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
init_stack_slab(void ** prealloc)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 } else {
87 stack_slabs[depot_index + 1] = *prealloc;
88 /*
89 * This smp_store_release pairs with smp_load_acquire() from
90 * |next_slab_inited| above and in stack_depot_save().
91 */
92 smp_store_release(&next_slab_inited, 1);
93 }
94 *prealloc = NULL;
95 return true;
96 }
97
98 /* Allocation of a new stack in raw storage */
depot_alloc_stack(unsigned long * entries,int size,u32 hash,void ** prealloc,gfp_t alloc_flags)99 static struct stack_record *depot_alloc_stack(unsigned long *entries, int size,
100 u32 hash, void **prealloc, gfp_t alloc_flags)
101 {
102 int required_size = offsetof(struct stack_record, entries) +
103 sizeof(unsigned long) * size;
104 struct stack_record *stack;
105
106 required_size = ALIGN(required_size, 1 << STACK_ALLOC_ALIGN);
107
108 if (unlikely(depot_offset + required_size > STACK_ALLOC_SIZE)) {
109 if (unlikely(depot_index + 1 >= STACK_ALLOC_MAX_SLABS)) {
110 WARN_ONCE(1, "Stack depot reached limit capacity");
111 return NULL;
112 }
113 depot_index++;
114 depot_offset = 0;
115 /*
116 * smp_store_release() here pairs with smp_load_acquire() from
117 * |next_slab_inited| in stack_depot_save() and
118 * init_stack_slab().
119 */
120 if (depot_index + 1 < STACK_ALLOC_MAX_SLABS)
121 smp_store_release(&next_slab_inited, 0);
122 }
123 init_stack_slab(prealloc);
124 if (stack_slabs[depot_index] == NULL)
125 return NULL;
126
127 stack = stack_slabs[depot_index] + depot_offset;
128
129 stack->hash = hash;
130 stack->size = size;
131 stack->handle.slabindex = depot_index;
132 stack->handle.offset = depot_offset >> STACK_ALLOC_ALIGN;
133 stack->handle.valid = 1;
134 memcpy(stack->entries, entries, size * sizeof(unsigned long));
135 depot_offset += required_size;
136
137 return stack;
138 }
139
140 #define STACK_HASH_ORDER 20
141 #define STACK_HASH_SIZE (1L << STACK_HASH_ORDER)
142 #define STACK_HASH_MASK (STACK_HASH_SIZE - 1)
143 #define STACK_HASH_SEED 0x9747b28c
144
145 static struct stack_record *stack_table[STACK_HASH_SIZE] = {
146 [0 ... STACK_HASH_SIZE - 1] = NULL
147 };
148
149 /* Calculate hash for a stack */
hash_stack(unsigned long * entries,unsigned int size)150 static inline u32 hash_stack(unsigned long *entries, unsigned int size)
151 {
152 return jhash2((u32 *)entries,
153 size * sizeof(unsigned long) / sizeof(u32),
154 STACK_HASH_SEED);
155 }
156
157 /* Use our own, non-instrumented version of memcmp().
158 *
159 * We actually don't care about the order, just the equality.
160 */
161 static inline
stackdepot_memcmp(const unsigned long * u1,const unsigned long * u2,unsigned int n)162 int stackdepot_memcmp(const unsigned long *u1, const unsigned long *u2,
163 unsigned int n)
164 {
165 for ( ; n-- ; u1++, u2++) {
166 if (*u1 != *u2)
167 return 1;
168 }
169 return 0;
170 }
171
172 /* Find a stack that is equal to the one stored in entries in the hash */
find_stack(struct stack_record * bucket,unsigned long * entries,int size,u32 hash)173 static inline struct stack_record *find_stack(struct stack_record *bucket,
174 unsigned long *entries, int size,
175 u32 hash)
176 {
177 struct stack_record *found;
178
179 for (found = bucket; found; found = found->next) {
180 if (found->hash == hash &&
181 found->size == size &&
182 !stackdepot_memcmp(entries, found->entries, size))
183 return found;
184 }
185 return NULL;
186 }
187
188 /**
189 * stack_depot_fetch - Fetch stack entries from a depot
190 *
191 * @handle: Stack depot handle which was returned from
192 * stack_depot_save().
193 * @entries: Pointer to store the entries address
194 *
195 * Return: The number of trace entries for this depot.
196 */
stack_depot_fetch(depot_stack_handle_t handle,unsigned long ** entries)197 unsigned int stack_depot_fetch(depot_stack_handle_t handle,
198 unsigned long **entries)
199 {
200 union handle_parts parts = { .handle = handle };
201 void *slab = stack_slabs[parts.slabindex];
202 size_t offset = parts.offset << STACK_ALLOC_ALIGN;
203 struct stack_record *stack = slab + offset;
204
205 *entries = stack->entries;
206 return stack->size;
207 }
208 EXPORT_SYMBOL_GPL(stack_depot_fetch);
209
210 /**
211 * stack_depot_save - Save a stack trace from an array
212 *
213 * @entries: Pointer to storage array
214 * @nr_entries: Size of the storage array
215 * @alloc_flags: Allocation gfp flags
216 *
217 * Return: The handle of the stack struct stored in depot
218 */
stack_depot_save(unsigned long * entries,unsigned int nr_entries,gfp_t alloc_flags)219 depot_stack_handle_t stack_depot_save(unsigned long *entries,
220 unsigned int nr_entries,
221 gfp_t alloc_flags)
222 {
223 struct stack_record *found = NULL, **bucket;
224 depot_stack_handle_t retval = 0;
225 struct page *page = NULL;
226 void *prealloc = NULL;
227 unsigned long flags;
228 u32 hash;
229
230 if (unlikely(nr_entries == 0))
231 goto fast_exit;
232
233 hash = hash_stack(entries, nr_entries);
234 bucket = &stack_table[hash & STACK_HASH_MASK];
235
236 /*
237 * Fast path: look the stack trace up without locking.
238 * The smp_load_acquire() here pairs with smp_store_release() to
239 * |bucket| below.
240 */
241 found = find_stack(smp_load_acquire(bucket), entries,
242 nr_entries, hash);
243 if (found)
244 goto exit;
245
246 /*
247 * Check if the current or the next stack slab need to be initialized.
248 * If so, allocate the memory - we won't be able to do that under the
249 * lock.
250 *
251 * The smp_load_acquire() here pairs with smp_store_release() to
252 * |next_slab_inited| in depot_alloc_stack() and init_stack_slab().
253 */
254 if (unlikely(!smp_load_acquire(&next_slab_inited))) {
255 /*
256 * Zero out zone modifiers, as we don't have specific zone
257 * requirements. Keep the flags related to allocation in atomic
258 * contexts and I/O.
259 */
260 alloc_flags &= ~GFP_ZONEMASK;
261 alloc_flags &= (GFP_ATOMIC | GFP_KERNEL);
262 alloc_flags |= __GFP_NOWARN;
263 page = alloc_pages(alloc_flags, STACK_ALLOC_ORDER);
264 if (page)
265 prealloc = page_address(page);
266 }
267
268 spin_lock_irqsave(&depot_lock, flags);
269
270 found = find_stack(*bucket, entries, nr_entries, hash);
271 if (!found) {
272 struct stack_record *new =
273 depot_alloc_stack(entries, nr_entries,
274 hash, &prealloc, alloc_flags);
275 if (new) {
276 new->next = *bucket;
277 /*
278 * This smp_store_release() pairs with
279 * smp_load_acquire() from |bucket| above.
280 */
281 smp_store_release(bucket, new);
282 found = new;
283 }
284 } else if (prealloc) {
285 /*
286 * We didn't need to store this stack trace, but let's keep
287 * the preallocated memory for the future.
288 */
289 WARN_ON(!init_stack_slab(&prealloc));
290 }
291
292 spin_unlock_irqrestore(&depot_lock, flags);
293 exit:
294 if (prealloc) {
295 /* Nobody used this memory, ok to free it. */
296 free_pages((unsigned long)prealloc, STACK_ALLOC_ORDER);
297 }
298 if (found)
299 retval = found->handle.handle;
300 fast_exit:
301 return retval;
302 }
303 EXPORT_SYMBOL_GPL(stack_depot_save);
304
