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Linux/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_NULL_PROTECTION_BITS 1
 46 #define STACK_ALLOC_ORDER 2 /* 'Slab' size order for stack depot, 4 pages */
 47 #define STACK_ALLOC_SIZE (1LL << (PAGE_SHIFT + STACK_ALLOC_ORDER))
 48 #define STACK_ALLOC_ALIGN 4
 49 #define STACK_ALLOC_OFFSET_BITS (STACK_ALLOC_ORDER + PAGE_SHIFT - \
 50                                         STACK_ALLOC_ALIGN)
 51 #define STACK_ALLOC_INDEX_BITS (DEPOT_STACK_BITS - \
 52                 STACK_ALLOC_NULL_PROTECTION_BITS - STACK_ALLOC_OFFSET_BITS)
 53 #define STACK_ALLOC_SLABS_CAP 8192
 54 #define STACK_ALLOC_MAX_SLABS \
 55         (((1LL << (STACK_ALLOC_INDEX_BITS)) < STACK_ALLOC_SLABS_CAP) ? \
 56          (1LL << (STACK_ALLOC_INDEX_BITS)) : STACK_ALLOC_SLABS_CAP)
 57 
 58 /* The compact structure to store the reference to stacks. */
 59 union handle_parts {
 60         depot_stack_handle_t handle;
 61         struct {
 62                 u32 slabindex : STACK_ALLOC_INDEX_BITS;
 63                 u32 offset : STACK_ALLOC_OFFSET_BITS;
 64                 u32 valid : STACK_ALLOC_NULL_PROTECTION_BITS;
 65         };
 66 };
 67 
 68 struct stack_record {
 69         struct stack_record *next;      /* Link in the hashtable */
 70         u32 hash;                       /* Hash in the hastable */
 71         u32 size;                       /* Number of frames in the stack */
 72         union handle_parts handle;
 73         unsigned long entries[1];       /* Variable-sized array of entries. */
 74 };
 75 
 76 static void *stack_slabs[STACK_ALLOC_MAX_SLABS];
 77 
 78 static int depot_index;
 79 static int next_slab_inited;
 80 static size_t depot_offset;
 81 static DEFINE_SPINLOCK(depot_lock);
 82 
 83 static bool init_stack_slab(void **prealloc)
 84 {
 85         if (!*prealloc)
 86                 return false;
 87         /*
 88          * This smp_load_acquire() pairs with smp_store_release() to
 89          * |next_slab_inited| below and in depot_alloc_stack().
 90          */
 91         if (smp_load_acquire(&next_slab_inited))
 92                 return true;
 93         if (stack_slabs[depot_index] == NULL) {
 94                 stack_slabs[depot_index] = *prealloc;
 95         } else {
 96                 stack_slabs[depot_index + 1] = *prealloc;
 97                 /*
 98                  * This smp_store_release pairs with smp_load_acquire() from
 99                  * |next_slab_inited| above and in depot_save_stack().
100                  */
101                 smp_store_release(&next_slab_inited, 1);
102         }
103         *prealloc = NULL;
104         return true;
105 }
106 
107 /* Allocation of a new stack in raw storage */
108 static struct stack_record *depot_alloc_stack(unsigned long *entries, int size,
109                 u32 hash, void **prealloc, gfp_t alloc_flags)
110 {
111         int required_size = offsetof(struct stack_record, entries) +
112                 sizeof(unsigned long) * size;
113         struct stack_record *stack;
114 
115         required_size = ALIGN(required_size, 1 << STACK_ALLOC_ALIGN);
116 
117         if (unlikely(depot_offset + required_size > STACK_ALLOC_SIZE)) {
118                 if (unlikely(depot_index + 1 >= STACK_ALLOC_MAX_SLABS)) {
119                         WARN_ONCE(1, "Stack depot reached limit capacity");
120                         return NULL;
121                 }
122                 depot_index++;
123                 depot_offset = 0;
124                 /*
125                  * smp_store_release() here pairs with smp_load_acquire() from
126                  * |next_slab_inited| in depot_save_stack() and
127                  * init_stack_slab().
128                  */
129                 if (depot_index + 1 < STACK_ALLOC_MAX_SLABS)
130                         smp_store_release(&next_slab_inited, 0);
131         }
132         init_stack_slab(prealloc);
133         if (stack_slabs[depot_index] == NULL)
134                 return NULL;
135 
136         stack = stack_slabs[depot_index] + depot_offset;
137 
138         stack->hash = hash;
139         stack->size = size;
140         stack->handle.slabindex = depot_index;
141         stack->handle.offset = depot_offset >> STACK_ALLOC_ALIGN;
142         stack->handle.valid = 1;
143         memcpy(stack->entries, entries, size * sizeof(unsigned long));
144         depot_offset += required_size;
145 
146         return stack;
147 }
148 
149 #define STACK_HASH_ORDER 20
150 #define STACK_HASH_SIZE (1L << STACK_HASH_ORDER)
151 #define STACK_HASH_MASK (STACK_HASH_SIZE - 1)
152 #define STACK_HASH_SEED 0x9747b28c
153 
154 static struct stack_record *stack_table[STACK_HASH_SIZE] = {
155         [0 ...  STACK_HASH_SIZE - 1] = NULL
156 };
157 
158 /* Calculate hash for a stack */
159 static inline u32 hash_stack(unsigned long *entries, unsigned int size)
160 {
161         return jhash2((u32 *)entries,
162                                size * sizeof(unsigned long) / sizeof(u32),
163                                STACK_HASH_SEED);
164 }
165 
166 /* Find a stack that is equal to the one stored in entries in the hash */
167 static inline struct stack_record *find_stack(struct stack_record *bucket,
168                                              unsigned long *entries, int size,
169                                              u32 hash)
170 {
171         struct stack_record *found;
172 
173         for (found = bucket; found; found = found->next) {
174                 if (found->hash == hash &&
175                     found->size == size &&
176                     !memcmp(entries, found->entries,
177                             size * sizeof(unsigned long))) {
178                         return found;
179                 }
180         }
181         return NULL;
182 }
183 
184 void depot_fetch_stack(depot_stack_handle_t handle, struct stack_trace *trace)
185 {
186         union handle_parts parts = { .handle = handle };
187         void *slab = stack_slabs[parts.slabindex];
188         size_t offset = parts.offset << STACK_ALLOC_ALIGN;
189         struct stack_record *stack = slab + offset;
190 
191         trace->nr_entries = trace->max_entries = stack->size;
192         trace->entries = stack->entries;
193         trace->skip = 0;
194 }
195 EXPORT_SYMBOL_GPL(depot_fetch_stack);
196 
197 /**
198  * depot_save_stack - save stack in a stack depot.
199  * @trace - the stacktrace to save.
200  * @alloc_flags - flags for allocating additional memory if required.
201  *
202  * Returns the handle of the stack struct stored in depot.
203  */
204 depot_stack_handle_t depot_save_stack(struct stack_trace *trace,
205                                     gfp_t alloc_flags)
206 {
207         u32 hash;
208         depot_stack_handle_t retval = 0;
209         struct stack_record *found = NULL, **bucket;
210         unsigned long flags;
211         struct page *page = NULL;
212         void *prealloc = NULL;
213 
214         if (unlikely(trace->nr_entries == 0))
215                 goto fast_exit;
216 
217         hash = hash_stack(trace->entries, trace->nr_entries);
218         bucket = &stack_table[hash & STACK_HASH_MASK];
219 
220         /*
221          * Fast path: look the stack trace up without locking.
222          * The smp_load_acquire() here pairs with smp_store_release() to
223          * |bucket| below.
224          */
225         found = find_stack(smp_load_acquire(bucket), trace->entries,
226                            trace->nr_entries, hash);
227         if (found)
228                 goto exit;
229 
230         /*
231          * Check if the current or the next stack slab need to be initialized.
232          * If so, allocate the memory - we won't be able to do that under the
233          * lock.
234          *
235          * The smp_load_acquire() here pairs with smp_store_release() to
236          * |next_slab_inited| in depot_alloc_stack() and init_stack_slab().
237          */
238         if (unlikely(!smp_load_acquire(&next_slab_inited))) {
239                 /*
240                  * Zero out zone modifiers, as we don't have specific zone
241                  * requirements. Keep the flags related to allocation in atomic
242                  * contexts and I/O.
243                  */
244                 alloc_flags &= ~GFP_ZONEMASK;
245                 alloc_flags &= (GFP_ATOMIC | GFP_KERNEL);
246                 alloc_flags |= __GFP_NOWARN;
247                 page = alloc_pages(alloc_flags, STACK_ALLOC_ORDER);
248                 if (page)
249                         prealloc = page_address(page);
250         }
251 
252         spin_lock_irqsave(&depot_lock, flags);
253 
254         found = find_stack(*bucket, trace->entries, trace->nr_entries, hash);
255         if (!found) {
256                 struct stack_record *new =
257                         depot_alloc_stack(trace->entries, trace->nr_entries,
258                                           hash, &prealloc, alloc_flags);
259                 if (new) {
260                         new->next = *bucket;
261                         /*
262                          * This smp_store_release() pairs with
263                          * smp_load_acquire() from |bucket| above.
264                          */
265                         smp_store_release(bucket, new);
266                         found = new;
267                 }
268         } else if (prealloc) {
269                 /*
270                  * We didn't need to store this stack trace, but let's keep
271                  * the preallocated memory for the future.
272                  */
273                 WARN_ON(!init_stack_slab(&prealloc));
274         }
275 
276         spin_unlock_irqrestore(&depot_lock, flags);
277 exit:
278         if (prealloc) {
279                 /* Nobody used this memory, ok to free it. */
280                 free_pages((unsigned long)prealloc, STACK_ALLOC_ORDER);
281         }
282         if (found)
283                 retval = found->handle.handle;
284 fast_exit:
285         return retval;
286 }
287 EXPORT_SYMBOL_GPL(depot_save_stack);
288 

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