28 files changed, 567 insertions, 1 deletions

diff --git a/Documentation/kcov.txt b/Documentation/kcov.txt
new file mode 100644
index 000000000000..779ff4ab1c1d
--- /dev/null
+++ b/Documentation/kcov.txt
@@ -0,0 +1,111 @@
+kcov: code coverage for fuzzing
+===============================
+
+kcov exposes kernel code coverage information in a form suitable for coverage-
+guided fuzzing (randomized testing). Coverage data of a running kernel is
+exported via the "kcov" debugfs file. Coverage collection is enabled on a task
+basis, and thus it can capture precise coverage of a single system call.
+
+Note that kcov does not aim to collect as much coverage as possible. It aims
+to collect more or less stable coverage that is function of syscall inputs.
+To achieve this goal it does not collect coverage in soft/hard interrupts
+and instrumentation of some inherently non-deterministic parts of kernel is
+disbled (e.g. scheduler, locking).
+
+Usage:
+======
+
+Configure kernel with:
+
+        CONFIG_KCOV=y
+
+CONFIG_KCOV requires gcc built on revision 231296 or later.
+Profiling data will only become accessible once debugfs has been mounted:
+
+        mount -t debugfs none /sys/kernel/debug
+
+The following program demonstrates kcov usage from within a test program:
+
+#include <stdio.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/ioctl.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#include <fcntl.h>
+
+#define KCOV_INIT_TRACE			_IOR('c', 1, unsigned long)
+#define KCOV_ENABLE			_IO('c', 100)
+#define KCOV_DISABLE			_IO('c', 101)
+#define COVER_SIZE			(64<<10)
+
+int main(int argc, char **argv)
+{
+	int fd;
+	unsigned long *cover, n, i;
+
+	/* A single fd descriptor allows coverage collection on a single
+	 * thread.
+	 */
+	fd = open("/sys/kernel/debug/kcov", O_RDWR);
+	if (fd == -1)
+		perror("open"), exit(1);
+	/* Setup trace mode and trace size. */
+	if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
+		perror("ioctl"), exit(1);
+	/* Mmap buffer shared between kernel- and user-space. */
+	cover = (unsigned long*)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
+				     PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if ((void*)cover == MAP_FAILED)
+		perror("mmap"), exit(1);
+	/* Enable coverage collection on the current thread. */
+	if (ioctl(fd, KCOV_ENABLE, 0))
+		perror("ioctl"), exit(1);
+	/* Reset coverage from the tail of the ioctl() call. */
+	__atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
+	/* That's the target syscal call. */
+	read(-1, NULL, 0);
+	/* Read number of PCs collected. */
+	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
+	for (i = 0; i < n; i++)
+		printf("0x%lx\n", cover[i + 1]);
+	/* Disable coverage collection for the current thread. After this call
+	 * coverage can be enabled for a different thread.
+	 */
+	if (ioctl(fd, KCOV_DISABLE, 0))
+		perror("ioctl"), exit(1);
+	/* Free resources. */
+	if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
+		perror("munmap"), exit(1);
+	if (close(fd))
+		perror("close"), exit(1);
+	return 0;
+}
+
+After piping through addr2line output of the program looks as follows:
+
+SyS_read
+fs/read_write.c:562
+__fdget_pos
+fs/file.c:774
+__fget_light
+fs/file.c:746
+__fget_light
+fs/file.c:750
+__fget_light
+fs/file.c:760
+__fdget_pos
+fs/file.c:784
+SyS_read
+fs/read_write.c:562
+
+If a program needs to collect coverage from several threads (independently),
+it needs to open /sys/kernel/debug/kcov in each thread separately.
+
+The interface is fine-grained to allow efficient forking of test processes.
+That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
+mmaps coverage buffer and then forks child processes in a loop. Child processes
+only need to enable coverage (disable happens automatically on thread end).
diff --git a/Makefile b/Makefile
index e055b969c325..b98a4f70d1b5 100644
--- a/Makefile
+++ b/Makefile
@@ -365,6 +365,7 @@ LDFLAGS_MODULE  =
 CFLAGS_KERNEL	=
 AFLAGS_KERNEL	=
 CFLAGS_GCOV	= -fprofile-arcs -ftest-coverage
+CFLAGS_KCOV	= -fsanitize-coverage=trace-pc
 
 
 # Use USERINCLUDE when you must reference the UAPI directories only.
@@ -411,7 +412,7 @@ export MAKE AWK GENKSYMS INSTALLKERNEL PERL PYTHON UTS_MACHINE
 export HOSTCXX HOSTCXXFLAGS LDFLAGS_MODULE CHECK CHECKFLAGS
 
 export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS LDFLAGS
-export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE CFLAGS_GCOV CFLAGS_KASAN CFLAGS_UBSAN
+export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE CFLAGS_GCOV CFLAGS_KCOV CFLAGS_KASAN CFLAGS_UBSAN
 export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
 export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_LDFLAGS_MODULE
 export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL
@@ -673,6 +674,14 @@ endif
 endif
 KBUILD_CFLAGS += $(stackp-flag)
 
+ifdef CONFIG_KCOV
+  ifeq ($(call cc-option, $(CFLAGS_KCOV)),)
+    $(warning Cannot use CONFIG_KCOV: \
+             -fsanitize-coverage=trace-pc is not supported by compiler)
+    CFLAGS_KCOV =
+  endif
+endif
+
 ifeq ($(cc-name),clang)
 KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
 KBUILD_CPPFLAGS += $(call cc-option,-Wno-unknown-warning-option,)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 8b680a5cb25b..54478b7635de 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -28,6 +28,7 @@ config X86
 	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_FAST_MULTIPLIER
 	select ARCH_HAS_GCOV_PROFILE_ALL
+	select ARCH_HAS_KCOV			if X86_64
 	select ARCH_HAS_PMEM_API		if X86_64
 	select ARCH_HAS_MMIO_FLUSH
 	select ARCH_HAS_SG_CHAIN
diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
index 0bf6749522d9..b1ef9e489084 100644
--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -12,6 +12,13 @@
 KASAN_SANITIZE			:= n
 OBJECT_FILES_NON_STANDARD	:= y
 
+# Kernel does not boot with kcov instrumentation here.
+# One of the problems observed was insertion of __sanitizer_cov_trace_pc()
+# callback into middle of per-cpu data enabling code. Thus the callback observed
+# inconsistent state and crashed. We are interested mostly in syscall coverage,
+# so boot code is not interesting anyway.
+KCOV_INSTRUMENT		:= n
+
 # If you want to preset the SVGA mode, uncomment the next line and
 # set SVGA_MODE to whatever number you want.
 # Set it to -DSVGA_MODE=NORMAL_VGA if you just want the EGA/VGA mode.
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 5e1d26e09407..6915ff2bd996 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -19,6 +19,9 @@
 KASAN_SANITIZE			:= n
 OBJECT_FILES_NON_STANDARD	:= y
 
+# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
+KCOV_INSTRUMENT		:= n
+
 targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \
 	vmlinux.bin.xz vmlinux.bin.lzo vmlinux.bin.lz4
 
diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index f9fb859c98b9..6874da5f67fc 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -7,6 +7,9 @@ KASAN_SANITIZE			:= n
 UBSAN_SANITIZE			:= n
 OBJECT_FILES_NON_STANDARD	:= y
 
+# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
+KCOV_INSTRUMENT		:= n
+
 VDSO64-$(CONFIG_X86_64)		:= y
 VDSOX32-$(CONFIG_X86_X32_ABI)	:= y
 VDSO32-$(CONFIG_X86_32)		:= y
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index d5fb0871aba3..adaae2c781c1 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -25,6 +25,12 @@ OBJECT_FILES_NON_STANDARD_relocate_kernel_$(BITS).o	:= y
 OBJECT_FILES_NON_STANDARD_mcount_$(BITS).o		:= y
 OBJECT_FILES_NON_STANDARD_test_nx.o			:= y
 
+# If instrumentation of this dir is enabled, boot hangs during first second.
+# Probably could be more selective here, but note that files related to irqs,
+# boot, dumpstack/stacktrace, etc are either non-interesting or can lead to
+# non-deterministic coverage.
+KCOV_INSTRUMENT		:= n
+
 CFLAGS_irq.o := -I$(src)/../include/asm/trace
 
 obj-y			:= process_$(BITS).o signal.o
diff --git a/arch/x86/kernel/apic/Makefile b/arch/x86/kernel/apic/Makefile
index 8bb12ddc5db8..8e63ebdcbd0b 100644
--- a/arch/x86/kernel/apic/Makefile
+++ b/arch/x86/kernel/apic/Makefile
@@ -2,6 +2,10 @@
 # Makefile for local APIC drivers and for the IO-APIC code
 #
 
+# Leads to non-deterministic coverage that is not a function of syscall inputs.
+# In particualr, smp_apic_timer_interrupt() is called in random places.
+KCOV_INSTRUMENT		:= n
+
 obj-$(CONFIG_X86_LOCAL_APIC)	+= apic.o apic_noop.o ipi.o vector.o
 obj-y				+= hw_nmi.o
 
diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
index 0d373d7affc8..4a8697f7d4ef 100644
--- a/arch/x86/kernel/cpu/Makefile
+++ b/arch/x86/kernel/cpu/Makefile
@@ -8,6 +8,10 @@ CFLAGS_REMOVE_common.o = -pg
 CFLAGS_REMOVE_perf_event.o = -pg
 endif
 
+# If these files are instrumented, boot hangs during the first second.
+KCOV_INSTRUMENT_common.o := n
+KCOV_INSTRUMENT_perf_event.o := n
+
 # Make sure load_percpu_segment has no stackprotector
 nostackp := $(call cc-option, -fno-stack-protector)
 CFLAGS_common.o		:= $(nostackp)
diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index a501fa25da41..72a576752a7e 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -2,6 +2,9 @@
 # Makefile for x86 specific library files.
 #
 
+# Produces uninteresting flaky coverage.
+KCOV_INSTRUMENT_delay.o	:= n
+
 inat_tables_script = $(srctree)/arch/x86/tools/gen-insn-attr-x86.awk
 inat_tables_maps = $(srctree)/arch/x86/lib/x86-opcode-map.txt
 quiet_cmd_inat_tables = GEN     $@
diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
index 67cf2e1e557b..f98913258c63 100644
--- a/arch/x86/mm/Makefile
+++ b/arch/x86/mm/Makefile
@@ -1,3 +1,6 @@
+# Kernel does not boot with instrumentation of tlb.c.
+KCOV_INSTRUMENT_tlb.o	:= n
+
 obj-y	:=  init.o init_$(BITS).o fault.o ioremap.o extable.o pageattr.o mmap.o \
 	    pat.o pgtable.o physaddr.o gup.o setup_nx.o
 
diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
index 053abe7b0ef7..b95964610ea7 100644
--- a/arch/x86/realmode/rm/Makefile
+++ b/arch/x86/realmode/rm/Makefile
@@ -9,6 +9,9 @@
 KASAN_SANITIZE			:= n
 OBJECT_FILES_NON_STANDARD	:= y
 
+# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
+KCOV_INSTRUMENT		:= n
+
 always := realmode.bin realmode.relocs
 
 wakeup-objs	:= wakeup_asm.o wakemain.o video-mode.o
diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index a15841eced4e..da99bbb74aeb 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -25,6 +25,9 @@ KASAN_SANITIZE			:= n
 UBSAN_SANITIZE			:= n
 OBJECT_FILES_NON_STANDARD	:= y
 
+# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
+KCOV_INSTRUMENT			:= n
+
 lib-y				:= efi-stub-helper.o
 
 # include the stub's generic dependencies from lib/ when building for ARM/arm64
diff --git a/include/linux/kcov.h b/include/linux/kcov.h
new file mode 100644
index 000000000000..2883ac98c280
--- /dev/null
+++ b/include/linux/kcov.h
@@ -0,0 +1,29 @@
+#ifndef _LINUX_KCOV_H
+#define _LINUX_KCOV_H
+
+#include <uapi/linux/kcov.h>
+
+struct task_struct;
+
+#ifdef CONFIG_KCOV
+
+void kcov_task_init(struct task_struct *t);
+void kcov_task_exit(struct task_struct *t);
+
+enum kcov_mode {
+	/* Coverage collection is not enabled yet. */
+	KCOV_MODE_DISABLED = 0,
+	/*
+	 * Tracing coverage collection mode.
+	 * Covered PCs are collected in a per-task buffer.
+	 */
+	KCOV_MODE_TRACE = 1,
+};
+
+#else
+
+static inline void kcov_task_init(struct task_struct *t) {}
+static inline void kcov_task_exit(struct task_struct *t) {}
+
+#endif /* CONFIG_KCOV */
+#endif /* _LINUX_KCOV_H */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 084ed9fba620..34495d2d2d7b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -51,6 +51,7 @@ struct sched_param {
 #include <linux/resource.h>
 #include <linux/timer.h>
 #include <linux/hrtimer.h>
+#include <linux/kcov.h>
 #include <linux/task_io_accounting.h>
 #include <linux/latencytop.h>
 #include <linux/cred.h>
@@ -1818,6 +1819,16 @@ struct task_struct {
 	/* bitmask and counter of trace recursion */
 	unsigned long trace_recursion;
 #endif /* CONFIG_TRACING */
+#ifdef CONFIG_KCOV
+	/* Coverage collection mode enabled for this task (0 if disabled). */
+	enum kcov_mode kcov_mode;
+	/* Size of the kcov_area. */
+	unsigned	kcov_size;
+	/* Buffer for coverage collection. */
+	void		*kcov_area;
+	/* kcov desciptor wired with this task or NULL. */
+	struct kcov	*kcov;
+#endif
 #ifdef CONFIG_MEMCG
 	struct mem_cgroup *memcg_in_oom;
 	gfp_t memcg_oom_gfp_mask;
diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
new file mode 100644
index 000000000000..574e22ec640d
--- /dev/null
+++ b/include/uapi/linux/kcov.h
@@ -0,0 +1,10 @@
+#ifndef _LINUX_KCOV_IOCTLS_H
+#define _LINUX_KCOV_IOCTLS_H
+
+#include <linux/types.h>
+
+#define KCOV_INIT_TRACE			_IOR('c', 1, unsigned long)
+#define KCOV_ENABLE			_IO('c', 100)
+#define KCOV_DISABLE			_IO('c', 101)
+
+#endif /* _LINUX_KCOV_IOCTLS_H */
diff --git a/kernel/Makefile b/kernel/Makefile
index baa55e50a315..f0c40bf49d9f 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -18,6 +18,17 @@ ifdef CONFIG_FUNCTION_TRACER
 CFLAGS_REMOVE_irq_work.o = $(CC_FLAGS_FTRACE)
 endif
 
+# Prevents flicker of uninteresting __do_softirq()/__local_bh_disable_ip()
+# in coverage traces.
+KCOV_INSTRUMENT_softirq.o := n
+# These are called from save_stack_trace() on slub debug path,
+# and produce insane amounts of uninteresting coverage.
+KCOV_INSTRUMENT_module.o := n
+KCOV_INSTRUMENT_extable.o := n
+# Don't self-instrument.
+KCOV_INSTRUMENT_kcov.o := n
+KASAN_SANITIZE_kcov.o := n
+
 # cond_syscall is currently not LTO compatible
 CFLAGS_sys_ni.o = $(DISABLE_LTO)
 
@@ -68,6 +79,7 @@ obj-$(CONFIG_AUDITSYSCALL) += auditsc.o
 obj-$(CONFIG_AUDIT_WATCH) += audit_watch.o audit_fsnotify.o
 obj-$(CONFIG_AUDIT_TREE) += audit_tree.o
 obj-$(CONFIG_GCOV_KERNEL) += gcov/
+obj-$(CONFIG_KCOV) += kcov.o
 obj-$(CONFIG_KPROBES) += kprobes.o
 obj-$(CONFIG_KGDB) += debug/
 obj-$(CONFIG_DETECT_HUNG_TASK) += hung_task.o
diff --git a/kernel/exit.c b/kernel/exit.c
index 10e088237fed..953d1a1c0387 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -53,6 +53,7 @@
 #include <linux/oom.h>
 #include <linux/writeback.h>
 #include <linux/shm.h>
+#include <linux/kcov.h>
 
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
@@ -655,6 +656,7 @@ void do_exit(long code)
 	TASKS_RCU(int tasks_rcu_i);
 
 	profile_task_exit(tsk);
+	kcov_task_exit(tsk);
 
 	WARN_ON(blk_needs_flush_plug(tsk));
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 5b8d1e7ceeea..d277e83ed3e0 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -75,6 +75,7 @@
 #include <linux/aio.h>
 #include <linux/compiler.h>
 #include <linux/sysctl.h>
+#include <linux/kcov.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -392,6 +393,8 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
 
 	account_kernel_stack(ti, 1);
 
+	kcov_task_init(tsk);
+
 	return tsk;
 
 free_ti:
diff --git a/kernel/kcov.c b/kernel/kcov.c
new file mode 100644
index 000000000000..3efbee0834a8
--- /dev/null
+++ b/kernel/kcov.c
@@ -0,0 +1,273 @@
+#define pr_fmt(fmt) "kcov: " fmt
+
+#include <linux/compiler.h>
+#include <linux/types.h>
+#include <linux/file.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/printk.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <linux/vmalloc.h>
+#include <linux/debugfs.h>
+#include <linux/uaccess.h>
+#include <linux/kcov.h>
+
+/*
+ * kcov descriptor (one per opened debugfs file).
+ * State transitions of the descriptor:
+ *  - initial state after open()
+ *  - then there must be a single ioctl(KCOV_INIT_TRACE) call
+ *  - then, mmap() call (several calls are allowed but not useful)
+ *  - then, repeated enable/disable for a task (only one task a time allowed)
+ */
+struct kcov {
+	/*
+	 * Reference counter. We keep one for:
+	 *  - opened file descriptor
+	 *  - task with enabled coverage (we can't unwire it from another task)
+	 */
+	atomic_t		refcount;
+	/* The lock protects mode, size, area and t. */
+	spinlock_t		lock;
+	enum kcov_mode		mode;
+	/* Size of arena (in long's for KCOV_MODE_TRACE). */
+	unsigned		size;
+	/* Coverage buffer shared with user space. */
+	void			*area;
+	/* Task for which we collect coverage, or NULL. */
+	struct task_struct	*t;
+};
+
+/*
+ * Entry point from instrumented code.
+ * This is called once per basic-block/edge.
+ */
+void __sanitizer_cov_trace_pc(void)
+{
+	struct task_struct *t;
+	enum kcov_mode mode;
+
+	t = current;
+	/*
+	 * We are interested in code coverage as a function of a syscall inputs,
+	 * so we ignore code executed in interrupts.
+	 */
+	if (!t || in_interrupt())
+		return;
+	mode = READ_ONCE(t->kcov_mode);
+	if (mode == KCOV_MODE_TRACE) {
+		unsigned long *area;
+		unsigned long pos;
+
+		/*
+		 * There is some code that runs in interrupts but for which
+		 * in_interrupt() returns false (e.g. preempt_schedule_irq()).
+		 * READ_ONCE()/barrier() effectively provides load-acquire wrt
+		 * interrupts, there are paired barrier()/WRITE_ONCE() in
+		 * kcov_ioctl_locked().
+		 */
+		barrier();
+		area = t->kcov_area;
+		/* The first word is number of subsequent PCs. */
+		pos = READ_ONCE(area[0]) + 1;
+		if (likely(pos < t->kcov_size)) {
+			area[pos] = _RET_IP_;
+			WRITE_ONCE(area[0], pos);
+		}
+	}
+}
+EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
+
+static void kcov_get(struct kcov *kcov)
+{
+	atomic_inc(&kcov->refcount);
+}
+
+static void kcov_put(struct kcov *kcov)
+{
+	if (atomic_dec_and_test(&kcov->refcount)) {
+		vfree(kcov->area);
+		kfree(kcov);
+	}
+}
+
+void kcov_task_init(struct task_struct *t)
+{
+	t->kcov_mode = KCOV_MODE_DISABLED;
+	t->kcov_size = 0;
+	t->kcov_area = NULL;
+	t->kcov = NULL;
+}
+
+void kcov_task_exit(struct task_struct *t)
+{
+	struct kcov *kcov;
+
+	kcov = t->kcov;
+	if (kcov == NULL)
+		return;
+	spin_lock(&kcov->lock);
+	if (WARN_ON(kcov->t != t)) {
+		spin_unlock(&kcov->lock);
+		return;
+	}
+	/* Just to not leave dangling references behind. */
+	kcov_task_init(t);
+	kcov->t = NULL;
+	spin_unlock(&kcov->lock);
+	kcov_put(kcov);
+}
+
+static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
+{
+	int res = 0;
+	void *area;
+	struct kcov *kcov = vma->vm_file->private_data;
+	unsigned long size, off;
+	struct page *page;
+
+	area = vmalloc_user(vma->vm_end - vma->vm_start);
+	if (!area)
+		return -ENOMEM;
+
+	spin_lock(&kcov->lock);
+	size = kcov->size * sizeof(unsigned long);
+	if (kcov->mode == KCOV_MODE_DISABLED || vma->vm_pgoff != 0 ||
+	    vma->vm_end - vma->vm_start != size) {
+		res = -EINVAL;
+		goto exit;
+	}
+	if (!kcov->area) {
+		kcov->area = area;
+		vma->vm_flags |= VM_DONTEXPAND;
+		spin_unlock(&kcov->lock);
+		for (off = 0; off < size; off += PAGE_SIZE) {
+			page = vmalloc_to_page(kcov->area + off);
+			if (vm_insert_page(vma, vma->vm_start + off, page))
+				WARN_ONCE(1, "vm_insert_page() failed");
+		}
+		return 0;
+	}
+exit:
+	spin_unlock(&kcov->lock);
+	vfree(area);
+	return res;
+}
+
+static int kcov_open(struct inode *inode, struct file *filep)
+{
+	struct kcov *kcov;
+
+	kcov = kzalloc(sizeof(*kcov), GFP_KERNEL);
+	if (!kcov)
+		return -ENOMEM;
+	atomic_set(&kcov->refcount, 1);
+	spin_lock_init(&kcov->lock);
+	filep->private_data = kcov;
+	return nonseekable_open(inode, filep);
+}
+
+static int kcov_close(struct inode *inode, struct file *filep)
+{
+	kcov_put(filep->private_data);
+	return 0;
+}
+
+static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
+			     unsigned long arg)
+{
+	struct task_struct *t;
+	unsigned long size, unused;
+
+	switch (cmd) {
+	case KCOV_INIT_TRACE:
+		/*
+		 * Enable kcov in trace mode and setup buffer size.
+		 * Must happen before anything else.
+		 */
+		if (kcov->mode != KCOV_MODE_DISABLED)
+			return -EBUSY;
+		/*
+		 * Size must be at least 2 to hold current position and one PC.
+		 * Later we allocate size * sizeof(unsigned long) memory,
+		 * that must not overflow.
+		 */
+		size = arg;
+		if (size < 2 || size > INT_MAX / sizeof(unsigned long))
+			return -EINVAL;
+		kcov->size = size;
+		kcov->mode = KCOV_MODE_TRACE;
+		return 0;
+	case KCOV_ENABLE:
+		/*
+		 * Enable coverage for the current task.
+		 * At this point user must have been enabled trace mode,
+		 * and mmapped the file. Coverage collection is disabled only
+		 * at task exit or voluntary by KCOV_DISABLE. After that it can
+		 * be enabled for another task.
+		 */
+		unused = arg;
+		if (unused != 0 || kcov->mode == KCOV_MODE_DISABLED ||
+		    kcov->area == NULL)
+			return -EINVAL;
+		if (kcov->t != NULL)
+			return -EBUSY;
+		t = current;
+		/* Cache in task struct for performance. */
+		t->kcov_size = kcov->size;
+		t->kcov_area = kcov->area;
+		/* See comment in __sanitizer_cov_trace_pc(). */
+		barrier();
+		WRITE_ONCE(t->kcov_mode, kcov->mode);
+		t->kcov = kcov;
+		kcov->t = t;
+		/* This is put either in kcov_task_exit() or in KCOV_DISABLE. */
+		kcov_get(kcov);
+		return 0;
+	case KCOV_DISABLE:
+		/* Disable coverage for the current task. */
+		unused = arg;
+		if (unused != 0 || current->kcov != kcov)
+			return -EINVAL;
+		t = current;
+		if (WARN_ON(kcov->t != t))
+			return -EINVAL;
+		kcov_task_init(t);
+		kcov->t = NULL;
+		kcov_put(kcov);
+		return 0;
+	default:
+		return -ENOTTY;
+	}
+}
+
+static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
+{
+	struct kcov *kcov;
+	int res;
+
+	kcov = filep->private_data;
+	spin_lock(&kcov->lock);
+	res = kcov_ioctl_locked(kcov, cmd, arg);
+	spin_unlock(&kcov->lock);
+	return res;
+}
+
+static const struct file_operations kcov_fops = {
+	.open		= kcov_open,
+	.unlocked_ioctl	= kcov_ioctl,
+	.mmap		= kcov_mmap,
+	.release        = kcov_close,
+};
+
+static int __init kcov_init(void)
+{
+	if (!debugfs_create_file("kcov", 0600, NULL, NULL, &kcov_fops)) {
+		pr_err("failed to create kcov in debugfs\n");
+		return -ENOMEM;
+	}
+	return 0;
+}
+
+device_initcall(kcov_init);
diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
index 8e96f6cc2a4a..31322a4275cd 100644
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -1,3 +1,6 @@
+# Any varying coverage in these files is non-deterministic
+# and is generally not a function of system call inputs.
+KCOV_INSTRUMENT		:= n
 
 obj-y += mutex.o semaphore.o rwsem.o percpu-rwsem.o
 
diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
index 61a16569ffbf..032b2c015beb 100644
--- a/kernel/rcu/Makefile
+++ b/kernel/rcu/Makefile
@@ -1,3 +1,7 @@
+# Any varying coverage in these files is non-deterministic
+# and is generally not a function of system call inputs.
+KCOV_INSTRUMENT := n
+
 obj-y += update.o sync.o
 obj-$(CONFIG_SRCU) += srcu.o
 obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
index 302d6ebd64f7..414d9c16da42 100644
--- a/kernel/sched/Makefile
+++ b/kernel/sched/Makefile
@@ -2,6 +2,10 @@ ifdef CONFIG_FUNCTION_TRACER
 CFLAGS_REMOVE_clock.o = $(CC_FLAGS_FTRACE)
 endif
 
+# These files are disabled because they produce non-interesting flaky coverage
+# that is not a function of syscall inputs. E.g. involuntary context switches.
+KCOV_INSTRUMENT := n
+
 ifneq ($(CONFIG_SCHED_OMIT_FRAME_POINTER),y)
 # According to Alan Modra <alan@linuxcare.com.au>, the -fno-omit-frame-pointer is
 # needed for x86 only.  Why this used to be enabled for all architectures is beyond
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 5a60f45cd9bb..532d4d52d1df 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -696,6 +696,27 @@ source "lib/Kconfig.kasan"
 
 endmenu # "Memory Debugging"
 
+config ARCH_HAS_KCOV
+	bool
+	help
+	  KCOV does not have any arch-specific code, but currently it is enabled
+	  only for x86_64. KCOV requires testing on other archs, and most likely
+	  disabling of instrumentation for some early boot code.
+
+config KCOV
+	bool "Code coverage for fuzzing"
+	depends on ARCH_HAS_KCOV
+	select DEBUG_FS
+	help
+	  KCOV exposes kernel code coverage information in a form suitable
+	  for coverage-guided fuzzing (randomized testing).
+
+	  If RANDOMIZE_BASE is enabled, PC values will not be stable across
+	  different machines and across reboots. If you need stable PC values,
+	  disable RANDOMIZE_BASE.
+
+	  For more details, see Documentation/kcov.txt.
+
 config DEBUG_SHIRQ
 	bool "Debug shared IRQ handlers"
 	depends on DEBUG_KERNEL
diff --git a/lib/Makefile b/lib/Makefile
index 4962d14c450f..a1de5b61ff40 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -7,6 +7,18 @@ ORIG_CFLAGS := $(KBUILD_CFLAGS)
 KBUILD_CFLAGS = $(subst $(CC_FLAGS_FTRACE),,$(ORIG_CFLAGS))
 endif
 
+# These files are disabled because they produce lots of non-interesting and/or
+# flaky coverage that is not a function of syscall inputs. For example,
+# rbtree can be global and individual rotations don't correlate with inputs.
+KCOV_INSTRUMENT_string.o := n
+KCOV_INSTRUMENT_rbtree.o := n
+KCOV_INSTRUMENT_list_debug.o := n
+KCOV_INSTRUMENT_debugobjects.o := n
+KCOV_INSTRUMENT_dynamic_debug.o := n
+# Kernel does not boot if we instrument this file as it uses custom calling
+# convention (see CONFIG_ARCH_HWEIGHT_CFLAGS).
+KCOV_INSTRUMENT_hweight.o := n
+
 lib-y := ctype.o string.o vsprintf.o cmdline.o \
 	 rbtree.o radix-tree.o dump_stack.o timerqueue.o\
 	 idr.o int_sqrt.o extable.o \
diff --git a/mm/Makefile b/mm/Makefile
index 6da300a1414b..f5e797cbd128 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -5,6 +5,21 @@
 KASAN_SANITIZE_slab_common.o := n
 KASAN_SANITIZE_slub.o := n
 
+# These files are disabled because they produce non-interesting and/or
+# flaky coverage that is not a function of syscall inputs. E.g. slab is out of
+# free pages, or a task is migrated between nodes.
+KCOV_INSTRUMENT_slab_common.o := n
+KCOV_INSTRUMENT_slob.o := n
+KCOV_INSTRUMENT_slab.o := n
+KCOV_INSTRUMENT_slub.o := n
+KCOV_INSTRUMENT_page_alloc.o := n
+KCOV_INSTRUMENT_debug-pagealloc.o := n
+KCOV_INSTRUMENT_kmemleak.o := n
+KCOV_INSTRUMENT_kmemcheck.o := n
+KCOV_INSTRUMENT_memcontrol.o := n
+KCOV_INSTRUMENT_mmzone.o := n
+KCOV_INSTRUMENT_vmstat.o := n
+
 mmu-y			:= nommu.o
 mmu-$(CONFIG_MMU)	:= gup.o highmem.o memory.o mincore.o \
 			   mlock.o mmap.o mprotect.o mremap.o msync.o rmap.o \
diff --git a/mm/kasan/Makefile b/mm/kasan/Makefile
index a61460d9f5b0..131daadf40e4 100644
--- a/mm/kasan/Makefile
+++ b/mm/kasan/Makefile
@@ -1,5 +1,6 @@
 KASAN_SANITIZE := n
 UBSAN_SANITIZE_kasan.o := n
+KCOV_INSTRUMENT := n
 
 CFLAGS_REMOVE_kasan.o = -pg
 # Function splitter causes unnecessary splits in __asan_load1/__asan_store1
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index ad50d5859ac4..ddf83d0181e7 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -136,6 +136,12 @@ _c_flags += $(if $(patsubst n%,, \
 		$(CFLAGS_UBSAN))
 endif
 
+ifeq ($(CONFIG_KCOV),y)
+_c_flags += $(if $(patsubst n%,, \
+	$(KCOV_INSTRUMENT_$(basetarget).o)$(KCOV_INSTRUMENT)y), \
+	$(CFLAGS_KCOV))
+endif
+
 # If building the kernel in a separate objtree expand all occurrences
 # of -Idir to -I$(srctree)/dir except for absolute paths (starting with '/').