lib/libc/string: add memset_explicit() for compliance with C23

Patterned after explicit_bzero, visible from C23 onwards.

Reviewed by:	emaste, kevans
Differential Revision:	https://reviews.freebsd.org/D47286

5 files changed, 52 insertions, 3 deletions

diff --git a/include/string.h b/include/string.h
index d9adcf4e0e41..73b675370c58 100644
--- a/include/string.h
+++ b/include/string.h
@@ -71,6 +71,9 @@ void	*(memmove)(void *, const void *, size_t);
 void	*(mempcpy)(void * __restrict, const void * __restrict, size_t);
 #endif
 void	*(memset)(void *, int, size_t);
+#if __BSD_VISIBLE || __ISO_C_VISIBLE >= 2023
+void	*memset_explicit(void *, int, size_t);
+#endif
 #if __POSIX_VISIBLE >= 200809
 char	*(stpcpy)(char * __restrict, const char * __restrict);
 char	*(stpncpy)(char * __restrict, const char * __restrict, size_t);
diff --git a/lib/libc/string/Makefile.inc b/lib/libc/string/Makefile.inc
index c5ded194c78a..5d4a9a6e3eed 100644
--- a/lib/libc/string/Makefile.inc
+++ b/lib/libc/string/Makefile.inc
@@ -11,6 +11,7 @@ MISRCS+=bcmp.c bcopy.c bzero.c explicit_bzero.c \
 	ffs.c ffsl.c ffsll.c fls.c flsl.c flsll.c \
 	memccpy.c memchr.c memrchr.c memcmp.c \
 	memcpy.c memmem.c memmove.c mempcpy.c memset.c memset_s.c \
+	memset_explicit.c \
 	stpcpy.c stpncpy.c strcasecmp.c \
 	strcat.c strcasestr.c strchr.c strchrnul.c strcmp.c strcoll.c strcpy.c\
 	strcspn.c strdup.c strerror.c strlcat.c strlcpy.c strlen.c strmode.c \
@@ -62,7 +63,8 @@ MLINKS+=ffs.3 ffsl.3 \
 MLINKS+=index.3 rindex.3
 MLINKS+=memchr.3 memrchr.3
 MLINKS+=memcpy.3 mempcpy.3
-MLINKS+=memset.3 memset_s.3
+MLINKS+=memset.3 memset_s.3 \
+	memset.3 memset_explicit.3
 MLINKS+=strcasecmp.3 strncasecmp.3 \
 	strcasecmp.3 strcasecmp_l.3 \
 	strcasecmp.3 strncasecmp_l.3
diff --git a/lib/libc/string/Symbol.map b/lib/libc/string/Symbol.map
index fd854d1f9479..6b2c41124adf 100644
--- a/lib/libc/string/Symbol.map
+++ b/lib/libc/string/Symbol.map
@@ -116,6 +116,10 @@ FBSD_1.7 {
 	wmempcpy;
 };
 
+FBSD_1.8 {
+	memset_explicit;
+};
+
 FBSDprivate_1.0 {
 	__strtok_r;
 };
diff --git a/lib/libc/string/memset.3 b/lib/libc/string/memset.3
index 3ae485f68a92..f2dba3ec5a48 100644
--- a/lib/libc/string/memset.3
+++ b/lib/libc/string/memset.3
@@ -29,7 +29,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd August 19, 2018
+.Dd October 24, 2024
 .Dt MEMSET 3
 .Os
 .Sh NAME
@@ -41,6 +41,8 @@
 .In string.h
 .Ft void *
 .Fn memset "void *dest" "int c" "size_t len"
+.Ft void *
+.Fn memset_explicit "void *dest" "int c" "size_t len"
 .Fd #define __STDC_WANT_LIB_EXT1__ 1
 .Ft errno_t
 .Fn memset_s "void *dest" "rsize_t destsz" "int c" "rsize_t len"
@@ -68,6 +70,13 @@ The behaviour is also undefined if
 is an invalid pointer.
 .Pp
 The
+.Fn memset_explicit
+function behaves the same as
+.Fn memset, but will not be removed by a compiler's dead store
+optimization pass, making it useful for clearing sensitive memory
+such as a password.
+.Pp
+The
 .Fn memset_s
 function behaves the same as
 .Fn memset
@@ -108,7 +117,9 @@ before
 .Sh RETURN VALUES
 The
 .Fn memset
-function returns its first argument.
+and
+.Fn memset_explicit
+functions return their first argument.
 The
 .Fn memset_s
 function returns zero on success, non-zero on error.
@@ -128,3 +139,6 @@ conforms to
 conforms to
 .St -isoC-2011
 K.3.7.4.1.
+.Fn memset_explicit
+conforms to
+.St -isoC-2024 .
diff --git a/lib/libc/string/memset_explicit.c b/lib/libc/string/memset_explicit.c
new file mode 100644
index 000000000000..ee6be0363981
--- /dev/null
+++ b/lib/libc/string/memset_explicit.c
@@ -0,0 +1,26 @@
+/*-
+ * SPDF-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2024 Robert Clausecker <fuz@FreeBSD.org>
+ */
+
+#include <string.h>
+
+__attribute__((weak)) void __memset_explicit_hook(void *, int, size_t);
+
+__attribute__((weak)) void
+__memset_explicit_hook(void *buf, int ch, size_t len)
+{
+	(void)buf;
+	(void)ch;
+	(void)len;
+}
+
+void *
+memset_explicit(void *buf, int ch, size_t len)
+{
+	memset(buf, ch, len);
+	__memset_explicit_hook(buf, ch, len);
+
+	return (buf);
+}