bzero(3) — Linux manual page


bzero(3)                Library Functions Manual                bzero(3)

NAME         top

       bzero, explicit_bzero - zero a byte string

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <strings.h>

       void bzero(void s[.n], size_t n);

       #include <string.h>

       void explicit_bzero(void s[.n], 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


ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ bzero(), explicit_bzero()           │ Thread safety │ MT-Safe │

STANDARDS         top


HISTORY         top

              glibc 2.25.

              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().


              Marked as LEGACY in POSIX.1-2001.  Removed in

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 be 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:

       •  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.

       •  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

SEE ALSO         top

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

Linux man-pages (unreleased)     (date)                         bzero(3)

Pages that refer to this page: bstring(3)memset(3)string_copying(7)