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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
BZERO(3) Linux Programmer's Manual BZERO(3)
bzero, explicit_bzero - zero a byte string
#include <strings.h>
void bzero(void *s, size_t n);
#include <string.h>
void explicit_bzero(void *s, size_t n);
The bzero() function erases the data in the n bytes of the memory
starting at the location pointed to by s, by writing zeroes (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".
None.
explicit_bzero() first appeared in glibc 2.25.
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├─────────────────┼───────────────┼─────────┤
│bzero(), │ Thread safety │ MT-Safe │
│explicit_bzero() │ │ │
└─────────────────┴───────────────┴─────────┘
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().
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.
bstring(3), memset(3), swab(3)
This page is part of release 4.12 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 2017-05-03 BZERO(3)
Pages that refer to this page: bstring(3), memset(3)