Section (3) bzero
bzero, explicit_bzero — zero a byte string
bzero() function erases
the data in the
bytes of the memory starting at the location pointed to by
s, by writing zeros
(bytes containing _zsingle_quotesz_ _zsingle_quotesz_) to that area.
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.
For an explanation of the terms used in this section, see attributes(7).
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() function first appeared in
function is a nonstandard extension that is also present on
some of the BSDs. Some other implementations have a similar
function, such as
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
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.
function does not solve all problems associated with erasing
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_zsingle_quotesz_t erase them.
In some circumstances,
decreasesecurity. 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
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.
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