misc/sbrk.c

   1 /* Copyright (C) 1991-2024 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 /* Mark symbols hidden in static PIE for early self relocation to work.  */
  19 #if BUILD_PIE_DEFAULT
  20 # pragma GCC visibility push(hidden)
  21 #endif
  22 #include <errno.h>
  23 #include <libc-internal.h>
  24 #include <stdbool.h>
  25 #include <stdint.h>
  26 #include <unistd.h>
  27
  28 /* Defined in brk.c.  */
  29 extern void *__curbrk;
  30 extern int __brk (void *addr);
  31
  32 /* Extend the process's data space by INCREMENT.
  33    If INCREMENT is negative, shrink data space by - INCREMENT.
  34    Return start of new space allocated, or -1 for errors.  */
  35 void *
  36 __sbrk (intptr_t increment)
  37 {
  38   /* Controls whether __brk (0) is called to read the brk value from
  39      the kernel.  */
  40   bool update_brk = __curbrk == NULL;
  41
  42 #if defined (SHARED) && ! IS_IN (rtld)
  43   if (!__libc_initial)
  44     {
  45       if (increment != 0)
  46         {
  47           /* Do not allow changing the brk from an inner libc because
  48              it cannot be synchronized with the outer libc's brk.  */
  49           __set_errno (ENOMEM);
  50           return (void *) -1;
  51         }
  52       /* Querying the kernel's brk value from an inner namespace is
  53          fine.  */
  54       update_brk = true;
  55     }
  56 #endif
  57
  58   if (update_brk)
  59     if (__brk (0) < 0)          /* Initialize the break.  */
  60       return (void *) -1;
  61
  62   if (increment == 0)
  63     return __curbrk;
  64
  65   void *oldbrk = __curbrk;
  66   if (increment > 0
  67       ? ((uintptr_t) oldbrk + (uintptr_t) increment < (uintptr_t) oldbrk)
  68       : ((uintptr_t) oldbrk < (uintptr_t) -increment))
  69     {
  70       __set_errno (ENOMEM);
  71       return (void *) -1;
  72     }
  73
  74   if (__brk (oldbrk + increment) < 0)
  75     return (void *) -1;
  76
  77   return oldbrk;
  78 }
  79 libc_hidden_def (__sbrk)
  80 weak_alias (__sbrk, sbrk)