BZERO
Section: Linux Programmer's Manual (3)
Updated: 2021-03-22
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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 5.11 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- VERSIONS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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Time: 06:22:48 GMT, May 09, 2021