BZERO(3) Linux Programmer's Manual BZERO(3)
bzero, explicit_bzero - zero a byte string
void bzero(void *s, size_t n);
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 con-
taining '\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".
explicit_bzero() first appeared in glibc 2.25.
For an explanation of the terms used in this section, see
|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-con-
scious 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 vari-
able), then the explicit_bzero() call will force the data to be
copied from the register to a location in RAM that is then immedi-
ately 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 vulner-
able 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)
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