/*
 * 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;
}