explicit_bzero(3) — Linux manual page


BZERO(3)                  Linux Programmer's Manual                 BZERO(3)

NAME         top

       bzero, explicit_bzero - zero a byte string

SYNOPSIS         top

       #include <strings.h>

       void bzero(void *s, size_t n);

       #include <string.h>

       void explicit_bzero(void *s, size_t n);

DESCRIPTION         top

       The bzero() function erases the data in the n bytes of the memory
       starting at the location pointed to by s, by writing zeros (bytes
       containing '\0') to that area.

       The explicit_bzero() function performs the same task as bzero().  It
       differs from bzero() in that it guarantees that compiler
       optimizations will not remove the erase operation if the compiler
       deduces that the operation is "unnecessary".

RETURN VALUE         top


VERSIONS         top

       explicit_bzero() first appeared in glibc 2.25.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface        Attribute     Value   │
       │bzero(),         │ Thread safety │ MT-Safe │
       │explicit_bzero() │               │         │

CONFORMING TO         top

       The bzero() function is deprecated (marked as LEGACY in
       POSIX.1-2001); use memset(3) in new programs.  POSIX.1-2008 removes
       the specification of bzero().  The bzero() function first appeared in

       The explicit_bzero() function is a nonstandard extension that is also
       present on some of the BSDs.  Some other implementations have a
       similar function, such as memset_explicit() or memset_s().

NOTES         top

       The explicit_bzero() function addresses a problem that security-
       conscious applications may run into when using bzero(): if the
       compiler can deduce that the location to zeroed will never again be
       touched by a correct program, then it may remove the bzero() call
       altogether.  This is a problem if the intent of the bzero() call was
       to erase sensitive data (e.g., passwords) to prevent the possibility
       that the data was leaked by an incorrect or compromised program.
       Calls to explicit_bzero() are never optimized away by the compiler.

       The explicit_bzero() function does not solve all problems associated
       with erasing sensitive data:

       1. The explicit_bzero() function does not guarantee that sensitive
          data is completely erased from memory.  (The same is true of
          bzero().)  For example, there may be copies of the sensitive data
          in a register and in "scratch" stack areas.  The explicit_bzero()
          function is not aware of these copies, and can't erase them.

       2. In some circumstances, explicit_bzero() can decrease security.  If
          the compiler determined that the variable containing the sensitive
          data could be optimized to be stored in a register (because it is
          small enough to fit in a register, and no operation other than the
          explicit_bzero() call would need to take the address of the
          variable), then the explicit_bzero() call will force the data to
          be copied from the register to a location in RAM that is then
          immediately erased (while the copy in the register remains
          unaffected).  The problem here is that data in RAM is more likely
          to be exposed by a bug than data in a register, and thus the
          explicit_bzero() call creates a brief time window where the
          sensitive data is more vulnerable than it would otherwise have
          been if no attempt had been made to erase the data.

       Note that declaring the sensitive variable with the volatile
       qualifier does not eliminate the above problems.  Indeed, it will
       make them worse, since, for example, it may force a variable that
       would otherwise have been optimized into a register to instead be
       maintained in (more vulnerable) RAM for its entire lifetime.

       Notwithstanding the above details, for security-conscious
       applications, using explicit_bzero() is generally preferable to not
       using it.  The developers of explicit_bzero() anticipate that future
       compilers will recognize calls to explicit_bzero() and take steps to
       ensure that all copies of the sensitive data are erased, including
       copies in registers or in "scratch" stack areas.

SEE ALSO         top

       bstring(3), memset(3), swab(3)

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2019-03-06                         BZERO(3)