-.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
+.\" Copyright (C) 2017 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
+.\" %%%LICENSE_START(VERBATIM)
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.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
-.\"
+.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" have taken the same level of care in the production of this manual,
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+.\" %%%LICENSE_END
.\"
-.\" References consulted:
-.\" Linux libc source code
-.\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
-.\" 386BSD man pages
-.\" Modified Sat Jul 24 21:28:17 1993 by Rik Faith <faith@cs.unc.edu>
-.\" Modified Tue Oct 22 23:49:37 1996 by Eric S. Raymond <esr@thyrsus.com>
-.TH BZERO 3 2002-12-31 "Linux" "Linux Programmer's Manual"
+.TH BZERO 3 2019-03-06 "Linux" "Linux Programmer's Manual"
.SH NAME
-bzero \- write zero bytes
+bzero, explicit_bzero \- zero a byte string
.SH SYNOPSIS
.nf
.B #include <strings.h>
-.sp
+.PP
.BI "void bzero(void *" s ", size_t " n );
+.PP
+.B #include <string.h>
+.PP
+.BI "void explicit_bzero(void *" s ", size_t " n );
.fi
.SH DESCRIPTION
The
-.B bzero()
-function sets the first
+.BR bzero ()
+function erases the data in the
.I n
-bytes of the byte area starting at
-.I s
-to zero.
-.SH "RETURN VALUE"
+bytes of the memory starting at the location pointed to by
+.IR s ,
+by writing zeros (bytes containing \(aq\e0\(aq) to that area.
+.PP
+The
+.BR explicit_bzero ()
+function performs the same task as
+.BR bzero ().
+It differs from
+.BR bzero ()
+in that it guarantees that compiler optimizations will not remove the
+erase operation if the compiler deduces that the operation is "unnecessary".
+.SH RETURN VALUE
None.
-.SH "CONFORMING TO"
-4.3BSD. This function is deprecated -- use
-.BR memset
-in new programs.
-.SH "SEE ALSO"
+.SH VERSIONS
+.BR explicit_bzero ()
+first appeared in glibc 2.25.
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lb lb lb
+l l l.
+Interface Attribute Value
+T{
+.BR bzero (),
+.br
+.BR explicit_bzero ()
+T} Thread safety MT-Safe
+.TE
+.SH CONFORMING TO
+The
+.BR bzero ()
+function is deprecated (marked as LEGACY in POSIX.1-2001); use
+.BR memset (3)
+in new programs.
+POSIX.1-2008 removes the specification of
+.BR bzero ().
+The
+.BR bzero ()
+function first appeared in 4.3BSD.
+.PP
+The
+.BR 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
+.BR memset_explicit ()
+or
+.BR memset_s ().
+.SH NOTES
+The
+.BR explicit_bzero ()
+function addresses a problem that security-conscious applications
+may run into when using
+.BR bzero ():
+if the compiler can deduce that the location to zeroed will
+never again be touched by a
+.I correct
+program, then it may remove the
+.BR bzero ()
+call altogether.
+This is a problem if the intent of the
+.BR 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
+.BR explicit_bzero ()
+are never optimized away by the compiler.
+.PP
+The
+.BR explicit_bzero ()
+function does not solve all problems associated with erasing sensitive data:
+.IP 1. 3
+The
+.BR explicit_bzero ()
+function does
+.I not
+guarantee that sensitive data is completely erased from memory.
+(The same is true of
+.BR bzero ().)
+For example, there may be copies of the sensitive data in
+a register and in "scratch" stack areas.
+The
+.BR explicit_bzero ()
+function is not aware of these copies, and can't erase them.
+.IP 2.
+In some circumstances,
+.BR explicit_bzero ()
+can
+.I 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
+.BR explicit_bzero ()
+call would need to take the address of the variable), then the
+.BR 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
+.BR 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.
+.PP
+Note that declaring the sensitive variable with the
+.B volatile
+qualifier does
+.I 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.
+.PP
+Notwithstanding the above details, for security-conscious applications, using
+.BR explicit_bzero ()
+is generally preferable to not using it.
+The developers of
+.BR explicit_bzero ()
+anticipate that future compilers will recognize calls to
+.BR 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.
+.SH SEE ALSO
+.BR bstring (3),
.BR memset (3),
.BR swab (3)
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