
> > + prog = skel->progs.prog_skb_verdict;
> > + map = bpf_map__fd(skel->maps.sock_map_rx);
> > +
> > + err = bpf_prog_attach(bpf_program__fd(prog), map, BPF_SK_SKB_STREAM_VERDICT, 0);
> > + if (!ASSERT_OK(err, "bpf_prog_attach verdict"))
> > + goto end;
> > +
> > + err = create_pair(AF_INET, sotype, &c, &p);
> > + if (!ASSERT_OK(err, "create_pair"))
> > + goto end;
> > +
> > + /* Step 1: Send data BEFORE sockmap insertion - lands in sk_receive_queue */
> > + sent = xsend(p, buf, sizeof(buf), 0);
> > + if (!ASSERT_EQ(sent, sizeof(buf), "xsend pre-insert"))
> > + goto end;
> > +
> > + /* Step 2: Verify FIONREAD reports data in sk_receive_queue */
> > + err = poll_read(c, IO_TIMEOUT_SEC);
> > + if (!ASSERT_OK(err, "poll_read pre-insert"))
> > + goto end;
> > + err = ioctl(c, FIONREAD, &avail);
> > + ASSERT_OK(err, "ioctl(FIONREAD) pre-insert error");
> > + ASSERT_EQ(avail, sizeof(buf), "ioctl(FIONREAD) pre-insert");
> > +
> > + /* Step 3: Insert socket into sockmap */
> > + err = bpf_map_update_elem(map, &zero, &c, BPF_ANY);
> > + if (!ASSERT_OK(err, "bpf_map_update_elem(c)"))
> > + goto end;
> > +
> > + /* Step 4: FIONREAD should still report the data in sk_receive_queue */
> > + err = ioctl(c, FIONREAD, &avail);
> > + ASSERT_OK(err, "ioctl(FIONREAD) post-insert error");
> > + ASSERT_EQ(avail, sizeof(buf), "ioctl(FIONREAD) post-insert");
> > +
> > + /* Verify we can still read the data */
> > + recvd = recv_timeout(c, rcv, sizeof(rcv), MSG_DONTWAIT, IO_TIMEOUT_SEC);
> > + ASSERT_EQ(recvd, sizeof(buf), "recv post-insert");
> > + ASSERT_OK(memcmp(buf, rcv, recvd), "data mismatch");
> > +
> > +end:
> > + if (c >= 0)
> > + close(c);
> > + if (p >= 0)
> > + close(p);
> > + test_sockmap_pass_prog__destroy(skel);
> > +}
> > +
> > /* it is used to send data to via native stack and BPF redirecting */
> > static void test_sockmap_multi_channels(int sotype)
> > {
> > @@ -1373,4 +1433,8 @@ void test_sockmap_basic(void)
> > test_sockmap_multi_channels(SOCK_STREAM);
> > if (test__start_subtest("sockmap udp multi channels"))
> > test_sockmap_multi_channels(SOCK_DGRAM);
> > + if (test__start_subtest("sockmap tcp fionread pre-insert"))
> > + test_sockmap_fionread_pre_insert(SOCK_STREAM);
> > + if (test__start_subtest("sockmap udp fionread pre-insert"))
> > + test_sockmap_fionread_pre_insert(SOCK_DGRAM);
> > }
> > --->8---
> >
> > >
> > > + *karg = sk_msg_first_len(sk);
> > > + return 0;
> > > +}
> > > +
> > >
> >
> I've been thinking about this some more and came to the conclusion that
> this udp_bpf_ioctl implementation is actually what we want, while
> tcp_bpf_ioctl *should not* be checking if the sk_receive_queue is
> non-empty.
>
> Why? Because the verdict prog might redirect or drop the skbs from
> sk_receive_queue once it actually runs. The messages might never appear
> on the msg_ingress queue.
>
> What I think we should be doing, in the end, is kicking the
> sk_receive_queue processing on bpf_map_update_elem, if there's data
> ready.
>
> The API semantics I'm proposing is:
>
> 1. ioctl(FIONREAD) -> reports N bytes
> 2. bpf_map_update_elem(sk) -> socket inserted into sockmap
> 3. poll() for POLLIN -> wait for socket to be ready to read
> 5. ioctl(FIONREAD) -> report N bytes if verdict prog didn't
> redirect or drop it
>
> We don't have to add the the queue kick on map update in this series.
>
> If you decide to leave it for later, can I ask that you open an issue at
> our GH project [1]?
>
> I don't want it to fall through the cracks. And I sometimes have people
> asking what they could help with in sockmap.
>
> Thanks,
> -jkbs
>
> [1] https://github.com/sockmap-project/sockmap-project/issues
>
Hi Jakub,
Thanks for taking the time to think through this carefully. I agree with your
analysis - reporting sk_receive_queue length is misleading since the verdict
prog might redirect or drop those skbs.
There's no rush to merge this patch.
Since the kick queue on bpf_map_update_elem addresses a closely related issue,
I think it makes sense to include it in this patchset for easier tracking rather
than splitting it out.
I'll spend more time looking into this and come back with an updated version.
Thanks,
Jiayuan
January 21, 2026 at 20:55, "Jiayuan Chen" <jiayuan.chen@linux.dev mailto:jiayuan.chen@linux.dev?to=%22Jiayuan%20Chen%22%20%3Cjiayuan.chen%40linux.dev%3E > wrote:
>
> January 21, 2026 at 17:36, "Jakub Sitnicki" <jakub@cloudflare.com mailto:jakub@cloudflare.com?to=%22Jakub%20Sitnicki%22%20%3Cjakub%40cloudflare.com%3E > wrote:
>
> >
> > On Tue, Jan 20, 2026 at 04:00 PM +01, Jakub Sitnicki wrote:
> >
> >
> > On Tue, Jan 13, 2026 at 10:50 AM +08, Jiayuan Chen wrote:
> >
> > [...]
> >
> >
> > >
> > > diff --git a/net/ipv4/udp_bpf.c b/net/ipv4/udp_bpf.c
> > > index 0735d820e413..91233e37cd97 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 +112,26 @@ 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)
> > > +{
> > > + if (cmd != SIOCINQ)
> > > + return udp_ioctl(sk, cmd, karg);
> > > +
> > > + /* Since we don't hold a lock, sk_receive_queue may contain data.
> > > + * BPF might only be processing this data at the moment. We only
> > > + * care about the data in the ingress_msg here.
> > > + */
> > >
> > I think we should strive for a design where FIONREAD does not go down
> > after you add your socket to sockmap, if there was no recvmsg call in
> > between. To show what I mean, I added this test that's currently failing
> > for udp:
> >
> > ---8<---
> > diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> > index 1f1289f5a8c2..123c96fcaef0 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> > @@ -1229,6 +1229,66 @@ static void test_sockmap_copied_seq(bool strp)
> > test_sockmap_pass_prog__destroy(skel);
> > }
> >
> > +/* Test FIONREAD when data exists in sk_receive_queue before sockmap insertion */
> > +static void test_sockmap_fionread_pre_insert(int sotype)
> > +{
> > + int map, err, sent, recvd, zero = 0, avail = 0;
> > + struct test_sockmap_pass_prog *skel = NULL;
> > + int c = -1, p = -1;
> > + char buf[10] = "0123456789", rcv[11];
> > + struct bpf_program *prog;
> > +
> > + skel = test_sockmap_pass_prog__open_and_load();
> > + if (!ASSERT_OK_PTR(skel, "open_and_load"))
> > + return;
> > +
> > + prog = skel->progs.prog_skb_verdict;
> > + map = bpf_map__fd(skel->maps.sock_map_rx);
> > +
> > + err = bpf_prog_attach(bpf_program__fd(prog), map, BPF_SK_SKB_STREAM_VERDICT, 0);
> > + if (!ASSERT_OK(err, "bpf_prog_attach verdict"))
> > + goto end;
> > +
> > + err = create_pair(AF_INET, sotype, &c, &p);
> > + if (!ASSERT_OK(err, "create_pair"))
> > + goto end;
> > +
> > + /* Step 1: Send data BEFORE sockmap insertion - lands in sk_receive_queue */
> > + sent = xsend(p, buf, sizeof(buf), 0);
> > + if (!ASSERT_EQ(sent, sizeof(buf), "xsend pre-insert"))
> > + goto end;
> > +
> > + /* Step 2: Verify FIONREAD reports data in sk_receive_queue */
> > + err = poll_read(c, IO_TIMEOUT_SEC);
> > + if (!ASSERT_OK(err, "poll_read pre-insert"))
> > + goto end;
> > + err = ioctl(c, FIONREAD, &avail);
> > + ASSERT_OK(err, "ioctl(FIONREAD) pre-insert error");
> > + ASSERT_EQ(avail, sizeof(buf), "ioctl(FIONREAD) pre-insert");
> > +
> > + /* Step 3: Insert socket into sockmap */
> > + err = bpf_map_update_elem(map, &zero, &c, BPF_ANY);
> > + if (!ASSERT_OK(err, "bpf_map_update_elem(c)"))
> > + goto end;
> > +
> > + /* Step 4: FIONREAD should still report the data in sk_receive_queue */
> > + err = ioctl(c, FIONREAD, &avail);
> > + ASSERT_OK(err, "ioctl(FIONREAD) post-insert error");
> > + ASSERT_EQ(avail, sizeof(buf), "ioctl(FIONREAD) post-insert");
> > +
> > + /* Verify we can still read the data */
> > + recvd = recv_timeout(c, rcv, sizeof(rcv), MSG_DONTWAIT, IO_TIMEOUT_SEC);
> > + ASSERT_EQ(recvd, sizeof(buf), "recv post-insert");
> > + ASSERT_OK(memcmp(buf, rcv, recvd), "data mismatch");
> > +
> > +end:
> > + if (c >= 0)
> > + close(c);
> > + if (p >= 0)
> > + close(p);
> > + test_sockmap_pass_prog__destroy(skel);
> > +}
> > +
> > /* it is used to send data to via native stack and BPF redirecting */
> > static void test_sockmap_multi_channels(int sotype)
> > {
> > @@ -1373,4 +1433,8 @@ void test_sockmap_basic(void)
> > test_sockmap_multi_channels(SOCK_STREAM);
> > if (test__start_subtest("sockmap udp multi channels"))
> > test_sockmap_multi_channels(SOCK_DGRAM);
> > + if (test__start_subtest("sockmap tcp fionread pre-insert"))
> > + test_sockmap_fionread_pre_insert(SOCK_STREAM);
> > + if (test__start_subtest("sockmap udp fionread pre-insert"))
> > + test_sockmap_fionread_pre_insert(SOCK_DGRAM);
> > }
> > --->8---
> >
> > >
> > > + *karg = sk_msg_first_len(sk);
> > > + return 0;
> > > +}
> > > +
> > >
> >
> > I've been thinking about this some more and came to the conclusion that
> > this udp_bpf_ioctl implementation is actually what we want, while
> > tcp_bpf_ioctl *should not* be checking if the sk_receive_queue is
> > non-empty.
> >
> > Why? Because the verdict prog might redirect or drop the skbs from
> > sk_receive_queue once it actually runs. The messages might never appear
> > on the msg_ingress queue.
> >
> > What I think we should be doing, in the end, is kicking the
> > sk_receive_queue processing on bpf_map_update_elem, if there's data
> > ready.
> >
> > The API semantics I'm proposing is:
> >
> > 1. ioctl(FIONREAD) -> reports N bytes
> > 2. bpf_map_update_elem(sk) -> socket inserted into sockmap
> > 3. poll() for POLLIN -> wait for socket to be ready to read
> > 5. ioctl(FIONREAD) -> report N bytes if verdict prog didn't
> > redirect or drop it
> >
> > We don't have to add the the queue kick on map update in this series.
> >
> > If you decide to leave it for later, can I ask that you open an issue at
> > our GH project [1]?
> >
> > I don't want it to fall through the cracks. And I sometimes have people
> > asking what they could help with in sockmap.
> >
> > Thanks,
> > -jkbs
> >
> > [1] https://github.com/sockmap-project/sockmap-project/issues
> >
> Hi Jakub,
>
> Thanks for taking the time to think through this carefully. I agree with your
> analysis - reporting sk_receive_queue length is misleading since the verdict
> prog might redirect or drop those skbs.
>
> There's no rush to merge this patch.
>
> Since the kick queue on bpf_map_update_elem addresses a closely related issue,
> I think it makes sense to include it in this patchset for easier tracking rather
> than splitting it out.
>
> I'll spend more time looking into this and come back with an updated version.
>
> Thanks,
> Jiayuan
>
Hi Jakub,
I've been thinking about this more, and I realize the problem is not as simple as it seems.
Regarding kicking the sk_receive_queue on bpf_map_update_elem: the BPF
program may not be fully initialized at that point. For example, with a
redirect program, the destination fd might not yet be inserted into the
map. If we kick the data through the BPF program immediately, the
redirect lookup would fail, leading to unexpected behavior (data being
dropped or passed to the wrong socket).
I also considered triggering the kick in poll/select via
sk_msg_is_readable(). However, this approach doesn't work for TCP
because tcp_poll() -> tcp_stream_is_readable() -> tcp_epollin_ready()
will return early when sk_receive_queue has data, before ever calling
sk_is_readable().
In the next version, I'll address your other nits and remove the
sk_receive_queue check from tcp_bpf_ioctl. I'll also open an issue on
the GH project to track this problem so we can continue exploring
better solutions.
Thanks,
Jiayuan
On Thu, Jan 22, 2026 at 03:56 AM GMT, Jiayuan Chen wrote: > January 21, 2026 at 20:55, "Jiayuan Chen" <jiayuan.chen@linux.dev > mailto:jiayuan.chen@linux.dev?to=%22Jiayuan%20Chen%22%20%3Cjiayuan.chen%40linux.dev%3E >> wrote: >> January 21, 2026 at 17:36, "Jakub Sitnicki" <jakub@cloudflare.com >> mailto:jakub@cloudflare.com?to=%22Jakub%20Sitnicki%22%20%3Cjakub%40cloudflare.com%3E >> > I've been thinking about this some more and came to the conclusion that >> > this udp_bpf_ioctl implementation is actually what we want, while >> > tcp_bpf_ioctl *should not* be checking if the sk_receive_queue is >> > non-empty. >> > >> > Why? Because the verdict prog might redirect or drop the skbs from >> > sk_receive_queue once it actually runs. The messages might never appear >> > on the msg_ingress queue. >> > >> > What I think we should be doing, in the end, is kicking the >> > sk_receive_queue processing on bpf_map_update_elem, if there's data >> > ready. >> > >> > The API semantics I'm proposing is: >> > >> > 1. ioctl(FIONREAD) -> reports N bytes >> > 2. bpf_map_update_elem(sk) -> socket inserted into sockmap >> > 3. poll() for POLLIN -> wait for socket to be ready to read >> > 5. ioctl(FIONREAD) -> report N bytes if verdict prog didn't >> > redirect or drop it >> > >> > We don't have to add the the queue kick on map update in this series. >> > >> > If you decide to leave it for later, can I ask that you open an issue at >> > our GH project [1]? >> > >> > I don't want it to fall through the cracks. And I sometimes have people >> > asking what they could help with in sockmap. >> > >> > Thanks, >> > -jkbs >> > >> > [1] https://github.com/sockmap-project/sockmap-project/issues >> > >> Hi Jakub, >> >> Thanks for taking the time to think through this carefully. I agree with your >> analysis - reporting sk_receive_queue length is misleading since the verdict >> prog might redirect or drop those skbs. >> >> There's no rush to merge this patch. >> >> Since the kick queue on bpf_map_update_elem addresses a closely related issue, >> I think it makes sense to include it in this patchset for easier tracking rather >> than splitting it out. >> >> I'll spend more time looking into this and come back with an updated version. >> >> Thanks, >> Jiayuan >> > > > Hi Jakub, > > I've been thinking about this more, and I realize the problem is not as simple as it seems. > > Regarding kicking the sk_receive_queue on bpf_map_update_elem: the BPF > program may not be fully initialized at that point. For example, with a > redirect program, the destination fd might not yet be inserted into the > map. If we kick the data through the BPF program immediately, the > redirect lookup would fail, leading to unexpected behavior (data being > dropped or passed to the wrong socket). I reckon there is not much we can do about it because we have no control over when inserts/removes sockets from sockmap. It can happen at any time. Also, a newly received segment can trigger sk_data_ready callback, and that would also cause the skbs to get processed. We don't have control of that either. Does this change break any of our existing tests/benchmarks or some other setup of yours? > I also considered triggering the kick in poll/select via > sk_msg_is_readable(). However, this approach doesn't work for TCP > because tcp_poll() -> tcp_stream_is_readable() -> tcp_epollin_ready() > will return early when sk_receive_queue has data, before ever calling > sk_is_readable(). > > In the next version, I'll address your other nits and remove the > sk_receive_queue check from tcp_bpf_ioctl. I'll also open an issue on > the GH project to track this problem so we can continue exploring > better solutions. Sounds like a plan. Thanks!
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