kernel/kcov.c

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