Add support for remote coverage to kcov. Remote coverage is collected

from threads other than the one currently having kcov enabled. A thread
with kcov enabled occasionally delegates work to another thread,
collecting coverage from such threads improves the ability of syzkaller
to correlate side effects in the kernel caused by issuing a syscall.

Remote coverage is divided into subsystems. The only supported subsystem
right now collects coverage from scheduled tasks and timeouts on behalf
of a kcov enabled thread. In order to make this work `struct task' and
`struct timeout' must be extended with a new field keeping track of the
process that scheduled the task/timeout. Both aforementioned structures
have therefore increased with the size of a pointer on all
architectures.

The kernel API is documented in a new kcov_remote_register(9) manual.

Remote coverage is also supported by kcov on NetBSD and Linux.

ok mpi@

2 files changed, 95 insertions, 3 deletions

diff --git a/share/man/man9/Makefile b/share/man/man9/Makefile
index 7b494dc20de..a04e8add7f5 100644
--- a/share/man/man9/Makefile
+++ b/share/man/man9/Makefile
@@ -1,4 +1,4 @@
-#	$OpenBSD: Makefile,v 1.305 2020/07/06 11:48:03 dlg Exp $
+#	$OpenBSD: Makefile,v 1.306 2020/08/01 08:40:20 anton Exp $
 #	$NetBSD: Makefile,v 1.4 1996/01/09 03:23:01 thorpej Exp $
 
 #	Makefile for section 9 (kernel function and variable) manual pages.
@@ -22,8 +22,8 @@ MAN=	aml_evalnode.9 atomic_add_int.9 atomic_cas_uint.9 \
 	ifiq_input.9 ifq_enqueue.9 \
 	ifq_deq_begin.9 imax.9 iic.9 intro.9 inittodr.9 \
 	intr_barrier.9 intrmap_create.9 \
-	KASSERT.9 km_alloc.9 knote.9 kstat_create.9 kstat_kv_init.9 \
-	kthread.9 ktrace.9 \
+	KASSERT.9 kcov_remote_register.9 km_alloc.9 knote.9 kstat_create.9 \
+	kstat_kv_init.9 kthread.9 ktrace.9 \
 	lim_cur.9 loadfirmware.9 log.9 \
 	malloc.9 membar_sync.9 memcmp.9 mbuf.9 mbuf_tags.9 md5.9 mi_switch.9 \
 	microtime.9 ml_init.9 mq_init.9 mutex.9 \
diff --git a/share/man/man9/kcov_remote_register.9 b/share/man/man9/kcov_remote_register.9
new file mode 100644
index 00000000000..5186d72db1d
--- /dev/null
+++ b/share/man/man9/kcov_remote_register.9
@@ -0,0 +1,92 @@
+.\"	$OpenBSD: kcov_remote_register.9,v 1.1 2020/08/01 08:40:20 anton Exp $
+.\"
+.\" Copyright (c) 2020 Anton Lindqvist <anton@openbsd.org>
+.\"
+.\" Permission to use, copy, modify, and distribute this software for any
+.\" purpose with or without fee is hereby granted, provided that the above
+.\" copyright notice and this permission notice appear in all copies.
+.\"
+.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+.\"
+.Dd $Mdocdate: August 1 2020 $
+.Dt KCOV_REMOTE_REGISTER 9
+.Os
+.Sh NAME
+.Nm kcov_remote_register ,
+.Nm kcov_remote_unregister ,
+.Nm kcov_remote_enter ,
+.Nm kcov_remote_leave
+.Nd Remote kernel code coverage collection interface
+.Sh SYNOPSIS
+.In sys/kcov.h
+.Ft void
+.Fn kcov_remote_register "int subsystem" "void *id"
+.Ft void
+.Fn kcov_remote_unregister "int subsystem" "void *id"
+.Ft void
+.Fn kcov_remote_enter "int subsystem" "void *id"
+.Ft void
+.Fn kcov_remote_leave "int subsystem" "void *id"
+.Sh DESCRIPTION
+These functions provide an interface to collect code coverage from various
+kernel subsystems using
+.Xr kcov 4 .
+The
+.Fa subsystem
+must be one of the following:
+.Bd -literal
+#define KCOV_REMOTE_COMMON	0
+.Ed
+.Pp
+The
+.Va id
+must be an unique identifier for a resource tied to
+.Va subsystem
+that is known both by kernel and user space.
+For instance, a network interface driver could form a subsystem using the
+unique interface identifier as the
+.Fa id .
+.Pp
+.Fn kcov_remote_register
+registers a new instance of the given
+.Fa subsystem .
+Calling this function more than once with the same arguments is idempotent.
+.Pp
+.Fn kcov_remote_unregister
+unregisters an existing instance of the given
+.Fa subsystem ,
+if present.
+If one or many threads are currently within a remote section,
+.Fn kcov_remote_unregister
+will sleep until all the sections has been left.
+.Pp
+.Fn kcov_remote_enter
+enters a remote section.
+If the current thread has
+.Xr kcov 4
+enabled, code coverage will collected inside the remote section.
+Remote sections may not be nested.
+.Pp
+.Fn kcov_remote_leave
+leaves a remote section.
+.Sh CONTEXT
+.Fn kcov_remote_register ,
+.Fn kcov_remote_unregister ,
+.Fn kcov_remote_enter
+and
+.Fn kcov_remote_leave
+can be called from process context.
+.Sh CODE REFERENCES
+These functions are implemented in
+.Pa sys/dev/kcov.c .
+.Sh SEE ALSO
+.Xr kcov 4
+.Sh HISTORY
+These functions first appeared in
+.Ox 6.8 .