[PATCH] connector: fix out-of-order cn_proc netlink message delivery

From: Aaron Campbell
Date: Fri Jun 24 2016 - 09:14:07 EST

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The proc connector messages include a sequence number, allowing userspace
programs to detect lost messages. However, performing this detection is
currently more difficult than necessary, since netlink messages can be
delivered to the application out-of-order. To fix this, leave pre-emption
disabled during cn_netlink_send(), and use GFP_NOWAIT.

The following was written as a test case. Building the kernel w/ make -j32
proved a reliable way to generate out-of-order cn_proc messages.

int
main(int argc, char *argv[])
{
static uint32_t last_seq[CPU_SETSIZE], seq;
int cpu, fd;
struct sockaddr_nl sa;
struct __attribute__((aligned(NLMSG_ALIGNTO))) {
struct nlmsghdr nl_hdr;
struct __attribute__((__packed__)) {
struct cn_msg cn_msg;
struct proc_event cn_proc;
};
} rmsg;
struct __attribute__((aligned(NLMSG_ALIGNTO))) {
struct nlmsghdr nl_hdr;
struct __attribute__((__packed__)) {
struct cn_msg cn_msg;
enum proc_cn_mcast_op cn_mcast;
};
} smsg;

fd = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
if (fd < 0) {
perror("socket");
}

sa.nl_family = AF_NETLINK;
sa.nl_groups = CN_IDX_PROC;
sa.nl_pid = getpid();
if (bind(fd, (struct sockaddr *)&sa, sizeof(sa)) < 0) {
perror("bind");
}

memset(&smsg, 0, sizeof(smsg));
smsg.nl_hdr.nlmsg_len = sizeof(smsg);
smsg.nl_hdr.nlmsg_pid = getpid();
smsg.nl_hdr.nlmsg_type = NLMSG_DONE;
smsg.cn_msg.id.idx = CN_IDX_PROC;
smsg.cn_msg.id.val = CN_VAL_PROC;
smsg.cn_msg.len = sizeof(enum proc_cn_mcast_op);
smsg.cn_mcast = PROC_CN_MCAST_LISTEN;
if (send(fd, &smsg, sizeof(smsg), 0) != sizeof(smsg)) {
perror("send");
}

while (recv(fd, &rmsg, sizeof(rmsg), 0) == sizeof(rmsg)) {
cpu = rmsg.cn_proc.cpu;
if (cpu < 0) {
continue;
}
seq = rmsg.cn_msg.seq;
if ((last_seq[cpu] != 0) && (seq != last_seq[cpu] + 1)) {
printf("out-of-order seq=%d on cpu=%d\n", seq, cpu);
}
last_seq[cpu] = seq;
}

/* NOTREACHED */

perror("recv");

return -1;
}

Signed-off-by: Aaron Campbell <aaron@xxxxxxxxxx>
---
drivers/connector/cn_proc.c | 43 ++++++++++++++++++++++---------------------
1 file changed, 22 insertions(+), 21 deletions(-)

diff --git a/drivers/connector/cn_proc.c b/drivers/connector/cn_proc.c
index 15d06fc..b02f9c6 100644
--- a/drivers/connector/cn_proc.c
+++ b/drivers/connector/cn_proc.c
@@ -56,11 +56,21 @@ static struct cb_id cn_proc_event_id = { CN_IDX_PROC, CN_VAL_PROC };
/* proc_event_counts is used as the sequence number of the netlink message */
static DEFINE_PER_CPU(__u32, proc_event_counts) = { 0 };

-static inline void get_seq(__u32 *ts, int *cpu)
+static inline void send_msg(struct cn_msg *msg)
{
preempt_disable();
- *ts = __this_cpu_inc_return(proc_event_counts) - 1;
- *cpu = smp_processor_id();
+
+ msg->seq = __this_cpu_inc_return(proc_event_counts) - 1;
+ ((struct proc_event *)msg->data)->cpu = smp_processor_id();
+
+ /*
+ * Preemption remains disabled during send to ensure the messages are
+ * ordered according to their sequence numbers.
+ *
+ * If cn_netlink_send() fails, the data is not sent.
+ */
+ cn_netlink_send(msg, 0, CN_IDX_PROC, GFP_NOWAIT);
+
preempt_enable();
}

@@ -77,7 +87,6 @@ void proc_fork_connector(struct task_struct *task)
msg = buffer_to_cn_msg(buffer);
ev = (struct proc_event *)msg->data;
memset(&ev->event_data, 0, sizeof(ev->event_data));
- get_seq(&msg->seq, &ev->cpu);
ev->timestamp_ns = ktime_get_ns();
ev->what = PROC_EVENT_FORK;
rcu_read_lock();
@@ -92,8 +101,7 @@ void proc_fork_connector(struct task_struct *task)
msg->ack = 0; /* not used */
msg->len = sizeof(*ev);
msg->flags = 0; /* not used */
- /* If cn_netlink_send() failed, the data is not sent */
- cn_netlink_send(msg, 0, CN_IDX_PROC, GFP_KERNEL);
+ send_msg(msg);
}

void proc_exec_connector(struct task_struct *task)
@@ -108,7 +116,6 @@ void proc_exec_connector(struct task_struct *task)
msg = buffer_to_cn_msg(buffer);
ev = (struct proc_event *)msg->data;
memset(&ev->event_data, 0, sizeof(ev->event_data));
- get_seq(&msg->seq, &ev->cpu);
ev->timestamp_ns = ktime_get_ns();
ev->what = PROC_EVENT_EXEC;
ev->event_data.exec.process_pid = task->pid;
@@ -118,7 +125,7 @@ void proc_exec_connector(struct task_struct *task)
msg->ack = 0; /* not used */
msg->len = sizeof(*ev);
msg->flags = 0; /* not used */
- cn_netlink_send(msg, 0, CN_IDX_PROC, GFP_KERNEL);
+ send_msg(msg);
}

void proc_id_connector(struct task_struct *task, int which_id)
@@ -150,14 +157,13 @@ void proc_id_connector(struct task_struct *task, int which_id)
return;
}
rcu_read_unlock();
- get_seq(&msg->seq, &ev->cpu);
ev->timestamp_ns = ktime_get_ns();

memcpy(&msg->id, &cn_proc_event_id, sizeof(msg->id));
msg->ack = 0; /* not used */
msg->len = sizeof(*ev);
msg->flags = 0; /* not used */
- cn_netlink_send(msg, 0, CN_IDX_PROC, GFP_KERNEL);
+ send_msg(msg);
}

void proc_sid_connector(struct task_struct *task)
@@ -172,7 +178,6 @@ void proc_sid_connector(struct task_struct *task)
msg = buffer_to_cn_msg(buffer);
ev = (struct proc_event *)msg->data;
memset(&ev->event_data, 0, sizeof(ev->event_data));
- get_seq(&msg->seq, &ev->cpu);
ev->timestamp_ns = ktime_get_ns();
ev->what = PROC_EVENT_SID;
ev->event_data.sid.process_pid = task->pid;
@@ -182,7 +187,7 @@ void proc_sid_connector(struct task_struct *task)
msg->ack = 0; /* not used */
msg->len = sizeof(*ev);
msg->flags = 0; /* not used */
- cn_netlink_send(msg, 0, CN_IDX_PROC, GFP_KERNEL);
+ send_msg(msg);
}

void proc_ptrace_connector(struct task_struct *task, int ptrace_id)
@@ -197,7 +202,6 @@ void proc_ptrace_connector(struct task_struct *task, int ptrace_id)
msg = buffer_to_cn_msg(buffer);
ev = (struct proc_event *)msg->data;
memset(&ev->event_data, 0, sizeof(ev->event_data));
- get_seq(&msg->seq, &ev->cpu);
ev->timestamp_ns = ktime_get_ns();
ev->what = PROC_EVENT_PTRACE;
ev->event_data.ptrace.process_pid = task->pid;
@@ -215,7 +219,7 @@ void proc_ptrace_connector(struct task_struct *task, int ptrace_id)
msg->ack = 0; /* not used */
msg->len = sizeof(*ev);
msg->flags = 0; /* not used */
- cn_netlink_send(msg, 0, CN_IDX_PROC, GFP_KERNEL);
+ send_msg(msg);
}

void proc_comm_connector(struct task_struct *task)
@@ -230,7 +234,6 @@ void proc_comm_connector(struct task_struct *task)
msg = buffer_to_cn_msg(buffer);
ev = (struct proc_event *)msg->data;
memset(&ev->event_data, 0, sizeof(ev->event_data));
- get_seq(&msg->seq, &ev->cpu);
ev->timestamp_ns = ktime_get_ns();
ev->what = PROC_EVENT_COMM;
ev->event_data.comm.process_pid = task->pid;
@@ -241,7 +244,7 @@ void proc_comm_connector(struct task_struct *task)
msg->ack = 0; /* not used */
msg->len = sizeof(*ev);
msg->flags = 0; /* not used */
- cn_netlink_send(msg, 0, CN_IDX_PROC, GFP_KERNEL);
+ send_msg(msg);
}

void proc_coredump_connector(struct task_struct *task)
@@ -256,7 +259,6 @@ void proc_coredump_connector(struct task_struct *task)
msg = buffer_to_cn_msg(buffer);
ev = (struct proc_event *)msg->data;
memset(&ev->event_data, 0, sizeof(ev->event_data));
- get_seq(&msg->seq, &ev->cpu);
ev->timestamp_ns = ktime_get_ns();
ev->what = PROC_EVENT_COREDUMP;
ev->event_data.coredump.process_pid = task->pid;
@@ -266,7 +268,7 @@ void proc_coredump_connector(struct task_struct *task)
msg->ack = 0; /* not used */
msg->len = sizeof(*ev);
msg->flags = 0; /* not used */
- cn_netlink_send(msg, 0, CN_IDX_PROC, GFP_KERNEL);
+ send_msg(msg);
}

void proc_exit_connector(struct task_struct *task)
@@ -281,7 +283,6 @@ void proc_exit_connector(struct task_struct *task)
msg = buffer_to_cn_msg(buffer);
ev = (struct proc_event *)msg->data;
memset(&ev->event_data, 0, sizeof(ev->event_data));
- get_seq(&msg->seq, &ev->cpu);
ev->timestamp_ns = ktime_get_ns();
ev->what = PROC_EVENT_EXIT;
ev->event_data.exit.process_pid = task->pid;
@@ -293,7 +294,7 @@ void proc_exit_connector(struct task_struct *task)
msg->ack = 0; /* not used */
msg->len = sizeof(*ev);
msg->flags = 0; /* not used */
- cn_netlink_send(msg, 0, CN_IDX_PROC, GFP_KERNEL);
+ send_msg(msg);
}

/*
@@ -325,7 +326,7 @@ static void cn_proc_ack(int err, int rcvd_seq, int rcvd_ack)
msg->ack = rcvd_ack + 1;
msg->len = sizeof(*ev);
msg->flags = 0; /* not used */
- cn_netlink_send(msg, 0, CN_IDX_PROC, GFP_KERNEL);
+ send_msg(msg);
}

/**
--
2.7.4

Next message: Gustavo Padovan: "Re: [RFC 0/5] rework fences on struct sync_file"
Previous message: Javier Martinez Canillas: "[PATCH 2/2] ARM: multi_v7_defconfig: Enable vivid driver as a module"
Next in thread: David Miller: "Re: [PATCH] connector: fix out-of-order cn_proc netlink message delivery"
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