Add a new userland-visible ktrace flag KTR_DROP and an internal ktrace flag

KTRFAC_DROP to track instances when ktrace events are dropped due to the
request pool being exhausted.  When a thread tries to post a ktrace event
and is unable to due to no available ktrace request objects, it sets
KTRFAC_DROP in its process' p_traceflag field.  The next trace event to
successfully post from that process will set the KTR_DROP flag in the
header of the request going out and clear KTRFAC_DROP.

The KTR_DROP flag is the high bit in the type field of the ktr_header
structure.  Older kdump binaries will simply complain about an unknown type
when seeing an entry with KTR_DROP set.  Note that KTR_DROP being set on a
record in a ktrace file does not tell you anything except that at least one
event from this process was dropped prior to this event.  The user has no
way of knowing what types of events were dropped nor how many were dropped.

Requested by:	phk

2 files changed, 13 insertions, 1 deletions

diff --git a/sys/kern/kern_ktrace.c b/sys/kern/kern_ktrace.c
index 8685eb1..a8aa2b9 100644
--- a/sys/kern/kern_ktrace.c
+++ b/sys/kern/kern_ktrace.c
@@ -226,6 +226,10 @@ ktr_getrequest(int type)
 	if (req != NULL) {
 		STAILQ_REMOVE_HEAD(&ktr_free, ktr_list);
 		req->ktr_header.ktr_type = type;
+		if (p->p_traceflag & KTRFAC_DROP) {
+			req->ktr_header.ktr_type |= KTR_DROP;
+			p->p_traceflag &= ~KTRFAC_DROP;
+		}
 		KASSERT(p->p_tracevp != NULL, ("ktrace: no trace vnode"));
 		KASSERT(p->p_tracecred != NULL, ("ktrace: no trace cred"));
 		req->ktr_vp = p->p_tracevp;
@@ -238,6 +242,7 @@ ktr_getrequest(int type)
 		req->ktr_header.ktr_buffer = NULL;
 		req->ktr_header.ktr_len = 0;
 	} else {
+		p->p_traceflag |= KTRFAC_DROP;
 		pm = print_message;
 		print_message = 0;
 		mtx_unlock(&ktrace_mtx);
@@ -762,7 +767,7 @@ ktr_writerequest(struct ktr_request *req)
 	if (vp == NULL)
 		return;
 	kth = &req->ktr_header;
-	datalen = data_lengths[kth->ktr_type];
+	datalen = data_lengths[(ushort)kth->ktr_type & ~KTR_DROP];
 	buflen = kth->ktr_len;
 	cred = req->ktr_cred;
 	td = curthread;
diff --git a/sys/sys/ktrace.h b/sys/sys/ktrace.h
index 2970ce0..8fffcaf 100644
--- a/sys/sys/ktrace.h
+++ b/sys/sys/ktrace.h
@@ -145,6 +145,12 @@ struct ktr_csw {
 #define KTR_USER	7
 
 /*
+ * KTR_DROP - If this bit is set in ktr_type, then at least one event
+ * between the previous record and this record was dropped.
+ */
+#define	KTR_DROP	0x8000
+
+/*
  * kernel trace points (in p_traceflag)
  */
 #define KTRFAC_MASK	0x00ffffff
@@ -160,6 +166,7 @@ struct ktr_csw {
  */
 #define KTRFAC_ROOT	0x80000000	/* root set this trace */
 #define KTRFAC_INHERIT	0x40000000	/* pass trace flags to children */
+#define	KTRFAC_DROP	0x20000000	/* last event was dropped */
 
 #ifdef	_KERNEL
 extern struct mtx ktrace_mtx;