lib/malloca.c

   1 /* Safe automatic memory allocation.
   2    Copyright (C) 2003, 2006-2007, 2009-2024 Free Software Foundation, Inc.
   3    Written by Bruno Haible <bruno@clisp.org>, 2003, 2018.
   4
   5    This file is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU Lesser General Public License as
   7    published by the Free Software Foundation; either version 2.1 of the
   8    License, or (at your option) any later version.
   9
  10    This file is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 #define _GL_USE_STDLIB_ALLOC 1
  19 #include <config.h>
  20
  21 /* Specification.  */
  22 #include "malloca.h"
  23
  24 #include <stdckdint.h>
  25 #if defined __CHERI_PURE_CAPABILITY__
  26 # include <cheri.h>
  27 #endif
  28
  29 #include "idx.h"
  30
  31 /* The speed critical point in this file is freea() applied to an alloca()
  32    result: it must be fast, to match the speed of alloca().  The speed of
  33    mmalloca() and freea() in the other case are not critical, because they
  34    are only invoked for big memory sizes.
  35    Here we use a bit in the address as an indicator, an idea by Ondřej Bílka.
  36    malloca() can return three types of pointers:
  37      - Pointers ≡ 0 mod 2*sa_alignment_max come from stack allocation.
  38      - Pointers ≡ sa_alignment_max mod 2*sa_alignment_max come from heap
  39        allocation.
  40      - NULL comes from a failed heap allocation.  */
  41
  42 #if defined __CHERI_PURE_CAPABILITY__
  43 /* Type for holding the original malloc() result.  */
  44 typedef uintptr_t small_t;
  45 #else
  46 /* Type for holding very small pointer differences.  */
  47 typedef unsigned char small_t;
  48 /* Verify that it is wide enough.  */
  49 static_assert (2 * sa_alignment_max - 1 <= (small_t) -1);
  50 #endif
  51
  52 void *
  53 mmalloca (size_t n)
  54 {
  55 #if HAVE_ALLOCA
  56   /* Allocate one more word, used to determine the address to pass to freea(),
  57      and room for the alignment ≡ sa_alignment_max mod 2*sa_alignment_max.  */
  58   uintptr_t alignment2_mask = 2 * sa_alignment_max - 1;
  59   int plus = sizeof (small_t) + alignment2_mask;
  60   idx_t nplus;
  61   if (!ckd_add (&nplus, n, plus) && !xalloc_oversized (nplus, 1))
  62     {
  63       char *mem = (char *) malloc (nplus);
  64
  65       if (mem != NULL)
  66         {
  67           uintptr_t umem = (uintptr_t) mem;
  68           /* The ckd_add avoids signed integer overflow on
  69              theoretical platforms where UINTPTR_MAX <= INT_MAX.  */
  70           uintptr_t umemplus;
  71           ckd_add (&umemplus, umem, sizeof (small_t) + sa_alignment_max - 1);
  72           idx_t offset = (umemplus - umemplus % (2 * sa_alignment_max)
  73                           + sa_alignment_max - umem);
  74           void *p = mem + offset;
  75           /* Here p >= mem + sizeof (small_t),
  76              and p <= mem + sizeof (small_t) + 2 * sa_alignment_max - 1
  77              hence p + n <= mem + nplus.
  78              So, the memory range [p, p+n) lies in the allocated memory range
  79              [mem, mem + nplus).  */
  80           small_t *sp = p;
  81 # if defined __CHERI_PURE_CAPABILITY__
  82           sp[-1] = umem;
  83           p = (char *) cheri_bounds_set ((char *) p - sizeof (small_t),
  84                                          sizeof (small_t) + n)
  85               + sizeof (small_t);
  86 # else
  87           sp[-1] = offset;
  88 # endif
  89           /* p ≡ sa_alignment_max mod 2*sa_alignment_max.  */
  90           return p;
  91         }
  92     }
  93   /* Out of memory.  */
  94   return NULL;
  95 #else
  96 # if !MALLOC_0_IS_NONNULL
  97   if (n == 0)
  98     n = 1;
  99 # endif
 100   return malloc (n);
 101 #endif
 102 }
 103
 104 #if HAVE_ALLOCA
 105 void
 106 freea (void *p)
 107 {
 108   /* Check argument.  */
 109   uintptr_t u = (uintptr_t) p;
 110   if (u & (sa_alignment_max - 1))
 111     {
 112       /* p was not the result of a malloca() call.  Invalid argument.  */
 113       abort ();
 114     }
 115   /* Determine whether p was a non-NULL pointer returned by mmalloca().  */
 116   if (u & sa_alignment_max)
 117     {
 118       char *cp = p;
 119       small_t *sp = p;
 120 # if defined __CHERI_PURE_CAPABILITY__
 121       void *mem = sp[-1];
 122 # else
 123       void *mem = cp - sp[-1];
 124 # endif
 125       free (mem);
 126     }
 127 }
 128 #endif
 129
 130 /*
 131  * Hey Emacs!
 132  * Local Variables:
 133  * coding: utf-8
 134  * End:
 135  */