xref: /openbmc/linux/kernel/kcov.c (revision a36954f5)
1 #define pr_fmt(fmt) "kcov: " fmt
2 
3 #define DISABLE_BRANCH_PROFILING
4 #include <linux/atomic.h>
5 #include <linux/compiler.h>
6 #include <linux/errno.h>
7 #include <linux/export.h>
8 #include <linux/types.h>
9 #include <linux/file.h>
10 #include <linux/fs.h>
11 #include <linux/init.h>
12 #include <linux/mm.h>
13 #include <linux/preempt.h>
14 #include <linux/printk.h>
15 #include <linux/sched.h>
16 #include <linux/slab.h>
17 #include <linux/spinlock.h>
18 #include <linux/vmalloc.h>
19 #include <linux/debugfs.h>
20 #include <linux/uaccess.h>
21 #include <linux/kcov.h>
22 #include <asm/setup.h>
23 
24 /*
25  * kcov descriptor (one per opened debugfs file).
26  * State transitions of the descriptor:
27  *  - initial state after open()
28  *  - then there must be a single ioctl(KCOV_INIT_TRACE) call
29  *  - then, mmap() call (several calls are allowed but not useful)
30  *  - then, repeated enable/disable for a task (only one task a time allowed)
31  */
32 struct kcov {
33 	/*
34 	 * Reference counter. We keep one for:
35 	 *  - opened file descriptor
36 	 *  - task with enabled coverage (we can't unwire it from another task)
37 	 */
38 	atomic_t		refcount;
39 	/* The lock protects mode, size, area and t. */
40 	spinlock_t		lock;
41 	enum kcov_mode		mode;
42 	/* Size of arena (in long's for KCOV_MODE_TRACE). */
43 	unsigned		size;
44 	/* Coverage buffer shared with user space. */
45 	void			*area;
46 	/* Task for which we collect coverage, or NULL. */
47 	struct task_struct	*t;
48 };
49 
50 /*
51  * Entry point from instrumented code.
52  * This is called once per basic-block/edge.
53  */
54 void notrace __sanitizer_cov_trace_pc(void)
55 {
56 	struct task_struct *t;
57 	enum kcov_mode mode;
58 
59 	t = current;
60 	/*
61 	 * We are interested in code coverage as a function of a syscall inputs,
62 	 * so we ignore code executed in interrupts.
63 	 */
64 	if (!t || !in_task())
65 		return;
66 	mode = READ_ONCE(t->kcov_mode);
67 	if (mode == KCOV_MODE_TRACE) {
68 		unsigned long *area;
69 		unsigned long pos;
70 		unsigned long ip = _RET_IP_;
71 
72 #ifdef CONFIG_RANDOMIZE_BASE
73 		ip -= kaslr_offset();
74 #endif
75 
76 		/*
77 		 * There is some code that runs in interrupts but for which
78 		 * in_interrupt() returns false (e.g. preempt_schedule_irq()).
79 		 * READ_ONCE()/barrier() effectively provides load-acquire wrt
80 		 * interrupts, there are paired barrier()/WRITE_ONCE() in
81 		 * kcov_ioctl_locked().
82 		 */
83 		barrier();
84 		area = t->kcov_area;
85 		/* The first word is number of subsequent PCs. */
86 		pos = READ_ONCE(area[0]) + 1;
87 		if (likely(pos < t->kcov_size)) {
88 			area[pos] = ip;
89 			WRITE_ONCE(area[0], pos);
90 		}
91 	}
92 }
93 EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
94 
95 static void kcov_get(struct kcov *kcov)
96 {
97 	atomic_inc(&kcov->refcount);
98 }
99 
100 static void kcov_put(struct kcov *kcov)
101 {
102 	if (atomic_dec_and_test(&kcov->refcount)) {
103 		vfree(kcov->area);
104 		kfree(kcov);
105 	}
106 }
107 
108 void kcov_task_init(struct task_struct *t)
109 {
110 	t->kcov_mode = KCOV_MODE_DISABLED;
111 	t->kcov_size = 0;
112 	t->kcov_area = NULL;
113 	t->kcov = NULL;
114 }
115 
116 void kcov_task_exit(struct task_struct *t)
117 {
118 	struct kcov *kcov;
119 
120 	kcov = t->kcov;
121 	if (kcov == NULL)
122 		return;
123 	spin_lock(&kcov->lock);
124 	if (WARN_ON(kcov->t != t)) {
125 		spin_unlock(&kcov->lock);
126 		return;
127 	}
128 	/* Just to not leave dangling references behind. */
129 	kcov_task_init(t);
130 	kcov->t = NULL;
131 	spin_unlock(&kcov->lock);
132 	kcov_put(kcov);
133 }
134 
135 static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
136 {
137 	int res = 0;
138 	void *area;
139 	struct kcov *kcov = vma->vm_file->private_data;
140 	unsigned long size, off;
141 	struct page *page;
142 
143 	area = vmalloc_user(vma->vm_end - vma->vm_start);
144 	if (!area)
145 		return -ENOMEM;
146 
147 	spin_lock(&kcov->lock);
148 	size = kcov->size * sizeof(unsigned long);
149 	if (kcov->mode == KCOV_MODE_DISABLED || vma->vm_pgoff != 0 ||
150 	    vma->vm_end - vma->vm_start != size) {
151 		res = -EINVAL;
152 		goto exit;
153 	}
154 	if (!kcov->area) {
155 		kcov->area = area;
156 		vma->vm_flags |= VM_DONTEXPAND;
157 		spin_unlock(&kcov->lock);
158 		for (off = 0; off < size; off += PAGE_SIZE) {
159 			page = vmalloc_to_page(kcov->area + off);
160 			if (vm_insert_page(vma, vma->vm_start + off, page))
161 				WARN_ONCE(1, "vm_insert_page() failed");
162 		}
163 		return 0;
164 	}
165 exit:
166 	spin_unlock(&kcov->lock);
167 	vfree(area);
168 	return res;
169 }
170 
171 static int kcov_open(struct inode *inode, struct file *filep)
172 {
173 	struct kcov *kcov;
174 
175 	kcov = kzalloc(sizeof(*kcov), GFP_KERNEL);
176 	if (!kcov)
177 		return -ENOMEM;
178 	atomic_set(&kcov->refcount, 1);
179 	spin_lock_init(&kcov->lock);
180 	filep->private_data = kcov;
181 	return nonseekable_open(inode, filep);
182 }
183 
184 static int kcov_close(struct inode *inode, struct file *filep)
185 {
186 	kcov_put(filep->private_data);
187 	return 0;
188 }
189 
190 static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
191 			     unsigned long arg)
192 {
193 	struct task_struct *t;
194 	unsigned long size, unused;
195 
196 	switch (cmd) {
197 	case KCOV_INIT_TRACE:
198 		/*
199 		 * Enable kcov in trace mode and setup buffer size.
200 		 * Must happen before anything else.
201 		 */
202 		if (kcov->mode != KCOV_MODE_DISABLED)
203 			return -EBUSY;
204 		/*
205 		 * Size must be at least 2 to hold current position and one PC.
206 		 * Later we allocate size * sizeof(unsigned long) memory,
207 		 * that must not overflow.
208 		 */
209 		size = arg;
210 		if (size < 2 || size > INT_MAX / sizeof(unsigned long))
211 			return -EINVAL;
212 		kcov->size = size;
213 		kcov->mode = KCOV_MODE_TRACE;
214 		return 0;
215 	case KCOV_ENABLE:
216 		/*
217 		 * Enable coverage for the current task.
218 		 * At this point user must have been enabled trace mode,
219 		 * and mmapped the file. Coverage collection is disabled only
220 		 * at task exit or voluntary by KCOV_DISABLE. After that it can
221 		 * be enabled for another task.
222 		 */
223 		unused = arg;
224 		if (unused != 0 || kcov->mode == KCOV_MODE_DISABLED ||
225 		    kcov->area == NULL)
226 			return -EINVAL;
227 		if (kcov->t != NULL)
228 			return -EBUSY;
229 		t = current;
230 		/* Cache in task struct for performance. */
231 		t->kcov_size = kcov->size;
232 		t->kcov_area = kcov->area;
233 		/* See comment in __sanitizer_cov_trace_pc(). */
234 		barrier();
235 		WRITE_ONCE(t->kcov_mode, kcov->mode);
236 		t->kcov = kcov;
237 		kcov->t = t;
238 		/* This is put either in kcov_task_exit() or in KCOV_DISABLE. */
239 		kcov_get(kcov);
240 		return 0;
241 	case KCOV_DISABLE:
242 		/* Disable coverage for the current task. */
243 		unused = arg;
244 		if (unused != 0 || current->kcov != kcov)
245 			return -EINVAL;
246 		t = current;
247 		if (WARN_ON(kcov->t != t))
248 			return -EINVAL;
249 		kcov_task_init(t);
250 		kcov->t = NULL;
251 		kcov_put(kcov);
252 		return 0;
253 	default:
254 		return -ENOTTY;
255 	}
256 }
257 
258 static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
259 {
260 	struct kcov *kcov;
261 	int res;
262 
263 	kcov = filep->private_data;
264 	spin_lock(&kcov->lock);
265 	res = kcov_ioctl_locked(kcov, cmd, arg);
266 	spin_unlock(&kcov->lock);
267 	return res;
268 }
269 
270 static const struct file_operations kcov_fops = {
271 	.open		= kcov_open,
272 	.unlocked_ioctl	= kcov_ioctl,
273 	.mmap		= kcov_mmap,
274 	.release        = kcov_close,
275 };
276 
277 static int __init kcov_init(void)
278 {
279 	/*
280 	 * The kcov debugfs file won't ever get removed and thus,
281 	 * there is no need to protect it against removal races. The
282 	 * use of debugfs_create_file_unsafe() is actually safe here.
283 	 */
284 	if (!debugfs_create_file_unsafe("kcov", 0600, NULL, NULL, &kcov_fops)) {
285 		pr_err("failed to create kcov in debugfs\n");
286 		return -ENOMEM;
287 	}
288 	return 0;
289 }
290 
291 device_initcall(kcov_init);
292