eBPF plugin improvements

* Check that the protocol is TCP
* Support all HTTP methods
* Clear out tasks from table when they exit (required using task ids as table
  keys). A nice side effect of this is that we can tell apart tasks receiving 0
  req/second (i.e. tasks which currently aren't receiving any requests but were
  receiving requests at some point) from tasks which don't exist anymore.
  We report metrics fro the former.
* Simplify hash table key (no need to use a struct)

examples/plugins/http-requests/http-requests.c

@@ -1,25 +1,22 @@
-#include <uapi/linux/ptrace.h>
 #include <linux/skbuff.h>
+#include <net/sock.h>
 
-struct received_http_requests_key_t {
-  u32 pid;
-};
-BPF_HASH(received_http_requests, struct received_http_requests_key_t, u64);
+/* Table from (Process id|Task id) to (Number of received http requests) */
+BPF_HASH(received_http_requests, u64, u64);
 
 
-/*
-  skb_copy_datagram_iter() (Kernels >= 3.19) is in charge of copying socket
-  buffers from kernel to userspace.
+/* skb_copy_datagram_iter() (Kernels >= 3.19) is in charge of copying socket
+   buffers from kernel to userspace.
 
-  skb_copy_datagram_iter() has an associated tracepoint
-  (trace_skb_copy_datagram_iovec), which would be more stable than a kprobe but
-  it lacks the offset argument.
+   skb_copy_datagram_iter() has an associated tracepoint
+   (trace_skb_copy_datagram_iovec), which would be more stable than a kprobe but
+   it lacks the offset argument.
  */
 int kprobe__skb_copy_datagram_iter(struct pt_regs *ctx, const struct sk_buff *skb, int offset, void *unused_iovec, int len)
 {
 
-  /* Inspect the beginning of socket buffers copied to user-space to determine if they
-     correspond to http requests.
+  /* Inspect the beginning of socket buffers copied to user-space to determine
+     if they correspond to http requests.
 
      Caveats:
 
@@ -30,32 +27,130 @@ int kprobe__skb_copy_datagram_iter(struct pt_regs *ctx, const struct sk_buff *sk
      We could inspect the full packet but:
      * It's very inefficient.
      * Examining the non-linear (paginated) area of a socket buffer would be
-     really tricky from ebpf.
-   */
+       really tricky from ebpf.
+  */
 
-  /* TODO: exit early if it's not TCP */
+  /* Verify it's a TCP socket */
+  struct sock *sk = skb->sk;
+  unsigned short skc_family = sk->__sk_common.skc_family;
+  switch (skc_family) {
+    case PF_INET:
+    case PF_INET6:
+    case PF_UNIX:
+      break;
+    default:
+      return 0;
+  }
+  /* The socket type and procotol are not directly addressable since they are
+     bitfields.  We access them by assuming sk_write_queue is immediately before
+     them (admittedly pretty hacky).
+  */
+  unsigned int flags = 0;
+  size_t flags_offset = offsetof(typeof(struct sock), sk_write_queue) + sizeof(sk->sk_write_queue);
+  bpf_probe_read(&flags, sizeof(flags), ((u8*)sk) + flags_offset);
+  u16 sk_type = flags >> 16;
+  if (sk_type != SOCK_STREAM) {
+    return 0;
+  }
+  u8 sk_protocol = flags >> 8 & 0xFF;
+  /* The protocol is unset (IPPROTO_IP) in Unix sockets */
+  if ( (sk_protocol != IPPROTO_TCP) && ((skc_family == PF_UNIX) && (sk_protocol != IPPROTO_IP)) ) {
+    return 0;
+  }
 
   /* Inline implementation of skb_headlen() */
   unsigned int head_len = skb->len - skb->data_len;
-
   /* http://stackoverflow.com/questions/25047905/http-request-minimum-size-in-bytes
-     minimum length of http request is always geater than 7 bytes
+     minimum length of http request is always greater than 7 bytes
   */
-  if (head_len - offset < 7) {
+  unsigned int available_data = head_len - offset;
+  if (available_data < 7) {
     return 0;
   }
 
-  u8 data[4] = {};
-  bpf_probe_read(&data, sizeof(data), skb->data + offset);
+  /* Check if buffer begins with a method name followed by a space.
+
+     To avoid false positives it would be good to do a deeper inspection
+     (i.e. fully ensure a 'Method SP Request-URI SP HTTP-Version CRLF'
+     structure) but loops are not allowed in ebpf, making variable-size-data
+     parsers infeasible.
+  */
+  u8 data[8] = {};
+  if (available_data >= 8) {
+    /* We have confirmed having access to 7 bytes, but need 8 bytes to check the
+       space after OPTIONS. bpf_probe_read() requires its second argument to be
+       an immediate, so we obtain the data in this unsexy way.
+    */
+    bpf_probe_read(&data, 8, skb->data + offset);
+  } else {
+    bpf_probe_read(&data, 7, skb->data + offset);
+  }
+
+  switch (data[0]) {
+    /* DELETE */
+    case 'D':
+      if ((data[1] != 'E') || (data[2] != 'L') || (data[3] != 'E') || (data[4] != 'T') || (data[5] != 'E') || (data[5] != ' ')) {
+        return 0;
+      }
+      break;
+
+    /* GET */
+    case 'G':
+      if ((data[1] != 'E') || (data[2] != 'T') || (data[3] != ' ')) {
+        return 0;
+      }
+      break;
+
+    /* HEAD */
+    case 'H':
+      if ((data[1] != 'E') || (data[2] != 'A') || (data[3] != 'D') || (data[4] != ' ')) {
+        return 0;
+      }
+      break;
+
+    /* OPTIONS */
+    case 'O':
+      if (available_data < 8 || (data[1] != 'P') || (data[2] != 'T') || (data[3] != 'I') || (data[4] != 'O') || (data[5] != 'N') || (data[6] != 'S') || (data[7] != ' ')) {
+        return 0;
+      }
+      break;
 
-  /* TODO: support other methods and optimize lookups */
-  if ((data[0] == 'G') && (data[1] == 'E') && (data[2] == 'T') && (data[3] == ' ')) {
-    /* Record request */
-    struct received_http_requests_key_t key = {};
-    key.pid = bpf_get_current_pid_tgid() >> 32;
-    received_http_requests.increment(key);
+    /* PATCH/POST/PUT */
+    case 'P':
+      switch (data[1]) {
+          case 'A':
+            if ((data[2] != 'T') || (data[3] != 'C') || (data[4] != 'H') || (data[5] != ' ')) {
+              return 0;
+            }
+            break;
+          case 'O':
+            if ((data[2] != 'S') || (data[3] != 'T') || (data[4] != ' ')) {
+              return 0;
+            }
+            break;
+          case 'U':
+            if ((data[2] != 'T') || (data[3] != ' ')) {
+              return 0;
+            }
+            break;
+      }
+      break;
+
+    default:
+      return 0;
   }
 
+  /* Finally, bump the request counter for current task */
+  u64 pid_tgid = bpf_get_current_pid_tgid();
+  received_http_requests.increment(pid_tgid);
+
+  return 0;
+}
+
 
+/* Clear out request count entries of tasks on exit */
+int kprobe__do_exit(struct pt_regs *ctx) {
+  u64 pid_tgid = bpf_get_current_pid_tgid();
+  received_http_requests.delete(&pid_tgid);
   return 0;
 }

examples/plugins/http-requests/http-requests.py

@@ -2,6 +2,7 @@
 import bcc
 
 import time
+import collections
 import datetime
 import os
 import signal
@@ -26,17 +27,20 @@ def __init__(self):
         self.lock = threading.Lock()
 
     def update_http_rate_per_pid(self, last_req_count_snapshot):
-        new_req_count_snapshot = dict()
-        new_http_rate_per_pid = dict()
+        # Aggregate per-task http request counts into per-process counts
         req_count_table = self.bpf.get_table(EBPF_TABLE_NAME)
-        for key, value in req_count_table.iteritems():
-            request_delta = value.value
-            if key.pid in last_req_count_snapshot:
-                 request_delta -= last_req_count_snapshot[key.pid]
-            if request_delta > 0:
-                new_http_rate_per_pid[key.pid] = request_delta
+        new_req_count_snapshot = collections.defaultdict(int)
+        for pid_tgid, req_count in req_count_table.iteritems():
+            pid = pid_tgid.value >> 32
+            new_req_count_snapshot[pid] += req_count.value
 
-            new_req_count_snapshot[key.pid] = value.value
+        # Compute request rate
+        new_http_rate_per_pid = dict()
+        for pid, req_count in new_req_count_snapshot.iteritems():
+            request_delta = req_count
+            if pid in last_req_count_snapshot:
+                 request_delta -= last_req_count_snapshot[pid]
+            new_http_rate_per_pid[pid] = request_delta
 
         self.lock.acquire()
         self.http_rate_per_pid = new_http_rate_per_pid
@@ -52,17 +56,11 @@ def on_http_rate_per_pid(self, f):
 
     def run(self):
         # Compute request rates based on the requests counts from the last
-        # second. It would be simpler to clear the table, wait one second
+        # second. It would be simpler to clear the table, wait one second but
         # clear() is expensive (each entry is individually cleared with a system
-        # call) and less robust (clearing contends with the increments done by
-        # the kernel probe).
-        # FIXME: we need a mechanism to garbage-collect old processes, either
-        #        here or on the probe. Some options are clearing the table once
-        #        in a while (not ideal for the reasons above) or adding another
-        #        probe to remove processes from the table when they die (this
-        #        will probably require keeping keeping track of tasks and not
-        #        just processes)
-        req_count_snapshot = dict()
+        # call) and less robust (it contends with the increments done by the
+        # kernel probe).
+        req_count_snapshot = collections.defaultdict(int)
         while True:
             time.sleep(1)
             req_count_snapshot = self.update_http_rate_per_pid(req_count_snapshot)