lib/memset_explicit.c

   1 /* Erase sensitive data from memory.
   2    Copyright 2022-2024 Free Software Foundation, Inc.
   3
   4    This file is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU Lesser General Public License as
   6    published by the Free Software Foundation; either version 2.1 of the
   7    License, or (at your option) any later version.
   8
   9    This file 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
  12    GNU Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 #include <config.h>
  18
  19 /* memset_s need this define */
  20 #if HAVE_MEMSET_S
  21 # define __STDC_WANT_LIB_EXT1__ 1
  22 #endif
  23
  24 #include <string.h>
  25
  26 /* Set S's bytes to C, where S has LEN bytes.  The compiler will not
  27    optimize effects away, even if S is dead after the call.  */
  28 void *
  29 memset_explicit (void *s, int c, size_t len)
  30 {
  31 #if HAVE_EXPLICIT_MEMSET
  32   return explicit_memset (s, c, len);
  33 #elif HAVE_MEMSET_S
  34   (void) memset_s (s, len, c, len);
  35   return s;
  36 #elif defined __GNUC__ && !defined __clang__
  37   memset (s, c, len);
  38   /* Compiler barrier.  */
  39   __asm__ volatile ("" ::: "memory");
  40   return s;
  41 #elif defined __clang__
  42   memset (s, c, len);
  43   /* Compiler barrier.  */
  44   /* With asm ("" ::: "memory") LLVM analyzes uses of 's' and finds that the
  45      whole thing is dead and eliminates it.  Use 'g' to work around this
  46      problem.  See <https://bugs.llvm.org/show_bug.cgi?id=15495#c11>.  */
  47   __asm__ volatile ("" : : "g"(s) : "memory");
  48   return s;
  49 #else
  50   /* Invoke memset through a volatile function pointer.  This defeats compiler
  51      optimizations.  */
  52   void * (* const volatile volatile_memset) (void *, int, size_t) = memset;
  53   return volatile_memset (s, c, len);
  54 #endif
  55 }