lib/memset_explicit.c

   1 /* Erase sensitive data from memory.
   2    Copyright 2022-2025 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 /* Specification.  */
  20 #include <string.h>
  21
  22 /* Set S's bytes to C, where S has LEN bytes.  The compiler will not
  23    optimize effects away, even if S is dead after the call.  */
  24 void *
  25 memset_explicit (void *s, int c, size_t len)
  26 {
  27 #if HAVE_EXPLICIT_MEMSET
  28   return explicit_memset (s, c, len);
  29 #elif HAVE_MEMSET_S
  30 # if !HAVE_MEMSET_S_SUPPORTS_ZERO
  31   if (len > 0)
  32 # endif
  33     (void) memset_s (s, len, c, len);
  34   return s;
  35 #elif defined __GNUC__ && !defined __clang__
  36   memset (s, c, len);
  37   /* Compiler barrier.  */
  38   __asm__ volatile ("" ::: "memory");
  39   return s;
  40 #elif defined __clang__
  41   memset (s, c, len);
  42   /* Compiler barrier.  */
  43   /* With asm ("" ::: "memory") LLVM analyzes uses of 's' and finds that the
  44      whole thing is dead and eliminates it.  Use 'g' to work around this
  45      problem.  See <https://bugs.llvm.org/show_bug.cgi?id=15495#c11>.  */
  46   __asm__ volatile ("" : : "g"(s) : "memory");
  47   return s;
  48 #else
  49   /* Invoke memset through a volatile function pointer.  This defeats compiler
  50      optimizations.  */
  51   void * (* const volatile volatile_memset) (void *, int, size_t) = memset;
  52   return volatile_memset (s, c, len);
  53 #endif
  54 }