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Re: [PATCH RFC] explicit_bzero, again

On 08/09/2015 07:29 PM, Zack Weinberg wrote:
> I'd like to resume the discussion of adding explicit_bzero or similar
> to glibc.  This was originally proposed by Nick Mathewson last
> December (https://sourceware.org/ml/libc-alpha/2014-12/msg00506.html)
> and I have prepared an updated patchset, which is attached to this
> message.  One patch adds the function, and the second adds several
> intra-libc uses; I suspect I haven't found all of the places where
> it should be used.

This updated patchset corrects a number of minor errors (thanks,
Joseph); adds tests for the fortification of explicit_bzero; and revises
the new manual section a little.

zw

New library function, explicit_bzero

[NEWS:]
The new function explicit_bzero is functionally identical to bzero,
but the compiler will not delete a call to it, even if the memory
region it's applied to is dead after the call.  You should use this
function, for instance, to erase cryptographic keys or similar when
you are done with them.  Taken from OpenBSD.

[ChangeLog:]
YYYY-MM-DD  Nick Mathewson <nickm@torproject.org>
            Zack Weinberg <zackw@panix.com>

	* manual/string.texi: Document new function explicit_bzero.

	* string/Versions [GLIBC_2.23]: Add explicit_bzero.
	* string/string.h [__USE_MISC]: Declare explicit_bzero.
	* string/bits/string2.h: Optimize explicit_bzero when possible.
	* string/bits/string3.h: Fortify explicit_bzero. Remove stray
        blank lines.
	* string/test-explicit_bzero.c: New file, basic test for
        explicit_bzero.
	* string/Makefile, string/test-memset.c: Support the new test.

	* string/bzero.c
	* sysdeps/aarch64/bzero.S
	* sysdeps/alpha/bzero.S
	* sysdeps/i386/bzero.c
	* sysdeps/i386/i586/bzero.S
	* sysdeps/i386/i686/bzero.S
	* sysdeps/powerpc/powerpc32/bzero.S
	* sysdeps/powerpc/powerpc32/power4/multiarch/bzero.c
	* sysdeps/powerpc/powerpc64/memset.S
	* sysdeps/powerpc/powerpc64/multiarch/bzero.c
	* sysdeps/powerpc/powerpc64/power4/memset.S
	* sysdeps/powerpc/powerpc64/power6/memset.S
	* sysdeps/powerpc/powerpc64/power7/memset.S
	* sysdeps/powerpc/powerpc64/power8/memset.S
	* sysdeps/s390/s390-32/bzero.S
	* sysdeps/s390/s390-64/bzero.S
	* sysdeps/sparc/sparc32/memset.S
	* sysdeps/sparc/sparc64/memset.S
	* sysdeps/sparc/sparc64/multiarch/memset.S
	* sysdeps/tile/bzero.S
	* sysdeps/x86_64/memset.S:
	Declare explicit_bzero as weak alias of __bzero.

	* debug/explicit_bzero_chk.c: New file.
	* debug/Makefile: Compile it.
	* debug/Versions [GLIBC_2.23]: Add __explicit_bzero_chk.
	* debug/tst-chk1.c: Test fortification of explicit_bzero,
        and also bzero and bcopy, while we're at it.

	* include/string.h: Add libc_hidden_proto for __memset_chk.
	* debug/memset_chk.c: Add libc_hidden_def for __memset_chk.
	En passant, convert to prototyped function definition and
	remove unnecessary #include.
	* sysdeps/i386/i586/memset.S
	* sysdeps/i386/i686/memset.S
	* sysdeps/i386/i686/multiarch/memset.S
	* sysdeps/x86_64/memset.S
	* sysdeps/x86_64/multiarch/memset.S:
	Add libc_hidden_builtin_def for __memset_chk.

	* sysdeps/arm/nacl/libc.abilist
	* sysdeps/unix/sysv/linux/aarch64/libc.abilist
	* sysdeps/unix/sysv/linux/alpha/libc.abilist
	* sysdeps/unix/sysv/linux/arm/libc.abilist
	* sysdeps/unix/sysv/linux/hppa/libc.abilist
	* sysdeps/unix/sysv/linux/i386/libc.abilist
	* sysdeps/unix/sysv/linux/ia64/libc.abilist
	* sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist
	* sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist
	* sysdeps/unix/sysv/linux/microblaze/libc.abilist
	* sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist
	* sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist
	* sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist
	* sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist
	* sysdeps/unix/sysv/linux/nios2/libc.abilist
	* sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist
	* sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist
	* sysdeps/unix/sysv/linux/powerpc/powerpc64/libc.abilist
	* sysdeps/unix/sysv/linux/powerpc/powerpc64/libc-le.abilist
	* sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist
	* sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist
	* sysdeps/unix/sysv/linux/sh/libc.abilist
	* sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist
	* sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist
	* sysdeps/unix/sysv/linux/tile/tilegx/tilegx32/libc.abilist
	* sysdeps/unix/sysv/linux/tile/tilegx/tilegx64/libc.abilist
	* sysdeps/unix/sysv/linux/tile/tilepro/libc.abilist
	* sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
	* sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist:
	Add GLIBC_2.23 stanza containing explicit_bzero and
	__explicit_bzero_chk.
---
 debug/Makefile                                     |  1 +
 debug/Versions                                     |  3 +
 debug/explicit_bzero_chk.c                         | 28 +++++++
 debug/memset_chk.c                                 |  8 +-
 debug/tst-chk1.c                                   | 89 ++++++++++++++++++++++
 include/string.h                                   |  4 +
 manual/string.texi                                 | 67 ++++++++++++++++
 string/Makefile                                    |  3 +-
 string/Versions                                    |  3 +
 string/bits/string2.h                              | 28 +++++++
 string/bits/string3.h                              | 43 +++++++++--
 string/bzero.c                                     |  1 +
 string/string.h                                    |  4 +
 string/test-explicit_bzero.c                       | 21 +++++
 string/test-memset.c                               | 10 ++-
 sysdeps/aarch64/bzero.S                            |  1 +
 sysdeps/alpha/bzero.S                              |  1 +
 sysdeps/arm/nacl/libc.abilist                      |  4 +
 sysdeps/i386/bzero.c                               |  1 +
 sysdeps/i386/i586/bzero.S                          |  1 +
 sysdeps/i386/i586/memset.S                         |  1 +
 sysdeps/i386/i686/bzero.S                          |  1 +
 sysdeps/i386/i686/memset.S                         |  1 +
 sysdeps/i386/i686/multiarch/memset.S               |  8 +-
 sysdeps/powerpc/powerpc32/bzero.S                  |  1 +
 sysdeps/powerpc/powerpc32/power4/multiarch/bzero.c |  1 +
 sysdeps/powerpc/powerpc64/memset.S                 |  1 +
 sysdeps/powerpc/powerpc64/multiarch/bzero.c        |  1 +
 sysdeps/powerpc/powerpc64/power4/memset.S          |  1 +
 sysdeps/powerpc/powerpc64/power6/memset.S          |  1 +
 sysdeps/powerpc/powerpc64/power7/memset.S          |  1 +
 sysdeps/powerpc/powerpc64/power8/memset.S          |  1 +
 sysdeps/s390/s390-32/bzero.S                       |  1 +
 sysdeps/s390/s390-64/bzero.S                       |  1 +
 sysdeps/sparc/sparc32/memset.S                     |  1 +
 sysdeps/sparc/sparc64/memset.S                     |  1 +
 sysdeps/sparc/sparc64/multiarch/memset.S           |  1 +
 sysdeps/tile/bzero.S                               |  1 +
 sysdeps/unix/sysv/linux/aarch64/libc.abilist       |  4 +
 sysdeps/unix/sysv/linux/alpha/libc.abilist         |  4 +
 sysdeps/unix/sysv/linux/arm/libc.abilist           |  4 +
 sysdeps/unix/sysv/linux/hppa/libc.abilist          |  4 +
 sysdeps/unix/sysv/linux/i386/libc.abilist          |  4 +
 sysdeps/unix/sysv/linux/ia64/libc.abilist          |  4 +
 sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist |  4 +
 sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist   |  4 +
 sysdeps/unix/sysv/linux/microblaze/libc.abilist    |  4 +
 .../unix/sysv/linux/mips/mips32/fpu/libc.abilist   |  4 +
 .../unix/sysv/linux/mips/mips32/nofpu/libc.abilist |  4 +
 .../unix/sysv/linux/mips/mips64/n32/libc.abilist   |  4 +
 .../unix/sysv/linux/mips/mips64/n64/libc.abilist   |  4 +
 sysdeps/unix/sysv/linux/nios2/libc.abilist         |  4 +
 .../sysv/linux/powerpc/powerpc32/fpu/libc.abilist  |  4 +
 .../linux/powerpc/powerpc32/nofpu/libc.abilist     |  4 +
 .../sysv/linux/powerpc/powerpc64/libc-le.abilist   |  4 +
 .../unix/sysv/linux/powerpc/powerpc64/libc.abilist |  4 +
 sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist  |  4 +
 sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist  |  4 +
 sysdeps/unix/sysv/linux/sh/libc.abilist            |  4 +
 sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist |  4 +
 sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist |  4 +
 .../sysv/linux/tile/tilegx/tilegx32/libc.abilist   |  4 +
 .../sysv/linux/tile/tilegx/tilegx64/libc.abilist   |  4 +
 sysdeps/unix/sysv/linux/tile/tilepro/libc.abilist  |  4 +
 sysdeps/unix/sysv/linux/x86_64/64/libc.abilist     |  4 +
 sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist    |  4 +
 sysdeps/x86_64/memset.S                            |  2 +
 sysdeps/x86_64/multiarch/memset.S                  |  2 +-
 68 files changed, 445 insertions(+), 17 deletions(-)
 create mode 100644 debug/explicit_bzero_chk.c
 create mode 100644 string/test-explicit_bzero.c

diff --git a/debug/Makefile b/debug/Makefile
index 9ff357b..93e044f 100644
--- a/debug/Makefile
+++ b/debug/Makefile
@@ -29,6 +29,7 @@ headers	:= execinfo.h
 # build, its _FORTIFY_SOURCE support will be too.
 routines  = backtrace backtracesyms backtracesymsfd noophooks \
 	    memcpy_chk memmove_chk mempcpy_chk memset_chk stpcpy_chk \
+	    explicit_bzero_chk \
 	    strcat_chk strcpy_chk strncat_chk strncpy_chk stpncpy_chk \
 	    sprintf_chk vsprintf_chk snprintf_chk vsnprintf_chk \
 	    printf_chk fprintf_chk vprintf_chk vfprintf_chk \
diff --git a/debug/Versions b/debug/Versions
index 0482c85..3c7feb1 100644
--- a/debug/Versions
+++ b/debug/Versions
@@ -55,6 +55,9 @@ libc {
   GLIBC_2.16 {
     __poll_chk; __ppoll_chk;
   }
+  GLIBC_2.23 {
+    __explicit_bzero_chk;
+  }
   GLIBC_PRIVATE {
     __fortify_fail;
   }
diff --git a/debug/explicit_bzero_chk.c b/debug/explicit_bzero_chk.c
new file mode 100644
index 0000000..15330c6
--- /dev/null
+++ b/debug/explicit_bzero_chk.c
@@ -0,0 +1,28 @@
+/* Copyright (C) 1991-2015 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <string.h>
+
+/* This has to be an exported symbol, unlike for plain bzero, because
+   bits/string3.h cannot always optimize explicit_bzero to memset.
+   See comments there, and "Erasing Sensitive Data" in the manual.  */
+
+void
+__explicit_bzero_chk (void *dstpp, size_t len, size_t dstlen)
+{
+  (void) __memset_chk (dstpp, 0, len, dstlen);
+}
diff --git a/debug/memset_chk.c b/debug/memset_chk.c
index 293172f..4df7c45 100644
--- a/debug/memset_chk.c
+++ b/debug/memset_chk.c
@@ -16,17 +16,13 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include <string.h>
-#include <memcopy.h>
 
 void *
-__memset_chk (dstpp, c, len, dstlen)
-     void *dstpp;
-     int c;
-     size_t len;
-     size_t dstlen;
+__memset_chk (void *dstpp, int c, size_t len, size_t dstlen)
 {
   if (__glibc_unlikely (dstlen < len))
     __chk_fail ();
 
   return memset (dstpp, c, len);
 }
+libc_hidden_def (__memset_chk)
diff --git a/debug/tst-chk1.c b/debug/tst-chk1.c
index bded583..4064eb9 100644
--- a/debug/tst-chk1.c
+++ b/debug/tst-chk1.c
@@ -143,6 +143,11 @@ do_test (void)
   if (memcmp (buf, "aabcdefghi", 10))
     FAIL ();
 
+  memcpy (buf, "abcdefghij", 10);
+  bcopy (buf, buf + 1, 9);
+  if (memcmp (buf, "aabcdefghi", 10))
+    FAIL ();
+
   if (mempcpy (buf + 5, "abcde", 5) != buf + 10
       || memcmp (buf, "aabcdabcde", 10))
     FAIL ();
@@ -151,6 +156,14 @@ do_test (void)
   if (memcmp (buf, "aabcdabcjj", 10))
     FAIL ();
 
+  bzero (buf + 8, 2);
+  if (memcmp (buf, "aabcdabc\0\0", 10))
+    FAIL ();
+
+  explicit_bzero (buf + 6, 4);
+  if (memcmp (buf, "aabcda\0\0\0\0", 10))
+    FAIL ();
+
   strcpy (buf + 4, "EDCBA");
   if (memcmp (buf, "aabcEDCBA", 10))
     FAIL ();
@@ -175,6 +188,11 @@ do_test (void)
   if (memcmp (buf, "aabcdefghi", 10))
     FAIL ();
 
+  memcpy (buf, "abcdefghij", l0 + 10);
+  bcopy (buf, buf + 1, l0 + 9);
+  if (memcmp (buf, "aabcdefghi", 10))
+    FAIL ();
+
   if (mempcpy (buf + 5, "abcde", l0 + 5) != buf + 10
       || memcmp (buf, "aabcdabcde", 10))
     FAIL ();
@@ -183,6 +201,14 @@ do_test (void)
   if (memcmp (buf, "aabcdabcjj", 10))
     FAIL ();
 
+  bzero (buf + 8, l0 + 2);
+  if (memcmp (buf, "aabcdabc\0\0", 10))
+    FAIL ();
+
+  explicit_bzero (buf + 6, l0 + 4);
+  if (memcmp (buf, "aabcda\0\0\0\0", 10))
+    FAIL ();
+
   strcpy (buf + 4, str1 + 5);
   if (memcmp (buf, "aabcEDCBA", 10))
     FAIL ();
@@ -214,11 +240,18 @@ do_test (void)
   if (memcmp (buf, "aabcEcdZY", 10))
     FAIL ();
 
+  /* The following tests are supposed to succeed at all fortify
+     levels, even though they overflow a.buf1 into a.buf2.  */
   memcpy (a.buf1, "abcdefghij", l0 + 10);
   memmove (a.buf1 + 1, a.buf1, l0 + 9);
   if (memcmp (a.buf1, "aabcdefghi", 10))
     FAIL ();
 
+  memcpy (a.buf1, "abcdefghij", l0 + 10);
+  bcopy (a.buf1, a.buf1 + 1, l0 + 9);
+  if (memcmp (a.buf1, "aabcdefghi", 10))
+    FAIL ();
+
   if (mempcpy (a.buf1 + 5, "abcde", l0 + 5) != a.buf1 + 10
       || memcmp (a.buf1, "aabcdabcde", 10))
     FAIL ();
@@ -227,6 +260,14 @@ do_test (void)
   if (memcmp (a.buf1, "aabcdabcjj", 10))
     FAIL ();
 
+  bzero (a.buf1 + 8, l0 + 2);
+  if (memcmp (a.buf1, "aabcdabc\0\0", 10))
+    FAIL ();
+
+  explicit_bzero (a.buf1 + 6, l0 + 4);
+  if (memcmp (a.buf1, "aabcda\0\0\0\0", 10))
+    FAIL ();
+
 #if __USE_FORTIFY_LEVEL < 2
   /* The following tests are supposed to crash with -D_FORTIFY_SOURCE=2
      and sufficient GCC support, as the string operations overflow
@@ -285,6 +326,14 @@ do_test (void)
   CHK_FAIL_END
 
   CHK_FAIL_START
+  bcopy (buf + 1, buf + 2, 9);
+  CHK_FAIL_END
+
+  CHK_FAIL_START
+  bcopy (buf + 1, buf + 2, l0 + 9);
+  CHK_FAIL_END
+
+  CHK_FAIL_START
   p = (char *) mempcpy (buf + 6, "abcde", 5);
   CHK_FAIL_END
 
@@ -301,6 +350,22 @@ do_test (void)
   CHK_FAIL_END
 
   CHK_FAIL_START
+  bzero (buf + 9, 2);
+  CHK_FAIL_END
+
+  CHK_FAIL_START
+  bzero (buf + 9, l0 + 2);
+  CHK_FAIL_END
+
+  CHK_FAIL_START
+  explicit_bzero (buf + 9, 2);
+  CHK_FAIL_END
+
+  CHK_FAIL_START
+  explicit_bzero (buf + 9, l0 + 2);
+  CHK_FAIL_END
+
+  CHK_FAIL_START
   strcpy (buf + 5, str1 + 5);
   CHK_FAIL_END
 
@@ -378,6 +443,14 @@ do_test (void)
   CHK_FAIL_END
 
   CHK_FAIL_START
+  bcopy (a.buf1 + 1, a.buf1 + 2, 9);
+  CHK_FAIL_END
+
+  CHK_FAIL_START
+  bcopy (a.buf1 + 1, a.buf1 + 2, l0 + 9);
+  CHK_FAIL_END
+
+  CHK_FAIL_START
   p = (char *) mempcpy (a.buf1 + 6, "abcde", 5);
   CHK_FAIL_END
 
@@ -393,6 +466,22 @@ do_test (void)
   memset (a.buf1 + 9, 'j', l0 + 2);
   CHK_FAIL_END
 
+  CHK_FAIL_START
+  bzero (a.buf1 + 9, 2);
+  CHK_FAIL_END
+
+  CHK_FAIL_START
+  bzero (a.buf1 + 9, l0 + 2);
+  CHK_FAIL_END
+
+  CHK_FAIL_START
+  explicit_bzero (a.buf1 + 9, 2);
+  CHK_FAIL_END
+
+  CHK_FAIL_START
+  explicit_bzero (a.buf1 + 9, l0 + 2);
+  CHK_FAIL_END
+
 # if __USE_FORTIFY_LEVEL >= 2
 #  define O 0
 # else
diff --git a/include/string.h b/include/string.h
index c57671e..3d0f7ca 100644
--- a/include/string.h
+++ b/include/string.h
@@ -160,6 +160,10 @@ extern char *__strcat_chk (char *__restrict __dest,
 extern char *__strncat_chk (char *__restrict __dest,
 			    const char *__restrict __src,
 			    size_t __len, size_t __destlen) __THROW;
+
+/* Used by __explicit_bzero_chk.  */
+libc_hidden_proto (__memset_chk)
+
 #endif
 
 #endif
diff --git a/manual/string.texi b/manual/string.texi
index 5f8a17e..1efcb55 100644
--- a/manual/string.texi
+++ b/manual/string.texi
@@ -33,6 +33,8 @@ too.
 * Search Functions::            Searching for a specific element or substring.
 * Finding Tokens in a String::  Splitting a string into tokens by looking
 				 for delimiters.
+* Erasing Sensitive Data::      Clearing memory which contains sensitive
+                                 data, after it's no longer needed.
 * strfry::                      Function for flash-cooking a string.
 * Trivial Encryption::          Obscuring data.
 * Encode Binary Data::          Encoding and Decoding of Binary Data.
@@ -2303,6 +2305,71 @@ contains no '/' characters, then "." is returned.  The prototype for this
 function can be found in @file{libgen.h}.
 @end deftypefun
 
+@node Erasing Sensitive Data
+@section Erasing Sensitive Data
+
+It is sometimes necessary to make sure that a block of data in memory
+is erased after use, even if no correct C program could access it
+again.  For instance, a cryptographic key should not be allowed to
+survive on the stack after the program is finished using it, because
+there might be a bug that causes junk stack data, including the key,
+to be revealed to the outside world.  @code{memset} and @code{bzero}
+are not safe to use for this, because the C compiler knows what they
+do, and can delete ``unnecessary'' calls to them.  For this situation,
+@theglibc{} provides @code{explicit_bzero}, which is functionally
+identical to @code{bzero}, except that the C compiler will @emph{not}
+delete apparently-unnecessary calls.  For example:
+
+@smallexample
+#include <string.h>
+
+extern void encrypt (const char *key, const char *in,
+                     char *out, size_t n);
+extern void genkey (const char *phrase, char *key);
+
+void encrypt_with_phrase (const char *phrase, const char *in,
+                          char *out, size_t n)
+@{
+  char key[16];
+  genkey (phrase, key);
+  encrypt (key, in, out, n);
+
+  explicit_bzero (key, 16);
+@}
+@end smallexample
+
+If @code{bzero} or @code{memset} had been used in this function, the C
+compiler might remove it as unnecessary, but it will not do this with
+@code{explicit_bzero}.
+
+@comment string.h
+@comment BSD
+@deftypefun void explicit_bzero (void *@var{block}, size_t @var{len})
+@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
+
+@code{explicit_bzero} writes zero into each of the first @var{len}
+bytes of the object beginning at @var{block}, just as @code{bzero}
+would.  The compiler will not delete a call to this function, even
+if the object beginning at @var{block} is never used again.
+
+@strong{Warning:} The compiler is free to make additional copies of
+any object, or parts of it, in temporary storage areas (such as
+registers and ``scratch'' stack space).  @code{explicit_bzero} does
+not guarantee that temporary copies of sensitive data are destroyed.
+
+@strong{Warning:} The @emph{only} optimization disabled by
+@code{explicit_bzero} is removal of ``unnecessary'' calls.  In all
+other respects, the compiler is allowed to optimize as it would for
+@code{memset}.  For instance, it may deduce that @var{block} cannot be
+a null pointer, and propagate this information both forward and
+backward in control flow.
+
+@strong{Portability Note:} This function first appeared in OpenBSD 5.5
+and has not been standardized.  @Theglibc{} declares this function in
+@file{string.h}, but on other systems it may be in @file{strings.h}
+instead.
+@end deftypefun
+
 @node strfry
 @section strfry
 
diff --git a/string/Makefile b/string/Makefile
index 8424a61..8246c10 100644
--- a/string/Makefile
+++ b/string/Makefile
@@ -47,7 +47,8 @@ strop-tests	:= memchr memcmp memcpy memmove mempcpy memset memccpy	\
 		   stpcpy stpncpy strcat strchr strcmp strcpy strcspn	\
 		   strlen strncmp strncpy strpbrk strrchr strspn memmem	\
 		   strstr strcasestr strnlen strcasecmp strncasecmp	\
-		   strncat rawmemchr strchrnul bcopy bzero memrchr
+		   strncat rawmemchr strchrnul bcopy bzero memrchr      \
+		   explicit_bzero
 tests		:= tester inl-tester noinl-tester testcopy test-ffs	\
 		   tst-strlen stratcliff tst-svc tst-inlcall		\
 		   bug-strncat1 bug-strspn1 bug-strpbrk1 tst-bswap	\
diff --git a/string/Versions b/string/Versions
index 59bf35a..0a02884 100644
--- a/string/Versions
+++ b/string/Versions
@@ -80,4 +80,7 @@ libc {
   GLIBC_2.6 {
     strerror_l;
   }
+  GLIBC_2.23 {
+    explicit_bzero;
+  }
 }
diff --git a/string/bits/string2.h b/string/bits/string2.h
index 7645176..a1efbe3 100644
--- a/string/bits/string2.h
+++ b/string/bits/string2.h
@@ -194,6 +194,34 @@ __STRING2_COPY_TYPE (8);
 #  define __bzero(s, n) __builtin_memset (s, '\0', n)
 # endif
 
+#endif /* !_HAVE_STRING_ARCH_memset */
+
+/* explicit_bzero can be optimized to memset plus a vacuous (but
+   non-deletable) use of the memory region, thus enabling all the
+   usual memset optimizations.
+
+   The __asm__ construct used below is known to work with GCC 4 and
+   later, when the size of the memory region is constant.  It's not
+   documented to work otherwise.  Moreover, even when guarded by
+   __builtin_constant_p, 'struct {char __x[n];}', where 'n' isn't
+   _syntactically_ an integer constant expression, is a hard error for
+   GCC when compiling C++, and a hard error for clang in both C and C++.  */
+
+#ifdef __USE_MISC
+# if __GNUC_PREREQ (4, 0) && !defined __clang__ && !defined __cplusplus
+__STRING_INLINE void
+__explicit_bzero_constn (void *__s, size_t __n)
+{
+  typedef struct {char __x[__n];} __memblk;
+  memset (__s, 0, __n);
+  __asm__ ("" : : "m" (*(__memblk __attribute__ ((__may_alias__)) *)__s));
+}
+
+#  define explicit_bzero(s, n)                          \
+  (__extension__ (__builtin_constant_p (n) && (n) > 0   \
+                  ? __explicit_bzero_constn (s, n)      \
+                  : explicit_bzero (s, n)))
+# endif
 #endif
 
 
diff --git a/string/bits/string3.h b/string/bits/string3.h
index 4d11aa6..c6b38d4 100644
--- a/string/bits/string3.h
+++ b/string/bits/string3.h
@@ -42,10 +42,10 @@ __warndecl (__warn_memset_zero_len,
 # ifdef __USE_MISC
 #  undef bcopy
 #  undef bzero
+#  undef explicit_bzero
 # endif
 #endif
 
-
 __fortify_function void *
 __NTH (memcpy (void *__restrict __dest, const void *__restrict __src,
 	       size_t __len))
@@ -68,7 +68,6 @@ __NTH (mempcpy (void *__restrict __dest, const void *__restrict __src,
 }
 #endif
 
-
 /* The first two tests here help to catch a somewhat common problem
    where the second and third parameter are transposed.  This is
    especially problematic if the intended fill value is zero.  In this
@@ -102,6 +101,43 @@ __NTH (bzero (void *__dest, size_t __len))
 {
   (void) __builtin___memset_chk (__dest, '\0', __len, __bos0 (__dest));
 }
+
+/* XXX We have no corresponding builtin yet.
+
+   __explicit_bzero_chk can be optimized to __builtin___memset_chk
+   plus a vacuous (but non-deletable) use of the memory region, thus
+   enabling all the usual memset optimizations.
+
+   The __asm__ construct used below is known to work with GCC 4 and
+   later, when the size of the memory region is constant.  It's not
+   documented to work otherwise.  Moreover, even when guarded by
+   __builtin_constant_p, 'struct {char __x[n];}', where 'n' isn't
+   _syntactically_ an integer constant expression, is a hard error for
+   GCC when compiling C++, and a hard error for clang in both C and C++.  */
+
+extern void __explicit_bzero_chk (void *__dest, size_t __len, size_t __dstlen);
+extern void __REDIRECT_NTH (__explicit_bzero_alias,
+                            (void *__dest, size_t __len),
+                            explicit_bzero);
+
+__fortify_function void
+__NTH (explicit_bzero (void *__dest, size_t __len))
+{
+# if __GNUC_PREREQ (4, 0) && !defined __clang__ && !defined __cplusplus
+  if (__builtin_constant_p (__len) && __len > 0)
+    {
+      (void) __builtin___memset_chk (__dest, '\0', __len, __bos0 (__dest));
+      typedef struct { char __x[__len]; } __memblk;
+      __asm__ ("" :: "m" (*(__memblk __attribute__ ((__may_alias__)) *)__dest));
+    }
+  else
+# endif
+  if (__bos0 (__dest) != (size_t) -1
+      && (!__builtin_constant_p (__len) || __len > __bos0 (__dest)))
+    __explicit_bzero_chk (__dest, __len, __bos0 (__dest));
+  else
+    __explicit_bzero_alias (__dest, __len);
+}
 #endif
 
 __fortify_function char *
@@ -118,7 +154,6 @@ __NTH (stpcpy (char *__restrict __dest, const char *__restrict __src))
 }
 #endif
 
-
 __fortify_function char *
 __NTH (strncpy (char *__restrict __dest, const char *__restrict __src,
 		size_t __len))
@@ -141,14 +176,12 @@ __NTH (stpncpy (char *__dest, const char *__src, size_t __n))
   return __stpncpy_alias (__dest, __src, __n);
 }
 
-
 __fortify_function char *
 __NTH (strcat (char *__restrict __dest, const char *__restrict __src))
 {
   return __builtin___strcat_chk (__dest, __src, __bos (__dest));
 }
 
-
 __fortify_function char *
 __NTH (strncat (char *__restrict __dest, const char *__restrict __src,
 		size_t __len))
diff --git a/string/bzero.c b/string/bzero.c
index 43e2b38..d60f494 100644
--- a/string/bzero.c
+++ b/string/bzero.c
@@ -27,3 +27,4 @@ __bzero (void *s, size_t len)
   memset (s, '\0', len);
 }
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
diff --git a/string/string.h b/string/string.h
index 3ab7103..0df7789 100644
--- a/string/string.h
+++ b/string/string.h
@@ -453,6 +453,10 @@ extern void bcopy (const void *__src, void *__dest, size_t __n)
 /* Set N bytes of S to 0.  */
 extern void bzero (void *__s, size_t __n) __THROW __nonnull ((1));
 
+/* As bzero, but the compiler will not delete a call to this
+   function, even if S is dead after the call. */
+extern void explicit_bzero (void *__s, size_t __n) __THROW __nonnull ((1));
+
 /* Compare N bytes of S1 and S2 (same as memcmp).  */
 extern int bcmp (const void *__s1, const void *__s2, size_t __n)
      __THROW __attribute_pure__ __nonnull ((1, 2));
diff --git a/string/test-explicit_bzero.c b/string/test-explicit_bzero.c
new file mode 100644
index 0000000..8e354c7
--- /dev/null
+++ b/string/test-explicit_bzero.c
@@ -0,0 +1,21 @@
+/* Test and measure explicit_bzero.
+   Copyright (C) 2012-2015 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#define TEST_EXPLICIT_BZERO
+#define TEST_BZERO
+#include "test-memset.c"
diff --git a/string/test-memset.c b/string/test-memset.c
index 9f3af46..ca56857 100644
--- a/string/test-memset.c
+++ b/string/test-memset.c
@@ -19,7 +19,11 @@
 
 #define TEST_MAIN
 #ifdef TEST_BZERO
-# define TEST_NAME "bzero"
+# ifdef TEST_EXPLICIT_BZERO
+#  define TEST_NAME "explicit_bzero"
+# else
+#  define TEST_NAME "bzero"
+# endif
 #else
 # define TEST_NAME "memset"
 #endif
@@ -35,7 +39,11 @@ void builtin_bzero (char *, size_t);
 
 IMPL (simple_bzero, 0)
 IMPL (builtin_bzero, 0)
+#ifdef TEST_EXPLICIT_BZERO
+IMPL (explicit_bzero, 1)
+#else
 IMPL (bzero, 1)
+#endif
 
 void
 simple_bzero (char *s, size_t n)
diff --git a/sysdeps/aarch64/bzero.S b/sysdeps/aarch64/bzero.S
index 7343896..375457b 100644
--- a/sysdeps/aarch64/bzero.S
+++ b/sysdeps/aarch64/bzero.S
@@ -25,3 +25,4 @@ ENTRY(__bzero)
 	b	__memset
 END(__bzero)
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
diff --git a/sysdeps/alpha/bzero.S b/sysdeps/alpha/bzero.S
index e3c015a..582b85a 100644
--- a/sysdeps/alpha/bzero.S
+++ b/sysdeps/alpha/bzero.S
@@ -108,3 +108,4 @@ $done:	ret
 
 	cfi_endproc
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
diff --git a/sysdeps/arm/nacl/libc.abilist b/sysdeps/arm/nacl/libc.abilist
index bef1259..f0c6a36 100644
--- a/sysdeps/arm/nacl/libc.abilist
+++ b/sysdeps/arm/nacl/libc.abilist
@@ -1835,3 +1835,7 @@ GLIBC_2.22
  write F
  writev F
  wscanf F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
diff --git a/sysdeps/i386/bzero.c b/sysdeps/i386/bzero.c
index 1a89444..c7575f8 100644
--- a/sysdeps/i386/bzero.c
+++ b/sysdeps/i386/bzero.c
@@ -76,6 +76,7 @@ __bzero (dstpp, len)
 		"memory");
 }
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
 
 #else
 #include <string/bzero.c>
diff --git a/sysdeps/i386/i586/bzero.S b/sysdeps/i386/i586/bzero.S
index 84d2f70..23ee2c9 100644
--- a/sysdeps/i386/i586/bzero.S
+++ b/sysdeps/i386/i586/bzero.S
@@ -1,3 +1,4 @@
 #define memset __bzero
 #include <sysdeps/i386/i586/memset.S>
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
diff --git a/sysdeps/i386/i586/memset.S b/sysdeps/i386/i586/memset.S
index bc26501..c15e4a7 100644
--- a/sysdeps/i386/i586/memset.S
+++ b/sysdeps/i386/i586/memset.S
@@ -119,3 +119,4 @@ L(2):	shrl	$2, %ecx	/* convert byte count to longword count */
 #endif
 END (memset)
 libc_hidden_builtin_def (memset)
+libc_hidden_builtin_def (__memset_chk)
diff --git a/sysdeps/i386/i686/bzero.S b/sysdeps/i386/i686/bzero.S
index 34b0faa..25a9811 100644
--- a/sysdeps/i386/i686/bzero.S
+++ b/sysdeps/i386/i686/bzero.S
@@ -1,3 +1,4 @@
 #define memset __bzero
 #include <sysdeps/i386/i686/memset.S>
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
diff --git a/sysdeps/i386/i686/memset.S b/sysdeps/i386/i686/memset.S
index b6dbf2a..01b79c0 100644
--- a/sysdeps/i386/i686/memset.S
+++ b/sysdeps/i386/i686/memset.S
@@ -98,6 +98,7 @@ ENTRY (memset)
 #endif
 END (memset)
 libc_hidden_builtin_def (memset)
+libc_hidden_builtin_def (__memset_chk)
 
 #if defined PIC && IS_IN (libc) && !BZERO_P
 strong_alias (__memset_chk, __memset_zero_constant_len_parameter)
diff --git a/sysdeps/i386/i686/multiarch/memset.S b/sysdeps/i386/i686/multiarch/memset.S
index 8015d57..21b6220 100644
--- a/sysdeps/i386/i686/multiarch/memset.S
+++ b/sysdeps/i386/i686/multiarch/memset.S
@@ -63,9 +63,11 @@ END(memset)
 #  undef libc_hidden_builtin_def
 /* IFUNC doesn't work with the hidden functions in shared library since
    they will be called without setting up EBX needed for PLT which is
-   used by IFUNC.  */
-#  define libc_hidden_builtin_def(name) \
-	.globl __GI_memset; __GI_memset = __memset_ia32
+   used by IFUNC.  ../memset.S will barf if we #define memset, so we
+   must instead create the necessary __GI_ symbols manually.  */
+#  define libc_hidden_builtin_def(name) /* disable */
+        .globl __GI_memset; __GI_memset = __memset_ia32
+        .globl __GI___memset_chk; __GI__memset_chk = __memset_chk_ia32
 # endif
 
 # undef strong_alias
diff --git a/sysdeps/powerpc/powerpc32/bzero.S b/sysdeps/powerpc/powerpc32/bzero.S
index ae21c53..2f9614e 100644
--- a/sysdeps/powerpc/powerpc32/bzero.S
+++ b/sysdeps/powerpc/powerpc32/bzero.S
@@ -25,3 +25,4 @@ ENTRY (__bzero)
 	b	memset@local
 END (__bzero)
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
diff --git a/sysdeps/powerpc/powerpc32/power4/multiarch/bzero.c b/sysdeps/powerpc/powerpc32/power4/multiarch/bzero.c
index e938643..88aec9c 100644
--- a/sysdeps/powerpc/powerpc32/power4/multiarch/bzero.c
+++ b/sysdeps/powerpc/powerpc32/power4/multiarch/bzero.c
@@ -34,4 +34,5 @@ libc_ifunc (__bzero,
             : __bzero_ppc);
 
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
 #endif
diff --git a/sysdeps/powerpc/powerpc64/memset.S b/sysdeps/powerpc/powerpc64/memset.S
index f8543e5..7acc1aa 100644
--- a/sysdeps/powerpc/powerpc64/memset.S
+++ b/sysdeps/powerpc/powerpc64/memset.S
@@ -258,4 +258,5 @@ ENTRY (__bzero)
 END_GEN_TB (__bzero,TB_TOCLESS)
 
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
 #endif
diff --git a/sysdeps/powerpc/powerpc64/multiarch/bzero.c b/sysdeps/powerpc/powerpc64/multiarch/bzero.c
index 4953284..df10f36 100644
--- a/sysdeps/powerpc/powerpc64/multiarch/bzero.c
+++ b/sysdeps/powerpc/powerpc64/multiarch/bzero.c
@@ -40,4 +40,5 @@ libc_ifunc (__bzero,
             : __bzero_ppc);
 
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
 #endif
diff --git a/sysdeps/powerpc/powerpc64/power4/memset.S b/sysdeps/powerpc/powerpc64/power4/memset.S
index 58877af..709db11 100644
--- a/sysdeps/powerpc/powerpc64/power4/memset.S
+++ b/sysdeps/powerpc/powerpc64/power4/memset.S
@@ -245,4 +245,5 @@ ENTRY (__bzero)
 END (__bzero)
 #ifndef __bzero
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
 #endif
diff --git a/sysdeps/powerpc/powerpc64/power6/memset.S b/sysdeps/powerpc/powerpc64/power6/memset.S
index 02f279d..b49b0a8 100644
--- a/sysdeps/powerpc/powerpc64/power6/memset.S
+++ b/sysdeps/powerpc/powerpc64/power6/memset.S
@@ -389,4 +389,5 @@ ENTRY (__bzero)
 END (__bzero)
 #ifndef __bzero
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
 #endif
diff --git a/sysdeps/powerpc/powerpc64/power7/memset.S b/sysdeps/powerpc/powerpc64/power7/memset.S
index 4c8c06f..adf4bfd 100644
--- a/sysdeps/powerpc/powerpc64/power7/memset.S
+++ b/sysdeps/powerpc/powerpc64/power7/memset.S
@@ -393,4 +393,5 @@ ENTRY (__bzero)
 END (__bzero)
 #ifndef __bzero
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
 #endif
diff --git a/sysdeps/powerpc/powerpc64/power8/memset.S b/sysdeps/powerpc/powerpc64/power8/memset.S
index eaff0e6..6d2ca37 100644
--- a/sysdeps/powerpc/powerpc64/power8/memset.S
+++ b/sysdeps/powerpc/powerpc64/power8/memset.S
@@ -451,4 +451,5 @@ ENTRY (__bzero)
 END (__bzero)
 #ifndef __bzero
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
 #endif
diff --git a/sysdeps/s390/s390-32/bzero.S b/sysdeps/s390/s390-32/bzero.S
index 7c03c3e..8a3c0cc 100644
--- a/sysdeps/s390/s390-32/bzero.S
+++ b/sysdeps/s390/s390-32/bzero.S
@@ -39,4 +39,5 @@ END(__bzero)
 
 #ifndef NO_WEAK_ALIAS
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
 #endif
diff --git a/sysdeps/s390/s390-64/bzero.S b/sysdeps/s390/s390-64/bzero.S
index 355142b..3b72e50 100644
--- a/sysdeps/s390/s390-64/bzero.S
+++ b/sysdeps/s390/s390-64/bzero.S
@@ -38,4 +38,5 @@ END(__bzero)
 
 #ifndef NO_WEAK_ALIAS
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
 #endif
diff --git a/sysdeps/sparc/sparc32/memset.S b/sysdeps/sparc/sparc32/memset.S
index 7021ce4..67a18f1 100644
--- a/sysdeps/sparc/sparc32/memset.S
+++ b/sysdeps/sparc/sparc32/memset.S
@@ -152,3 +152,4 @@ END(memset)
 libc_hidden_builtin_def (memset)
 
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
diff --git a/sysdeps/sparc/sparc64/memset.S b/sysdeps/sparc/sparc64/memset.S
index d3293fc..85a7a1f 100644
--- a/sysdeps/sparc/sparc64/memset.S
+++ b/sysdeps/sparc/sparc64/memset.S
@@ -312,3 +312,4 @@ ENTRY(__bzero)
 END(__bzero)
 
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
diff --git a/sysdeps/sparc/sparc64/multiarch/memset.S b/sysdeps/sparc/sparc64/multiarch/memset.S
index 45de5ab..ca26cb0 100644
--- a/sysdeps/sparc/sparc64/multiarch/memset.S
+++ b/sysdeps/sparc/sparc64/multiarch/memset.S
@@ -107,6 +107,7 @@ ENTRY(__bzero)
 END(__bzero)
 
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
 
 # undef weak_alias
 # define weak_alias(a, b)
diff --git a/sysdeps/tile/bzero.S b/sysdeps/tile/bzero.S
index 412dc97..3472738 100644
--- a/sysdeps/tile/bzero.S
+++ b/sysdeps/tile/bzero.S
@@ -28,3 +28,4 @@ ENTRY(__bzero)
 	j __memset
 END(__bzero)
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
diff --git a/sysdeps/unix/sysv/linux/aarch64/libc.abilist b/sysdeps/unix/sysv/linux/aarch64/libc.abilist
index 3e0f329..c2b077f 100644
--- a/sysdeps/unix/sysv/linux/aarch64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/aarch64/libc.abilist
@@ -2084,3 +2084,7 @@ GLIBC_2.18
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
diff --git a/sysdeps/unix/sysv/linux/alpha/libc.abilist b/sysdeps/unix/sysv/linux/alpha/libc.abilist
index 58c8b32..dd75663 100644
--- a/sysdeps/unix/sysv/linux/alpha/libc.abilist
+++ b/sysdeps/unix/sysv/linux/alpha/libc.abilist
@@ -1825,6 +1825,10 @@ GLIBC_2.18
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.2
  GLIBC_2.2 A
  _IO_adjust_wcolumn F
diff --git a/sysdeps/unix/sysv/linux/arm/libc.abilist b/sysdeps/unix/sysv/linux/arm/libc.abilist
index f2b20ad..6014129 100644
--- a/sysdeps/unix/sysv/linux/arm/libc.abilist
+++ b/sysdeps/unix/sysv/linux/arm/libc.abilist
@@ -92,6 +92,10 @@ GLIBC_2.18
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.4
  GLIBC_2.4 A
  _Exit F
diff --git a/sysdeps/unix/sysv/linux/hppa/libc.abilist b/sysdeps/unix/sysv/linux/hppa/libc.abilist
index cf0ad90..a563c40 100644
--- a/sysdeps/unix/sysv/linux/hppa/libc.abilist
+++ b/sysdeps/unix/sysv/linux/hppa/libc.abilist
@@ -98,6 +98,10 @@ GLIBC_2.19
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.2
  GLIBC_2.2 A
  _Exit F
diff --git a/sysdeps/unix/sysv/linux/i386/libc.abilist b/sysdeps/unix/sysv/linux/i386/libc.abilist
index fcf1b72..5c4972f 100644
--- a/sysdeps/unix/sysv/linux/i386/libc.abilist
+++ b/sysdeps/unix/sysv/linux/i386/libc.abilist
@@ -2026,6 +2026,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/ia64/libc.abilist b/sysdeps/unix/sysv/linux/ia64/libc.abilist
index 16c2e3d..16c82e9 100644
--- a/sysdeps/unix/sysv/linux/ia64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/ia64/libc.abilist
@@ -1884,6 +1884,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist b/sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist
index 902b0c3..945dc34 100644
--- a/sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist
+++ b/sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist
@@ -93,6 +93,10 @@ GLIBC_2.18
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.4
  GLIBC_2.4 A
  _Exit F
diff --git a/sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist b/sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist
index 4db00b0..fd50ed5 100644
--- a/sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist
+++ b/sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist
@@ -1982,6 +1982,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/microblaze/libc.abilist b/sysdeps/unix/sysv/linux/microblaze/libc.abilist
index f1f76a1..8c3b4aa 100644
--- a/sysdeps/unix/sysv/linux/microblaze/libc.abilist
+++ b/sysdeps/unix/sysv/linux/microblaze/libc.abilist
@@ -2083,3 +2083,7 @@ GLIBC_2.18
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
diff --git a/sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist b/sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist
index 2d51989..2cf852c 100644
--- a/sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist
+++ b/sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist
@@ -1954,6 +1954,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist b/sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist
index b012bdf..86c2f78 100644
--- a/sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist
+++ b/sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist
@@ -1952,6 +1952,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist b/sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist
index 9db0e8b..8f6890d 100644
--- a/sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist
+++ b/sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist
@@ -1950,6 +1950,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist b/sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist
index eecfcfe..d1975c7 100644
--- a/sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist
@@ -1944,6 +1944,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/nios2/libc.abilist b/sysdeps/unix/sysv/linux/nios2/libc.abilist
index b10bf62..0bc6c5c 100644
--- a/sysdeps/unix/sysv/linux/nios2/libc.abilist
+++ b/sysdeps/unix/sysv/linux/nios2/libc.abilist
@@ -2124,3 +2124,7 @@ GLIBC_2.21
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist
index 0f15463..e80f798 100644
--- a/sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist
+++ b/sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist
@@ -1986,6 +1986,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist
index 7bcaa07..bcc8383 100644
--- a/sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist
+++ b/sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist
@@ -1992,6 +1992,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc64/libc-le.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc64/libc-le.abilist
index c0b3985..c66effb 100644
--- a/sysdeps/unix/sysv/linux/powerpc/powerpc64/libc-le.abilist
+++ b/sysdeps/unix/sysv/linux/powerpc/powerpc64/libc-le.abilist
@@ -2172,3 +2172,7 @@ GLIBC_2.18
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc64/libc.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc64/libc.abilist
index e58a00d..2dc2424 100644
--- a/sysdeps/unix/sysv/linux/powerpc/powerpc64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/powerpc/powerpc64/libc.abilist
@@ -93,6 +93,10 @@ GLIBC_2.18
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  _Exit F
diff --git a/sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist b/sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist
index 0ce7824..eaca001 100644
--- a/sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist
+++ b/sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist
@@ -1987,6 +1987,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist b/sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist
index dc79912..7513fda 100644
--- a/sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist
@@ -1883,6 +1883,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/sh/libc.abilist b/sysdeps/unix/sysv/linux/sh/libc.abilist
index 9ed1b45..c5625b6 100644
--- a/sysdeps/unix/sysv/linux/sh/libc.abilist
+++ b/sysdeps/unix/sysv/linux/sh/libc.abilist
@@ -1867,6 +1867,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist b/sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist
index 5a40ff3..ea7aa50 100644
--- a/sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist
+++ b/sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist
@@ -1978,6 +1978,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist b/sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist
index fa70645..0da655f 100644
--- a/sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist
@@ -1911,6 +1911,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/tile/tilegx/tilegx32/libc.abilist b/sysdeps/unix/sysv/linux/tile/tilegx/tilegx32/libc.abilist
index dd215d7..f968929 100644
--- a/sysdeps/unix/sysv/linux/tile/tilegx/tilegx32/libc.abilist
+++ b/sysdeps/unix/sysv/linux/tile/tilegx/tilegx32/libc.abilist
@@ -2094,3 +2094,7 @@ GLIBC_2.18
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
diff --git a/sysdeps/unix/sysv/linux/tile/tilegx/tilegx64/libc.abilist b/sysdeps/unix/sysv/linux/tile/tilegx/tilegx64/libc.abilist
index ef4747a..4305450 100644
--- a/sysdeps/unix/sysv/linux/tile/tilegx/tilegx64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/tile/tilegx/tilegx64/libc.abilist
@@ -2094,3 +2094,7 @@ GLIBC_2.18
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
diff --git a/sysdeps/unix/sysv/linux/tile/tilepro/libc.abilist b/sysdeps/unix/sysv/linux/tile/tilepro/libc.abilist
index dd215d7..f968929 100644
--- a/sysdeps/unix/sysv/linux/tile/tilepro/libc.abilist
+++ b/sysdeps/unix/sysv/linux/tile/tilepro/libc.abilist
@@ -2094,3 +2094,7 @@ GLIBC_2.18
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
diff --git a/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist b/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
index b377b04..fd7c07e 100644
--- a/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
@@ -1857,6 +1857,10 @@ GLIBC_2.2.6
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
 GLIBC_2.3
  GLIBC_2.3 A
  __ctype_b_loc F
diff --git a/sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist b/sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist
index 5f70329..c632178 100644
--- a/sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist
+++ b/sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist
@@ -2092,3 +2092,7 @@ GLIBC_2.18
 GLIBC_2.22
  GLIBC_2.22 A
  fmemopen F
+GLIBC_2.23
+ GLIBC_2.23 A
+ __explicit_bzero_chk F
+ explicit_bzero F
diff --git a/sysdeps/x86_64/memset.S b/sysdeps/x86_64/memset.S
index e496254..7f1e136 100644
--- a/sysdeps/x86_64/memset.S
+++ b/sysdeps/x86_64/memset.S
@@ -28,6 +28,7 @@ ENTRY(__bzero)
 	jmp	L(entry_from_bzero)
 END(__bzero)
 weak_alias (__bzero, bzero)
+weak_alias (__bzero, explicit_bzero)
 
 /* Like memset but takes additional parameter with return value.  */
 ENTRY(__memset_tail)
@@ -124,6 +125,7 @@ L(between8_16bytes):
 
 END (memset)
 libc_hidden_builtin_def (memset)
+libc_hidden_builtin_def (__memset_chk)
 
 #if defined PIC && IS_IN (libc) && !defined USE_MULTIARCH
 strong_alias (__memset_chk, __memset_zero_constant_len_parameter)
diff --git a/sysdeps/x86_64/multiarch/memset.S b/sysdeps/x86_64/multiarch/memset.S
index 16fefa7..03e62e5 100644
--- a/sysdeps/x86_64/multiarch/memset.S
+++ b/sysdeps/x86_64/multiarch/memset.S
@@ -48,7 +48,7 @@ END(memset)
    The speedup we get from using GPR instruction is likely eaten away
    by the indirect call in the PLT.  */
 #  define libc_hidden_builtin_def(name) \
-	.globl __GI_memset; __GI_memset = __memset_sse2
+	.globl __GI_##name; __GI_##name = name
 #  endif
 
 #  undef strong_alias
-- 
2.5.0


Use explicit_bzero where appropriate

I *believe* these are the only places where memset was being used
to clear buffers containing sensitive data.  The compiler probably
couldn't optimize *all* of them out but it seems best to change them all.

The legacy DES implementation wasn't bothering to clear its buffers,
so I added that, mostly for consistency's sake.

[ChangeLog:]
YYYY-MM-DD  Zack Weinberg <zackw@panix.com>

	* crypt/crypt-entry.c (__crypt_r): Clear key-dependent intermediate
	data before returning, using explicit_bzero.
	* crypt/md5-crypt.c (__md5_crypt_r): Likewise.
	* crypt/sha256-crypt.c (__sha256_crypt_r): Likewise.
	* crypt/sha512-crypt.c (__sha512_crypt_r): Likewise.
---
 crypt/crypt-entry.c  | 11 +++++++++++
 crypt/md5-crypt.c    |  8 ++++----
 crypt/sha256-crypt.c | 14 +++++++-------
 crypt/sha512-crypt.c | 14 +++++++-------
 4 files changed, 29 insertions(+), 18 deletions(-)

diff --git a/crypt/crypt-entry.c b/crypt/crypt-entry.c
index 7e655ba..47db32c 100644
--- a/crypt/crypt-entry.c
+++ b/crypt/crypt-entry.c
@@ -143,6 +143,17 @@ __crypt_r (key, salt, data)
    * And convert back to 6 bit ASCII
    */
   _ufc_output_conversion_r (res[0], res[1], salt, data);
+
+#ifdef _LIBC
+  /*
+   * Erase key-dependent intermediate data.  Data dependent only on
+   * the salt is not considered sensitive.
+   */
+  explicit_bzero (ktab, sizeof (ktab));
+  explicit_bzero (data->keysched, sizeof (data->keysched));
+  explicit_bzero (res, sizeof (res));
+#endif
+
   return data->crypt_3_buf;
 }
 weak_alias (__crypt_r, crypt_r)
diff --git a/crypt/md5-crypt.c b/crypt/md5-crypt.c
index 1b890bc..b95766d 100644
--- a/crypt/md5-crypt.c
+++ b/crypt/md5-crypt.c
@@ -292,13 +292,13 @@ __md5_crypt_r (key, salt, buffer, buflen)
 #ifndef USE_NSS
   __md5_init_ctx (&ctx);
   __md5_finish_ctx (&ctx, alt_result);
-  memset (&ctx, '\0', sizeof (ctx));
-  memset (&alt_ctx, '\0', sizeof (alt_ctx));
+  explicit_bzero (&ctx, sizeof (ctx));
+  explicit_bzero (&alt_ctx, sizeof (alt_ctx));
 #endif
   if (copied_key != NULL)
-    memset (copied_key, '\0', key_len);
+    explicit_bzero (copied_key, key_len);
   if (copied_salt != NULL)
-    memset (copied_salt, '\0', salt_len);
+    explicit_bzero (copied_salt, salt_len);
 
   free (free_key);
   return buffer;
diff --git a/crypt/sha256-crypt.c b/crypt/sha256-crypt.c
index d90e291..189183a 100644
--- a/crypt/sha256-crypt.c
+++ b/crypt/sha256-crypt.c
@@ -375,16 +375,16 @@ __sha256_crypt_r (key, salt, buffer, buflen)
 #ifndef USE_NSS
   __sha256_init_ctx (&ctx);
   __sha256_finish_ctx (&ctx, alt_result);
-  memset (&ctx, '\0', sizeof (ctx));
-  memset (&alt_ctx, '\0', sizeof (alt_ctx));
+  explicit_bzero (&ctx, sizeof (ctx));
+  explicit_bzero (&alt_ctx, sizeof (alt_ctx));
 #endif
-  memset (temp_result, '\0', sizeof (temp_result));
-  memset (p_bytes, '\0', key_len);
-  memset (s_bytes, '\0', salt_len);
+  explicit_bzero (temp_result, sizeof (temp_result));
+  explicit_bzero (p_bytes, key_len);
+  explicit_bzero (s_bytes, salt_len);
   if (copied_key != NULL)
-    memset (copied_key, '\0', key_len);
+    explicit_bzero (copied_key, key_len);
   if (copied_salt != NULL)
-    memset (copied_salt, '\0', salt_len);
+    explicit_bzero (copied_salt, salt_len);
 
   free (free_key);
   free (free_pbytes);
diff --git a/crypt/sha512-crypt.c b/crypt/sha512-crypt.c
index 9c581ab..bbd5e24 100644
--- a/crypt/sha512-crypt.c
+++ b/crypt/sha512-crypt.c
@@ -397,16 +397,16 @@ __sha512_crypt_r (key, salt, buffer, buflen)
 #ifndef USE_NSS
   __sha512_init_ctx (&ctx);
   __sha512_finish_ctx (&ctx, alt_result);
-  memset (&ctx, '\0', sizeof (ctx));
-  memset (&alt_ctx, '\0', sizeof (alt_ctx));
+  explicit_bzero (&ctx, sizeof (ctx));
+  explicit_bzero (&alt_ctx, sizeof (alt_ctx));
 #endif
-  memset (temp_result, '\0', sizeof (temp_result));
-  memset (p_bytes, '\0', key_len);
-  memset (s_bytes, '\0', salt_len);
+  explicit_bzero (temp_result, sizeof (temp_result));
+  explicit_bzero (p_bytes, key_len);
+  explicit_bzero (s_bytes, salt_len);
   if (copied_key != NULL)
-    memset (copied_key, '\0', key_len);
+    explicit_bzero (copied_key, key_len);
   if (copied_salt != NULL)
-    memset (copied_salt, '\0', salt_len);
+    explicit_bzero (copied_salt, salt_len);
 
   free (free_key);
   free (free_pbytes);
-- 
2.5.0

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