string: Add memset_explicit from C23

This is a non-eliable version of memset, suitable for use before
calling free to avoid leaving sensitive data visible on the heap.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

newlib/libc/include/string.h

@@ -92,6 +92,9 @@ void    *mempcpy (void *, const void *, size_t);
 void    *memrchr (const void *, int, size_t);
 #endif
 void    *memset (void *, int, size_t);
+#if __ISO_C_VISIBLE >= 2023
+void    *memset_explicit(void *, int, size_t);
+#endif
 #if __GNU_VISIBLE
 void    *rawmemchr (const void *, int);
 #endif

newlib/libc/string/CMakeLists.txt

@@ -53,6 +53,7 @@ picolibc_sources(
   mempcpy.c
   memrchr.c
   memset.c
+  memset_explicit.c
   rawmemchr.c
   rindex.c
   stpcpy.c

newlib/libc/string/memset_explicit.c

@@ -0,0 +1,46 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright © 2024 Keith Packard
+ *
+ * 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. Neither the name of the copyright holder nor the names of its
+ *    contributors may be used to endorse or promote products derived
+ *    from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS 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
+ * COPYRIGHT HOLDER OR 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.
+ */
+
+#define _ISOC23_SOURCE
+#include <string.h>
+
+void
+*memset_explicit(void *dest, int ch, size_t count)
+{
+    memset(dest, ch, count);
+    /* Compiler barrier.  */
+    __asm__ __volatile__ ("" ::"r" (dest): "memory");
+    return dest;
+}

newlib/libc/string/meson.build

@@ -53,6 +53,7 @@ srcs_string = [
     'mempcpy.c',
     'memrchr.c',
     'memset.c',
+    'memset_explicit.c',
     'rawmemchr.c',
     'rindex.c',
     'stpcpy.c',