/* * Generic stack depot for storing stack traces. * * Some debugging tools need to save stack traces of certain events which can * be later presented to the user. For example, KASAN needs to safe alloc and * free stacks for each object, but storing two stack traces per object * requires too much memory (e.g. SLUB_DEBUG needs 256 bytes per object for * that). * * Instead, stack depot maintains a hashtable of unique stacktraces. Since alloc * and free stacks repeat a lot, we save about 100x space. * Stacks are never removed from depot, so we store them contiguously one after * another in a contiguos memory allocation. * * Author: Alexander Potapenko <glider@google.com> * Copyright (C) 2016 Google, Inc. * * Based on code by Dmitry Chernenkov. * * This program is free software; you can redistribute it and/or * modify it under the terms of the GNU General Public License * version 2 as published by the Free Software Foundation. * * This program is distributed in the hope that it will be useful, but * WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU * General Public License for more details. * */ #include <linux/gfp.h> #include <linux/jhash.h> #include <linux/kernel.h> #include <linux/mm.h> #include <linux/percpu.h> #include <linux/printk.h> #include <linux/slab.h> #include <linux/stacktrace.h> #include <linux/stackdepot.h> #include <linux/string.h> #include <linux/types.h> #define DEPOT_STACK_BITS (sizeof(depot_stack_handle_t) * 8) #define STACK_ALLOC_NULL_PROTECTION_BITS 1 #define STACK_ALLOC_ORDER 2 /* 'Slab' size order for stack depot, 4 pages */ #define STACK_ALLOC_SIZE (1LL << (PAGE_SHIFT + STACK_ALLOC_ORDER)) #define STACK_ALLOC_ALIGN 4 #define STACK_ALLOC_OFFSET_BITS (STACK_ALLOC_ORDER + PAGE_SHIFT - \ STACK_ALLOC_ALIGN) #define STACK_ALLOC_INDEX_BITS (DEPOT_STACK_BITS - \ STACK_ALLOC_NULL_PROTECTION_BITS - STACK_ALLOC_OFFSET_BITS) #define STACK_ALLOC_SLABS_CAP 1024 #define STACK_ALLOC_MAX_SLABS \ (((1LL << (STACK_ALLOC_INDEX_BITS)) < STACK_ALLOC_SLABS_CAP) ? \ (1LL << (STACK_ALLOC_INDEX_BITS)) : STACK_ALLOC_SLABS_CAP) /* The compact structure to store the reference to stacks. */ union handle_parts { depot_stack_handle_t handle; struct { u32 slabindex : STACK_ALLOC_INDEX_BITS; u32 offset : STACK_ALLOC_OFFSET_BITS; u32 valid : STACK_ALLOC_NULL_PROTECTION_BITS; }; }; struct stack_record { struct stack_record *next; /* Link in the hashtable */ u32 hash; /* Hash in the hastable */ u32 size; /* Number of frames in the stack */ union handle_parts handle; unsigned long entries[1]; /* Variable-sized array of entries. */ }; static void *stack_slabs[STACK_ALLOC_MAX_SLABS]; static int depot_index; static int next_slab_inited; static size_t depot_offset; static DEFINE_SPINLOCK(depot_lock); static bool init_stack_slab(void **prealloc) { if (!*prealloc) return false; /* * This smp_load_acquire() pairs with smp_store_release() to * |next_slab_inited| below and in depot_alloc_stack(). */ if (smp_load_acquire(&next_slab_inited)) return true; if (stack_slabs[depot_index] == NULL) { stack_slabs[depot_index] = *prealloc; } else { stack_slabs[depot_index + 1] = *prealloc; /* * This smp_store_release pairs with smp_load_acquire() from * |next_slab_inited| above and in depot_save_stack(). */ smp_store_release(&next_slab_inited, 1); } *prealloc = NULL; return true; } /* Allocation of a new stack in raw storage */ static struct stack_record *depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc, gfp_t alloc_flags) { int required_size = offsetof(struct stack_record, entries) + sizeof(unsigned long) * size; struct stack_record *stack; required_size = ALIGN(required_size, 1 << STACK_ALLOC_ALIGN); if (unlikely(depot_offset + required_size > STACK_ALLOC_SIZE)) { if (unlikely(depot_index + 1 >= STACK_ALLOC_MAX_SLABS)) { WARN_ONCE(1, "Stack depot reached limit capacity"); return NULL; } depot_index++; depot_offset = 0; /* * smp_store_release() here pairs with smp_load_acquire() from * |next_slab_inited| in depot_save_stack() and * init_stack_slab(). */ if (depot_index + 1 < STACK_ALLOC_MAX_SLABS) smp_store_release(&next_slab_inited, 0); } init_stack_slab(prealloc); if (stack_slabs[depot_index] == NULL) return NULL; stack = stack_slabs[depot_index] + depot_offset; stack->hash = hash; stack->size = size; stack->handle.slabindex = depot_index; stack->handle.offset = depot_offset >> STACK_ALLOC_ALIGN; stack->handle.valid = 1; memcpy(stack->entries, entries, size * sizeof(unsigned long)); depot_offset += required_size; return stack; } #define STACK_HASH_ORDER 20 #define STACK_HASH_SIZE (1L << STACK_HASH_ORDER) #define STACK_HASH_MASK (STACK_HASH_SIZE - 1) #define STACK_HASH_SEED 0x9747b28c static struct stack_record *stack_table[STACK_HASH_SIZE] = { [0 ... STACK_HASH_SIZE - 1] = NULL }; /* Calculate hash for a stack */ static inline u32 hash_stack(unsigned long *entries, unsigned int size) { return jhash2((u32 *)entries, size * sizeof(unsigned long) / sizeof(u32), STACK_HASH_SEED); } /* Find a stack that is equal to the one stored in entries in the hash */ static inline struct stack_record *find_stack(struct stack_record *bucket, unsigned long *entries, int size, u32 hash) { struct stack_record *found; for (found = bucket; found; found = found->next) { if (found->hash == hash && found->size == size && !memcmp(entries, found->entries, size * sizeof(unsigned long))) { return found; } } return NULL; } void depot_fetch_stack(depot_stack_handle_t handle, struct stack_trace *trace) { union handle_parts parts = { .handle = handle }; void *slab = stack_slabs[parts.slabindex]; size_t offset = parts.offset << STACK_ALLOC_ALIGN; struct stack_record *stack = slab + offset; trace->nr_entries = trace->max_entries = stack->size; trace->entries = stack->entries; trace->skip = 0; } /** * depot_save_stack - save stack in a stack depot. * @trace - the stacktrace to save. * @alloc_flags - flags for allocating additional memory if required. * * Returns the handle of the stack struct stored in depot. */ depot_stack_handle_t depot_save_stack(struct stack_trace *trace, gfp_t alloc_flags) { u32 hash; depot_stack_handle_t retval = 0; struct stack_record *found = NULL, **bucket; unsigned long flags; struct page *page = NULL; void *prealloc = NULL; if (unlikely(trace->nr_entries == 0)) goto fast_exit; hash = hash_stack(trace->entries, trace->nr_entries); bucket = &stack_table[hash & STACK_HASH_MASK]; /* * Fast path: look the stack trace up without locking. * The smp_load_acquire() here pairs with smp_store_release() to * |bucket| below. */ found = find_stack(smp_load_acquire(bucket), trace->entries, trace->nr_entries, hash); if (found) goto exit; /* * Check if the current or the next stack slab need to be initialized. * If so, allocate the memory - we won't be able to do that under the * lock. * * The smp_load_acquire() here pairs with smp_store_release() to * |next_slab_inited| in depot_alloc_stack() and init_stack_slab(). */ if (unlikely(!smp_load_acquire(&next_slab_inited))) { /* * Zero out zone modifiers, as we don't have specific zone * requirements. Keep the flags related to allocation in atomic * contexts and I/O. */ alloc_flags &= ~GFP_ZONEMASK; alloc_flags &= (GFP_ATOMIC | GFP_KERNEL); page = alloc_pages(alloc_flags, STACK_ALLOC_ORDER); if (page) prealloc = page_address(page); } spin_lock_irqsave(&depot_lock, flags); found = find_stack(*bucket, trace->entries, trace->nr_entries, hash); if (!found) { struct stack_record *new = depot_alloc_stack(trace->entries, trace->nr_entries, hash, &prealloc, alloc_flags); if (new) { new->next = *bucket; /* * This smp_store_release() pairs with * smp_load_acquire() from |bucket| above. */ smp_store_release(bucket, new); found = new; } } else if (prealloc) { /* * We didn't need to store this stack trace, but let's keep * the preallocated memory for the future. */ WARN_ON(!init_stack_slab(&prealloc)); } spin_unlock_irqrestore(&depot_lock, flags); exit: if (prealloc) { /* Nobody used this memory, ok to free it. */ free_pages((unsigned long)prealloc, STACK_ALLOC_ORDER); } if (found) retval = found->handle.handle; fast_exit: return retval; }