lib/stackdepot.c

   1 /*
   2  * Generic stack depot for storing stack traces.
   3  *
   4  * Some debugging tools need to save stack traces of certain events which can
   5  * be later presented to the user. For example, KASAN needs to safe alloc and
   6  * free stacks for each object, but storing two stack traces per object
   7  * requires too much memory (e.g. SLUB_DEBUG needs 256 bytes per object for
   8  * that).
   9  *
  10  * Instead, stack depot maintains a hashtable of unique stacktraces. Since alloc
  11  * and free stacks repeat a lot, we save about 100x space.
  12  * Stacks are never removed from depot, so we store them contiguously one after
  13  * another in a contiguos memory allocation.
  14  *
  15  * Author: Alexander Potapenko <glider@google.com>
  16  * Copyright (C) 2016 Google, Inc.
  17  *
  18  * Based on code by Dmitry Chernenkov.
  19  *
  20  * This program is free software; you can redistribute it and/or
  21  * modify it under the terms of the GNU General Public License
  22  * version 2 as published by the Free Software Foundation.
  23  *
  24  * This program is distributed in the hope that it will be useful, but
  25  * WITHOUT ANY WARRANTY; without even the implied warranty of
  26  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  27  * General Public License for more details.
  28  *
  29  */
  30
  31 #include <linux/gfp.h>
  32 #include <linux/jhash.h>
  33 #include <linux/kernel.h>
  34 #include <linux/mm.h>
  35 #include <linux/percpu.h>
  36 #include <linux/printk.h>
  37 #include <linux/slab.h>
  38 #include <linux/stacktrace.h>
  39 #include <linux/stackdepot.h>
  40 #include <linux/string.h>
  41 #include <linux/types.h>
  42
  43 #define DEPOT_STACK_BITS (sizeof(depot_stack_handle_t) * 8)
  44
  45 #define STACK_ALLOC_ORDER 2 /* 'Slab' size order for stack depot, 4 pages */
  46 #define STACK_ALLOC_SIZE (1LL << (PAGE_SHIFT + STACK_ALLOC_ORDER))
  47 #define STACK_ALLOC_ALIGN 4
  48 #define STACK_ALLOC_OFFSET_BITS (STACK_ALLOC_ORDER + PAGE_SHIFT - \
  49                                         STACK_ALLOC_ALIGN)
  50 #define STACK_ALLOC_INDEX_BITS (DEPOT_STACK_BITS - STACK_ALLOC_OFFSET_BITS)
  51 #define STACK_ALLOC_SLABS_CAP 1024
  52 #define STACK_ALLOC_MAX_SLABS \
  53         (((1LL << (STACK_ALLOC_INDEX_BITS)) < STACK_ALLOC_SLABS_CAP) ? \
  54          (1LL << (STACK_ALLOC_INDEX_BITS)) : STACK_ALLOC_SLABS_CAP)
  55
  56 /* The compact structure to store the reference to stacks. */
  57 union handle_parts {
  58         depot_stack_handle_t handle;
  59         struct {
  60                 u32 slabindex : STACK_ALLOC_INDEX_BITS;
  61                 u32 offset : STACK_ALLOC_OFFSET_BITS;
  62         };
  63 };
  64
  65 struct stack_record {
  66         struct stack_record *next;      /* Link in the hashtable */
  67         u32 hash;                       /* Hash in the hastable */
  68         u32 size;                       /* Number of frames in the stack */
  69         union handle_parts handle;
  70         unsigned long entries[1];       /* Variable-sized array of entries. */
  71 };
  72
  73 static void *stack_slabs[STACK_ALLOC_MAX_SLABS];
  74
  75 static int depot_index;
  76 static int next_slab_inited;
  77 static size_t depot_offset;
  78 static DEFINE_SPINLOCK(depot_lock);
  79
  80 static bool init_stack_slab(void **prealloc)
  81 {
  82         if (!*prealloc)
  83                 return false;
  84         /*
  85          * This smp_load_acquire() pairs with smp_store_release() to
  86          * |next_slab_inited| below and in depot_alloc_stack().
  87          */
  88         if (smp_load_acquire(&next_slab_inited))
  89                 return true;
  90         if (stack_slabs[depot_index] == NULL) {
  91                 stack_slabs[depot_index] = *prealloc;
  92         } else {
  93                 stack_slabs[depot_index + 1] = *prealloc;
  94                 /*
  95                  * This smp_store_release pairs with smp_load_acquire() from
  96                  * |next_slab_inited| above and in depot_save_stack().
  97                  */
  98                 smp_store_release(&next_slab_inited, 1);
  99         }
 100         *prealloc = NULL;
 101         return true;
 102 }
 103
 104 /* Allocation of a new stack in raw storage */
 105 static struct stack_record *depot_alloc_stack(unsigned long *entries, int size,
 106                 u32 hash, void **prealloc, gfp_t alloc_flags)
 107 {
 108         int required_size = offsetof(struct stack_record, entries) +
 109                 sizeof(unsigned long) * size;
 110         struct stack_record *stack;
 111
 112         required_size = ALIGN(required_size, 1 << STACK_ALLOC_ALIGN);
 113
 114         if (unlikely(depot_offset + required_size > STACK_ALLOC_SIZE)) {
 115                 if (unlikely(depot_index + 1 >= STACK_ALLOC_MAX_SLABS)) {
 116                         WARN_ONCE(1, "Stack depot reached limit capacity");
 117                         return NULL;
 118                 }
 119                 depot_index++;
 120                 depot_offset = 0;
 121                 /*
 122                  * smp_store_release() here pairs with smp_load_acquire() from
 123                  * |next_slab_inited| in depot_save_stack() and
 124                  * init_stack_slab().
 125                  */
 126                 if (depot_index + 1 < STACK_ALLOC_MAX_SLABS)
 127                         smp_store_release(&next_slab_inited, 0);
 128         }
 129         init_stack_slab(prealloc);
 130         if (stack_slabs[depot_index] == NULL)
 131                 return NULL;
 132
 133         stack = stack_slabs[depot_index] + depot_offset;
 134
 135         stack->hash = hash;
 136         stack->size = size;
 137         stack->handle.slabindex = depot_index;
 138         stack->handle.offset = depot_offset >> STACK_ALLOC_ALIGN;
 139         memcpy(stack->entries, entries, size * sizeof(unsigned long));
 140         depot_offset += required_size;
 141
 142         return stack;
 143 }
 144
 145 #define STACK_HASH_ORDER 20
 146 #define STACK_HASH_SIZE (1L << STACK_HASH_ORDER)
 147 #define STACK_HASH_MASK (STACK_HASH_SIZE - 1)
 148 #define STACK_HASH_SEED 0x9747b28c
 149
 150 static struct stack_record *stack_table[STACK_HASH_SIZE] = {
 151         [0 ...  STACK_HASH_SIZE - 1] = NULL
 152 };
 153
 154 /* Calculate hash for a stack */
 155 static inline u32 hash_stack(unsigned long *entries, unsigned int size)
 156 {
 157         return jhash2((u32 *)entries,
 158                                size * sizeof(unsigned long) / sizeof(u32),
 159                                STACK_HASH_SEED);
 160 }
 161
 162 /* Find a stack that is equal to the one stored in entries in the hash */
 163 static inline struct stack_record *find_stack(struct stack_record *bucket,
 164                                              unsigned long *entries, int size,
 165                                              u32 hash)
 166 {
 167         struct stack_record *found;
 168
 169         for (found = bucket; found; found = found->next) {
 170                 if (found->hash == hash &&
 171                     found->size == size &&
 172                     !memcmp(entries, found->entries,
 173                             size * sizeof(unsigned long))) {
 174                         return found;
 175                 }
 176         }
 177         return NULL;
 178 }
 179
 180 void depot_fetch_stack(depot_stack_handle_t handle, struct stack_trace *trace)
 181 {
 182         union handle_parts parts = { .handle = handle };
 183         void *slab = stack_slabs[parts.slabindex];
 184         size_t offset = parts.offset << STACK_ALLOC_ALIGN;
 185         struct stack_record *stack = slab + offset;
 186
 187         trace->nr_entries = trace->max_entries = stack->size;
 188         trace->entries = stack->entries;
 189         trace->skip = 0;
 190 }
 191
 192 /**
 193  * depot_save_stack - save stack in a stack depot.
 194  * @trace - the stacktrace to save.
 195  * @alloc_flags - flags for allocating additional memory if required.
 196  *
 197  * Returns the handle of the stack struct stored in depot.
 198  */
 199 depot_stack_handle_t depot_save_stack(struct stack_trace *trace,
 200                                     gfp_t alloc_flags)
 201 {
 202         u32 hash;
 203         depot_stack_handle_t retval = 0;
 204         struct stack_record *found = NULL, **bucket;
 205         unsigned long flags;
 206         struct page *page = NULL;
 207         void *prealloc = NULL;
 208
 209         if (unlikely(trace->nr_entries == 0))
 210                 goto fast_exit;
 211
 212         hash = hash_stack(trace->entries, trace->nr_entries);
 213         /* Bad luck, we won't store this stack. */
 214         if (hash == 0)
 215                 goto exit;
 216
 217         bucket = &stack_table[hash & STACK_HASH_MASK];
 218
 219         /*
 220          * Fast path: look the stack trace up without locking.
 221          * The smp_load_acquire() here pairs with smp_store_release() to
 222          * |bucket| below.
 223          */
 224         found = find_stack(smp_load_acquire(bucket), trace->entries,
 225                            trace->nr_entries, hash);
 226         if (found)
 227                 goto exit;
 228
 229         /*
 230          * Check if the current or the next stack slab need to be initialized.
 231          * If so, allocate the memory - we won't be able to do that under the
 232          * lock.
 233          *
 234          * The smp_load_acquire() here pairs with smp_store_release() to
 235          * |next_slab_inited| in depot_alloc_stack() and init_stack_slab().
 236          */
 237         if (unlikely(!smp_load_acquire(&next_slab_inited))) {
 238                 /*
 239                  * Zero out zone modifiers, as we don't have specific zone
 240                  * requirements. Keep the flags related to allocation in atomic
 241                  * contexts and I/O.
 242                  */
 243                 alloc_flags &= ~GFP_ZONEMASK;
 244                 alloc_flags &= (GFP_ATOMIC | GFP_KERNEL);
 245                 page = alloc_pages(alloc_flags, STACK_ALLOC_ORDER);
 246                 if (page)
 247                         prealloc = page_address(page);
 248         }
 249
 250         spin_lock_irqsave(&depot_lock, flags);
 251
 252         found = find_stack(*bucket, trace->entries, trace->nr_entries, hash);
 253         if (!found) {
 254                 struct stack_record *new =
 255                         depot_alloc_stack(trace->entries, trace->nr_entries,
 256                                           hash, &prealloc, alloc_flags);
 257                 if (new) {
 258                         new->next = *bucket;
 259                         /*
 260                          * This smp_store_release() pairs with
 261                          * smp_load_acquire() from |bucket| above.
 262                          */
 263                         smp_store_release(bucket, new);
 264                         found = new;
 265                 }
 266         } else if (prealloc) {
 267                 /*
 268                  * We didn't need to store this stack trace, but let's keep
 269                  * the preallocated memory for the future.
 270                  */
 271                 WARN_ON(!init_stack_slab(&prealloc));
 272         }
 273
 274         spin_unlock_irqrestore(&depot_lock, flags);
 275 exit:
 276         if (prealloc) {
 277                 /* Nobody used this memory, ok to free it. */
 278                 free_pages((unsigned long)prealloc, STACK_ALLOC_ORDER);
 279         }
 280         if (found)
 281                 retval = found->handle.handle;
 282 fast_exit:
 283         return retval;
 284 }