kcov: initial implementation

Implement a device (/dev/kcov) that exposes coverage info for the
kernel. This will be especially useful for coverage-based fuzzing (e.g.
syzkaller).

Exclude a lot of functions which would crash/otherwise interfere in the
coverage, including only processing coverage callbacks when not handling
an irq.

kernel-src/include/kernel/kcov.h

@@ -0,0 +1,43 @@
+#ifndef _KCOV_H
+#define _KCOV_H
+
+#include <stdint.h>
+#include <stddef.h>
+#include <mutex.h>
+
+#ifdef KCOV_ENABLED
+#define KCOV_DISABLED __attribute__((no_sanitize_coverage))
+#else
+#define KCOV_DISABLED
+#endif
+
+#define KCOV_MODE_DISABLED 0
+#define KCOV_MODE_PC 1
+#define KCOV_MODE_CMP 2
+
+#define KCOV_CMP_FLAGS_CONST 1
+#define KCOV_CMP_FLAGS_8BIT 0
+#define KCOV_CMP_FLAGS_16BIT 2
+#define KCOV_CMP_FLAGS_32BIT 4
+#define KCOV_CMP_FLAGS_64BIT 6
+
+typedef struct {
+	unsigned int mode;
+	mutex_t mapping_mutex;
+	unsigned int entry_count; // protected by mutex
+	uint64_t *buffer; // pointer protected by mutex
+	size_t mappings; // protected by mutex
+	unsigned int refcount;
+	unsigned int minor;
+	void *vnode_handle;
+} kcov_t;
+
+void kcov_publish_pc(uint64_t pc);
+void kcov_publish_cmp(uint64_t pc, uint64_t v1, uint64_t v2, uint64_t flags);
+int kcov_set_buffer_size(unsigned int entry_count);
+int kcov_set_mode(unsigned int mode);
+void kcov_hold(kcov_t *kcov);
+void kcov_release(kcov_t *kcov);
+void kcov_free_buffer(kcov_t *kcov);
+
+#endif