bzero

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



NAME
       bzero, explicit_bzero - zero a byte string

SYNOPSIS
       #include <strings.h>

       void bzero(void *s, size_t n);

       #include <string.h>

       void explicit_bzero(void *s, size_t n);

DESCRIPTION
       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
       None.

VERSIONS
       explicit_bzero() first appeared in glibc 2.25.

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

COLOPHON
       This page is part of release 5.12 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)