bzero(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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

       None.

VERSIONS         top

       explicit_bzero() first appeared in glibc 2.25.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌──────────────────────────────────────┬───────────────┬─────────┐
       │Interface                             Attribute     Value   │
       ├──────────────────────────────────────┼───────────────┼─────────┤
       │bzero(), explicit_bzero()             │ Thread safety │ MT-Safe │
       └──────────────────────────────────────┴───────────────┴─────────┘

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 4.3BSD.

       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 5.11 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
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                       BZERO(3)

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