lib/stackdepot.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Generic stack depot for storing stack traces.
   4  *
   5  * Some debugging tools need to save stack traces of certain events which can
   6  * be later presented to the user. For example, KASAN needs to safe alloc and
   7  * free stacks for each object, but storing two stack traces per object
   8  * requires too much memory (e.g. SLUB_DEBUG needs 256 bytes per object for
   9  * that).
  10  *
  11  * Instead, stack depot maintains a hashtable of unique stacktraces. Since alloc
  12  * and free stacks repeat a lot, we save about 100x space.
  13  * Stacks are never removed from depot, so we store them contiguously one after
  14  * another in a contiguos memory allocation.
  15  *
  16  * Author: Alexander Potapenko <glider@google.com>
  17  * Copyright (C) 2016 Google, Inc.
  18  *
  19  * Based on code by Dmitry Chernenkov.
  20  */
  21
  22 #include <linux/gfp.h>
  23 #include <linux/jhash.h>
  24 #include <linux/kernel.h>
  25 #include <linux/mm.h>
  26 #include <linux/percpu.h>
  27 #include <linux/printk.h>
  28 #include <linux/slab.h>
  29 #include <linux/stacktrace.h>
  30 #include <linux/stackdepot.h>
  31 #include <linux/string.h>
  32 #include <linux/types.h>
  33
  34 #define DEPOT_STACK_BITS (sizeof(depot_stack_handle_t) * 8)
  35
  36 #define STACK_ALLOC_NULL_PROTECTION_BITS 1
  37 #define STACK_ALLOC_ORDER 2 /* 'Slab' size order for stack depot, 4 pages */
  38 #define STACK_ALLOC_SIZE (1LL << (PAGE_SHIFT + STACK_ALLOC_ORDER))
  39 #define STACK_ALLOC_ALIGN 4
  40 #define STACK_ALLOC_OFFSET_BITS (STACK_ALLOC_ORDER + PAGE_SHIFT - \
  41                                         STACK_ALLOC_ALIGN)
  42 #define STACK_ALLOC_INDEX_BITS (DEPOT_STACK_BITS - \
  43                 STACK_ALLOC_NULL_PROTECTION_BITS - STACK_ALLOC_OFFSET_BITS)
  44 #define STACK_ALLOC_SLABS_CAP 8192
  45 #define STACK_ALLOC_MAX_SLABS \
  46         (((1LL << (STACK_ALLOC_INDEX_BITS)) < STACK_ALLOC_SLABS_CAP) ? \
  47          (1LL << (STACK_ALLOC_INDEX_BITS)) : STACK_ALLOC_SLABS_CAP)
  48
  49 /* The compact structure to store the reference to stacks. */
  50 union handle_parts {
  51         depot_stack_handle_t handle;
  52         struct {
  53                 u32 slabindex : STACK_ALLOC_INDEX_BITS;
  54                 u32 offset : STACK_ALLOC_OFFSET_BITS;
  55                 u32 valid : STACK_ALLOC_NULL_PROTECTION_BITS;
  56         };
  57 };
  58
  59 struct stack_record {
  60         struct stack_record *next;      /* Link in the hashtable */
  61         u32 hash;                       /* Hash in the hastable */
  62         u32 size;                       /* Number of frames in the stack */
  63         union handle_parts handle;
  64         unsigned long entries[1];       /* Variable-sized array of entries. */
  65 };
  66
  67 static void *stack_slabs[STACK_ALLOC_MAX_SLABS];
  68
  69 static int depot_index;
  70 static int next_slab_inited;
  71 static size_t depot_offset;
  72 static DEFINE_SPINLOCK(depot_lock);
  73
  74 static bool init_stack_slab(void **prealloc)
  75 {
  76         if (!*prealloc)
  77                 return false;
  78         /*
  79          * This smp_load_acquire() pairs with smp_store_release() to
  80          * |next_slab_inited| below and in depot_alloc_stack().
  81          */
  82         if (smp_load_acquire(&next_slab_inited))
  83                 return true;
  84         if (stack_slabs[depot_index] == NULL) {
  85                 stack_slabs[depot_index] = *prealloc;
  86                 *prealloc = NULL;
  87         } else {
  88                 /* If this is the last depot slab, do not touch the next one. */
  89                 if (depot_index + 1 < STACK_ALLOC_MAX_SLABS) {
  90                         stack_slabs[depot_index + 1] = *prealloc;
  91                         *prealloc = NULL;
  92                 }
  93                 /*
  94                  * This smp_store_release pairs with smp_load_acquire() from
  95                  * |next_slab_inited| above and in stack_depot_save().
  96                  */
  97                 smp_store_release(&next_slab_inited, 1);
  98         }
  99         return true;
 100 }
 101
 102 /* Allocation of a new stack in raw storage */
 103 static struct stack_record *depot_alloc_stack(unsigned long *entries, int size,
 104                 u32 hash, void **prealloc, gfp_t alloc_flags)
 105 {
 106         int required_size = offsetof(struct stack_record, entries) +
 107                 sizeof(unsigned long) * size;
 108         struct stack_record *stack;
 109
 110         required_size = ALIGN(required_size, 1 << STACK_ALLOC_ALIGN);
 111
 112         if (unlikely(depot_offset + required_size > STACK_ALLOC_SIZE)) {
 113                 if (unlikely(depot_index + 1 >= STACK_ALLOC_MAX_SLABS)) {
 114                         WARN_ONCE(1, "Stack depot reached limit capacity");
 115                         return NULL;
 116                 }
 117                 depot_index++;
 118                 depot_offset = 0;
 119                 /*
 120                  * smp_store_release() here pairs with smp_load_acquire() from
 121                  * |next_slab_inited| in stack_depot_save() and
 122                  * init_stack_slab().
 123                  */
 124                 if (depot_index + 1 < STACK_ALLOC_MAX_SLABS)
 125                         smp_store_release(&next_slab_inited, 0);
 126         }
 127         init_stack_slab(prealloc);
 128         if (stack_slabs[depot_index] == NULL)
 129                 return NULL;
 130
 131         stack = stack_slabs[depot_index] + depot_offset;
 132
 133         stack->hash = hash;
 134         stack->size = size;
 135         stack->handle.slabindex = depot_index;
 136         stack->handle.offset = depot_offset >> STACK_ALLOC_ALIGN;
 137         stack->handle.valid = 1;
 138         memcpy(stack->entries, entries, size * sizeof(unsigned long));
 139         depot_offset += required_size;
 140
 141         return stack;
 142 }
 143
 144 #define STACK_HASH_ORDER 20
 145 #define STACK_HASH_SIZE (1L << STACK_HASH_ORDER)
 146 #define STACK_HASH_MASK (STACK_HASH_SIZE - 1)
 147 #define STACK_HASH_SEED 0x9747b28c
 148
 149 static struct stack_record *stack_table[STACK_HASH_SIZE] = {
 150         [0 ...  STACK_HASH_SIZE - 1] = NULL
 151 };
 152
 153 /* Calculate hash for a stack */
 154 static inline u32 hash_stack(unsigned long *entries, unsigned int size)
 155 {
 156         return jhash2((u32 *)entries,
 157                                size * sizeof(unsigned long) / sizeof(u32),
 158                                STACK_HASH_SEED);
 159 }
 160
 161 /* Use our own, non-instrumented version of memcmp().
 162  *
 163  * We actually don't care about the order, just the equality.
 164  */
 165 static inline
 166 int stackdepot_memcmp(const unsigned long *u1, const unsigned long *u2,
 167                         unsigned int n)
 168 {
 169         for ( ; n-- ; u1++, u2++) {
 170                 if (*u1 != *u2)
 171                         return 1;
 172         }
 173         return 0;
 174 }
 175
 176 /* Find a stack that is equal to the one stored in entries in the hash */
 177 static inline struct stack_record *find_stack(struct stack_record *bucket,
 178                                              unsigned long *entries, int size,
 179                                              u32 hash)
 180 {
 181         struct stack_record *found;
 182
 183         for (found = bucket; found; found = found->next) {
 184                 if (found->hash == hash &&
 185                     found->size == size &&
 186                     !stackdepot_memcmp(entries, found->entries, size))
 187                         return found;
 188         }
 189         return NULL;
 190 }
 191
 192 /**
 193  * stack_depot_fetch - Fetch stack entries from a depot
 194  *
 195  * @handle:             Stack depot handle which was returned from
 196  *                      stack_depot_save().
 197  * @entries:            Pointer to store the entries address
 198  *
 199  * Return: The number of trace entries for this depot.
 200  */
 201 unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 202                                unsigned long **entries)
 203 {
 204         union handle_parts parts = { .handle = handle };
 205         void *slab = stack_slabs[parts.slabindex];
 206         size_t offset = parts.offset << STACK_ALLOC_ALIGN;
 207         struct stack_record *stack = slab + offset;
 208
 209         *entries = stack->entries;
 210         return stack->size;
 211 }
 212 EXPORT_SYMBOL_GPL(stack_depot_fetch);
 213
 214 /**
 215  * stack_depot_save - Save a stack trace from an array
 216  *
 217  * @entries:            Pointer to storage array
 218  * @nr_entries:         Size of the storage array
 219  * @alloc_flags:        Allocation gfp flags
 220  *
 221  * Return: The handle of the stack struct stored in depot
 222  */
 223 depot_stack_handle_t stack_depot_save(unsigned long *entries,
 224                                       unsigned int nr_entries,
 225                                       gfp_t alloc_flags)
 226 {
 227         struct stack_record *found = NULL, **bucket;
 228         depot_stack_handle_t retval = 0;
 229         struct page *page = NULL;
 230         void *prealloc = NULL;
 231         unsigned long flags;
 232         u32 hash;
 233
 234         if (unlikely(nr_entries == 0))
 235                 goto fast_exit;
 236
 237         hash = hash_stack(entries, nr_entries);
 238         bucket = &stack_table[hash & STACK_HASH_MASK];
 239
 240         /*
 241          * Fast path: look the stack trace up without locking.
 242          * The smp_load_acquire() here pairs with smp_store_release() to
 243          * |bucket| below.
 244          */
 245         found = find_stack(smp_load_acquire(bucket), entries,
 246                            nr_entries, hash);
 247         if (found)
 248                 goto exit;
 249
 250         /*
 251          * Check if the current or the next stack slab need to be initialized.
 252          * If so, allocate the memory - we won't be able to do that under the
 253          * lock.
 254          *
 255          * The smp_load_acquire() here pairs with smp_store_release() to
 256          * |next_slab_inited| in depot_alloc_stack() and init_stack_slab().
 257          */
 258         if (unlikely(!smp_load_acquire(&next_slab_inited))) {
 259                 /*
 260                  * Zero out zone modifiers, as we don't have specific zone
 261                  * requirements. Keep the flags related to allocation in atomic
 262                  * contexts and I/O.
 263                  */
 264                 alloc_flags &= ~GFP_ZONEMASK;
 265                 alloc_flags &= (GFP_ATOMIC | GFP_KERNEL);
 266                 alloc_flags |= __GFP_NOWARN;
 267                 page = alloc_pages(alloc_flags, STACK_ALLOC_ORDER);
 268                 if (page)
 269                         prealloc = page_address(page);
 270         }
 271
 272         spin_lock_irqsave(&depot_lock, flags);
 273
 274         found = find_stack(*bucket, entries, nr_entries, hash);
 275         if (!found) {
 276                 struct stack_record *new =
 277                         depot_alloc_stack(entries, nr_entries,
 278                                           hash, &prealloc, alloc_flags);
 279                 if (new) {
 280                         new->next = *bucket;
 281                         /*
 282                          * This smp_store_release() pairs with
 283                          * smp_load_acquire() from |bucket| above.
 284                          */
 285                         smp_store_release(bucket, new);
 286                         found = new;
 287                 }
 288         } else if (prealloc) {
 289                 /*
 290                  * We didn't need to store this stack trace, but let's keep
 291                  * the preallocated memory for the future.
 292                  */
 293                 WARN_ON(!init_stack_slab(&prealloc));
 294         }
 295
 296         spin_unlock_irqrestore(&depot_lock, flags);
 297 exit:
 298         if (prealloc) {
 299                 /* Nobody used this memory, ok to free it. */
 300                 free_pages((unsigned long)prealloc, STACK_ALLOC_ORDER);
 301         }
 302         if (found)
 303                 retval = found->handle.handle;
 304 fast_exit:
 305         return retval;
 306 }
 307 EXPORT_SYMBOL_GPL(stack_depot_save);