text: /* crypto/bn/bn_ctx.c */
  /* Written by Ulf Moeller for the OpenSSL project. */
  /* ====================================================================
   * Copyright (c) 1998-2019 The OpenSSL Project.  All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions
   * are met:
   *
   * 1. Redistributions of source code must retain the above copyright
   *    notice, this list of conditions and the following disclaimer.
   *
   * 2. Redistributions in binary form must reproduce the above copyright
   *    notice, this list of conditions and the following disclaimer in
   *    the documentation and/or other materials provided with the
   *    distribution.
   *
   * 3. All advertising materials mentioning features or use of this
   *    software must display the following acknowledgment:
   *    "This product includes software developed by the OpenSSL Project
   *    for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
   *
   * 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
   *    endorse or promote products derived from this software without
   *    prior written permission. For written permission, please contact
   *    openssl-core@openssl.org.
   *
   * 5. Products derived from this software may not be called "OpenSSL"
   *    nor may "OpenSSL" appear in their names without prior written
   *    permission of the OpenSSL Project.
   *
   * 6. Redistributions of any form whatsoever must retain the following
   *    acknowledgment:
   *    "This product includes software developed by the OpenSSL Project
   *    for use in the OpenSSL Toolkit (http://www.openssl.org/)"
   *
   * THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
   * EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
   * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE OpenSSL PROJECT OR
   * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
   * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
   * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
   * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
   * OF THE POSSIBILITY OF SUCH DAMAGE.
   * ====================================================================
   *
   * This product includes cryptographic software written by Eric Young
   * (eay@cryptsoft.com).  This product includes software written by Tim
   * Hudson (tjh@cryptsoft.com).
   *
   */
  
  #if !defined(BN_CTX_DEBUG) && !defined(BN_DEBUG)
  # ifndef NDEBUG
  #  define NDEBUG
  # endif
  #endif
  
  #include <stdio.h>
  #include <assert.h>
  
  #include "cryptlib.h"
  #include "bn_lcl.h"
  
  /*-
   * TODO list
   *
   * 1. Check a bunch of "(words+1)" type hacks in various bignum functions and
   * check they can be safely removed.
   *  - Check +1 and other ugliness in BN_from_montgomery()
   *
   * 2. Consider allowing a BN_new_ex() that, at least, lets you specify an
   * appropriate 'block' size that will be honoured by bn_expand_internal() to
   * prevent piddly little reallocations. OTOH, profiling bignum expansions in
   * BN_CTX doesn't show this to be a big issue.
   */
  
  /* How many bignums are in each "pool item"; */
  #define BN_CTX_POOL_SIZE        16
  /* The stack frame info is resizing, set a first-time expansion size; */
  #define BN_CTX_START_FRAMES     32
  
  /***********/
  /* BN_POOL */
  /***********/
  
  /* A bundle of bignums that can be linked with other bundles */
  typedef struct bignum_pool_item {
      /* The bignum values */
      BIGNUM vals[BN_CTX_POOL_SIZE];
      /* Linked-list admin */
      struct bignum_pool_item *prev, *next;
  } BN_POOL_ITEM;
  /* A linked-list of bignums grouped in bundles */
  typedef struct bignum_pool {
      /* Linked-list admin */
      BN_POOL_ITEM *head, *current, *tail;
      /* Stack depth and allocation size */
      unsigned used, size;
  } BN_POOL;
  static void BN_POOL_init(BN_POOL *);
  static void BN_POOL_finish(BN_POOL *);
  #ifndef OPENSSL_NO_DEPRECATED
  static void BN_POOL_reset(BN_POOL *);
  #endif
  static BIGNUM *BN_POOL_get(BN_POOL *);
  static void BN_POOL_release(BN_POOL *, unsigned int);
  
  /************/
  /* BN_STACK */
  /************/
  
  /* A wrapper to manage the "stack frames" */
  typedef struct bignum_ctx_stack {
      /* Array of indexes into the bignum stack */
      unsigned int *indexes;
      /* Number of stack frames, and the size of the allocated array */
      unsigned int depth, size;
  } BN_STACK;
  static void BN_STACK_init(BN_STACK *);
  static void BN_STACK_finish(BN_STACK *);
  #ifndef OPENSSL_NO_DEPRECATED
  static void BN_STACK_reset(BN_STACK *);
  #endif
  static int BN_STACK_push(BN_STACK *, unsigned int);
  static unsigned int BN_STACK_pop(BN_STACK *);
  
  /**********/
  /* BN_CTX */
  /**********/
  
  /* The opaque BN_CTX type */
  struct bignum_ctx {
      /* The bignum bundles */
      BN_POOL pool;
      /* The "stack frames", if you will */
      BN_STACK stack;
      /* The number of bignums currently assigned */
      unsigned int used;
      /* Depth of stack overflow */
      int err_stack;
      /* Block "gets" until an "end" (compatibility behaviour) */
      int too_many;
  };
  
  /* Enable this to find BN_CTX bugs */
  #ifdef BN_CTX_DEBUG
  static const char *ctxdbg_cur = NULL;
  static void ctxdbg(BN_CTX *ctx)
  {
      unsigned int bnidx = 0, fpidx = 0;
      BN_POOL_ITEM *item = ctx->pool.head;
      BN_STACK *stack = &ctx->stack;
      fprintf(stderr, "(%16p): ", ctx);
      while (bnidx < ctx->used) {
          fprintf(stderr, "%03x ", item->vals[bnidx++ % BN_CTX_POOL_SIZE].dmax);
          if (!(bnidx % BN_CTX_POOL_SIZE))
              item = item->next;
      }
      fprintf(stderr, "\n");
      bnidx = 0;
      fprintf(stderr, "          : ");
      while (fpidx < stack->depth) {
          while (bnidx++ < stack->indexes[fpidx])
              fprintf(stderr, "    ");
          fprintf(stderr, "^^^ ");
          bnidx++;
          fpidx++;
      }
      fprintf(stderr, "\n");
  }
  
  # define CTXDBG_ENTRY(str, ctx)  do { \
                                  ctxdbg_cur = (str); \
                                  fprintf(stderr,"Starting %s\n", ctxdbg_cur); \
                                  ctxdbg(ctx); \
                                  } while(0)
  # define CTXDBG_EXIT(ctx)        do { \
                                  fprintf(stderr,"Ending %s\n", ctxdbg_cur); \
                                  ctxdbg(ctx); \
                                  } while(0)
  # define CTXDBG_RET(ctx,ret)
  #else
  # define CTXDBG_ENTRY(str, ctx)
  # define CTXDBG_EXIT(ctx)
  # define CTXDBG_RET(ctx,ret)
  #endif
  
  /*
   * This function is an evil legacy and should not be used. This
   * implementation is WYSIWYG, though I've done my best.
   */
  #ifndef OPENSSL_NO_DEPRECATED
  void BN_CTX_init(BN_CTX *ctx)
  {
      /*
       * Assume the caller obtained the context via BN_CTX_new() and so is
       * trying to reset it for use. Nothing else makes sense, least of all
       * binary compatibility from a time when they could declare a static
       * variable.
       */
      BN_POOL_reset(&ctx->pool);
      BN_STACK_reset(&ctx->stack);
      ctx->used = 0;
      ctx->err_stack = 0;
      ctx->too_many = 0;
  }
  #endif
  
  BN_CTX *BN_CTX_new(void)
  {
      BN_CTX *ret = OPENSSL_malloc(sizeof(BN_CTX));
      if (!ret) {
          BNerr(BN_F_BN_CTX_NEW, ERR_R_MALLOC_FAILURE);
          return NULL;
      }
      /* Initialise the structure */
      BN_POOL_init(&ret->pool);
      BN_STACK_init(&ret->stack);
      ret->used = 0;
      ret->err_stack = 0;
      ret->too_many = 0;
      return ret;
  }
  
  void BN_CTX_free(BN_CTX *ctx)
  {
      if (ctx == NULL)
          return;
  #ifdef BN_CTX_DEBUG
      {
          BN_POOL_ITEM *pool = ctx->pool.head;
          fprintf(stderr, "BN_CTX_free, stack-size=%d, pool-bignums=%d\n",
                  ctx->stack.size, ctx->pool.size);
          fprintf(stderr, "dmaxs: ");
          while (pool) {
              unsigned loop = 0;
              while (loop < BN_CTX_POOL_SIZE)
                  fprintf(stderr, "%02x ", pool->vals[loop++].dmax);
              pool = pool->next;
          }
          fprintf(stderr, "\n");
      }
  #endif
      BN_STACK_finish(&ctx->stack);
      BN_POOL_finish(&ctx->pool);
      OPENSSL_free(ctx);
  }
  
  void BN_CTX_start(BN_CTX *ctx)
  {
      CTXDBG_ENTRY("BN_CTX_start", ctx);
      /* If we're already overflowing ... */
      if (ctx->err_stack || ctx->too_many)
          ctx->err_stack++;
      /* (Try to) get a new frame pointer */
      else if (!BN_STACK_push(&ctx->stack, ctx->used)) {
          BNerr(BN_F_BN_CTX_START, BN_R_TOO_MANY_TEMPORARY_VARIABLES);
          ctx->err_stack++;
      }
      CTXDBG_EXIT(ctx);
  }
  
  void BN_CTX_end(BN_CTX *ctx)
  {
      CTXDBG_ENTRY("BN_CTX_end", ctx);
      if (ctx->err_stack)
          ctx->err_stack--;
      else {
          unsigned int fp = BN_STACK_pop(&ctx->stack);
          /* Does this stack frame have anything to release? */
          if (fp < ctx->used)
              BN_POOL_release(&ctx->pool, ctx->used - fp);
          ctx->used = fp;
          /* Unjam "too_many" in case "get" had failed */
          ctx->too_many = 0;
      }
      CTXDBG_EXIT(ctx);
  }
  
  BIGNUM *BN_CTX_get(BN_CTX *ctx)
  {
      BIGNUM *ret;
      CTXDBG_ENTRY("BN_CTX_get", ctx);
      if (ctx->err_stack || ctx->too_many)
          return NULL;
      if ((ret = BN_POOL_get(&ctx->pool)) == NULL) {
          /*
           * Setting too_many prevents repeated "get" attempts from cluttering
           * the error stack.
           */
          ctx->too_many = 1;
          BNerr(BN_F_BN_CTX_GET, BN_R_TOO_MANY_TEMPORARY_VARIABLES);
          return NULL;
      }
      /* OK, make sure the returned bignum is "zero" */
      BN_zero(ret);
      /* clear BN_FLG_CONSTTIME if leaked from previous frames */
      ret->flags &= (~BN_FLG_CONSTTIME);
      ctx->used++;
      CTXDBG_RET(ctx, ret);
      return ret;
  }
  
  /************/
  /* BN_STACK */
  /************/
  
  static void BN_STACK_init(BN_STACK *st)
  {
      st->indexes = NULL;
      st->depth = st->size = 0;
  }
  
  static void BN_STACK_finish(BN_STACK *st)
  {
      if (st->size)
          OPENSSL_free(st->indexes);
  }
  
  #ifndef OPENSSL_NO_DEPRECATED
  static void BN_STACK_reset(BN_STACK *st)
  {
      st->depth = 0;
  }
  #endif
  
  static int BN_STACK_push(BN_STACK *st, unsigned int idx)
  {
      if (st->depth == st->size)
          /* Need to expand */
      {
          unsigned int newsize = (st->size ?
                                  (st->size * 3 / 2) : BN_CTX_START_FRAMES);
          unsigned int *newitems = OPENSSL_malloc(newsize *
                                                  sizeof(unsigned int));
          if (!newitems)
              return 0;
          if (st->depth)
              memcpy(newitems, st->indexes, st->depth * sizeof(unsigned int));
          if (st->size)
              OPENSSL_free(st->indexes);
          st->indexes = newitems;
          st->size = newsize;
      }
      st->indexes[(st->depth)++] = idx;
      return 1;
  }
  
  static unsigned int BN_STACK_pop(BN_STACK *st)
  {
      return st->indexes[--(st->depth)];
  }
  
  /***********/
  /* BN_POOL */
  /***********/
  
  static void BN_POOL_init(BN_POOL *p)
  {
      p->head = p->current = p->tail = NULL;
      p->used = p->size = 0;
  }
  
  static void BN_POOL_finish(BN_POOL *p)
  {
      while (p->head) {
          unsigned int loop = 0;
          BIGNUM *bn = p->head->vals;
          while (loop++ < BN_CTX_POOL_SIZE) {
              if (bn->d)
                  BN_clear_free(bn);
              bn++;
          }
          p->current = p->head->next;
          OPENSSL_free(p->head);
          p->head = p->current;
      }
  }
  
  #ifndef OPENSSL_NO_DEPRECATED
  static void BN_POOL_reset(BN_POOL *p)
  {
      BN_POOL_ITEM *item = p->head;
      while (item) {
          unsigned int loop = 0;
          BIGNUM *bn = item->vals;
          while (loop++ < BN_CTX_POOL_SIZE) {
              if (bn->d)
                  BN_clear(bn);
              bn++;
          }
          item = item->next;
      }
      p->current = p->head;
      p->used = 0;
  }
  #endif
  
  static BIGNUM *BN_POOL_get(BN_POOL *p)
  {
      if (p->used == p->size) {
          BIGNUM *bn;
          unsigned int loop = 0;
          BN_POOL_ITEM *item = OPENSSL_malloc(sizeof(BN_POOL_ITEM));
          if (!item)
              return NULL;
          /* Initialise the structure */
          bn = item->vals;
          while (loop++ < BN_CTX_POOL_SIZE)
              BN_init(bn++);
          item->prev = p->tail;
          item->next = NULL;
          /* Link it in */
          if (!p->head)
              p->head = p->current = p->tail = item;
          else {
              p->tail->next = item;
              p->tail = item;
              p->current = item;
          }
          p->size += BN_CTX_POOL_SIZE;
          p->used++;
          /* Return the first bignum from the new pool */
          return item->vals;
      }
      if (!p->used)
          p->current = p->head;
      else if ((p->used % BN_CTX_POOL_SIZE) == 0)
          p->current = p->current->next;
      return p->current->vals + ((p->used++) % BN_CTX_POOL_SIZE);
  }
  
  static void BN_POOL_release(BN_POOL *p, unsigned int num)
  {
      unsigned int offset = (p->used - 1) % BN_CTX_POOL_SIZE;
      p->used -= num;
      while (num--) {
          bn_check_top(p->current->vals + offset);
          if (!offset) {
              offset = BN_CTX_POOL_SIZE - 1;
              p->current = p->current->prev;
          } else
              offset--;
      }
  }
  