man3/bzero.3

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  25 .TH BZERO 3  2017-09-15 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 bzero, explicit_bzero \- zero a byte string
  28 .SH SYNOPSIS
  29 .nf
  30 .B #include <strings.h>
  31 .PP
  32 .BI "void bzero(void *" s ", size_t " n );
  33 .PP
  34 .B #include <string.h>
  35 .PP
  36 .BI "void explicit_bzero(void *" s ", size_t " n );
  37 .fi
  38 .SH DESCRIPTION
  39 The
  40 .BR bzero ()
  41 function erases the data in the
  42 .I n
  43 bytes of the memory starting at the location pointed to by
  44 .IR s ,
  45 by writing zeros (bytes containing \(aq\\0\(aq) to that area.
  46 .PP
  47 The
  48 .BR explicit_bzero ()
  49 function performs the same task as
  50 .BR bzero ().
  51 It differs from
  52 .BR bzero ()
  53 in that it guarantees that compiler optimizations will not remove the
  54 erase operation if the compiler deduces that the operation is "unnecessary".
  55 .SH RETURN VALUE
  56 None.
  57 .SH VERSIONS
  58 .BR explicit_bzero ()
  59 first appeared in glibc 2.25.
  60 .SH ATTRIBUTES
  61 For an explanation of the terms used in this section, see
  62 .BR attributes (7).
  63 .TS
  64 allbox;
  65 lb lb lb
  66 l l l.
  67 Interface       Attribute       Value
  68 T{
  69 .BR bzero (),
  70 .br
  71 .BR explicit_bzero ()
  72 T}      Thread safety   MT-Safe
  73 .TE
  74 .SH CONFORMING TO
  75 The
  76 .BR bzero ()
  77 function is deprecated (marked as LEGACY in POSIX.1-2001); use
  78 .BR memset (3)
  79 in new programs.
  80 POSIX.1-2008 removes the specification of
  81 .BR bzero ().
  82 The
  83 .BR bzero ()
  84 function first appeared in 4.3BSD.
  85 .PP
  86 The
  87 .BR explicit_bzero ()
  88 function is a nonstandard extension that is also present on some of the BSDs.
  89 Some other implementations have a similar function, such as
  90 .BR memset_explicit ()
  91 or
  92 .BR memset_s ().
  93 .SH NOTES
  94 The
  95 .BR explicit_bzero ()
  96 function addresses a problem that security-conscious applications
  97 may run into when using
  98 .BR bzero ():
  99 if the compiler can deduce that the location to zeroed will
 100 never again be touched by a
 101 .I correct
 102 program, then it may remove the
 103 .BR bzero ()
 104 call altogether.
 105 This is a problem if the intent of the
 106 .BR bzero ()
 107 call was to erase sensitive data (e.g., passwords)
 108 to prevent the possibility that the data was leaked
 109 by an incorrect or compromised program.
 110 Calls to
 111 .BR explicit_bzero ()
 112 are never optimized away by the compiler.
 113 .PP
 114 The
 115 .BR explicit_bzero ()
 116 function does not solve all problems associated with erasing sensitive data:
 117 .IP 1. 3
 118 The
 119 .BR explicit_bzero ()
 120 function does
 121 .I not
 122 guarantee that sensitive data is completely erased from memory.
 123 (The same is true of
 124 .BR bzero ().)
 125 For example, there may be copies of the sensitive data in
 126 a register and in "scratch" stack areas.
 127 The
 128 .BR explicit_bzero ()
 129 function is not aware of these copies, and can't erase them.
 130 .IP 2.
 131 In some circumstances,
 132 .BR explicit_bzero ()
 133 can
 134 .I decrease
 135 security.
 136 If the compiler determined that the variable containing the
 137 sensitive data could be optimized to be stored in a register
 138 (because it is small enough to fit in a register,
 139 and no operation other than the
 140 .BR explicit_bzero ()
 141 call would need to take the address of the variable), then the
 142 .BR explicit_bzero ()
 143 call will force the data to be copied from the register
 144 to a location in RAM that is then immediately erased
 145 (while the copy in the register remains unaffected).
 146 The problem here is that data in RAM is more likely to be exposed
 147 by a bug than data in a register, and thus the
 148 .BR explicit_bzero ()
 149 call creates a brief time window where the sensitive data is more
 150 vulnerable than it would otherwise have been
 151 if no attempt had been made to erase the data.
 152 .PP
 153 Note that declaring the sensitive variable with the
 154 .B volatile
 155 qualifier does
 156 .I not
 157 eliminate the above problems.
 158 Indeed, it will make them worse, since, for example,
 159 it may force a variable that would otherwise have been optimized
 160 into a register to instead be maintained in (more vulnerable)
 161 RAM for its entire lifetime.
 162 .PP
 163 Notwithstanding the above details, for security-conscious applications, using
 164 .BR explicit_bzero ()
 165 is generally preferable to not using it.
 166 The developers of
 167 .BR explicit_bzero ()
 168 anticipate that future compilers will recognize calls to
 169 .BR explicit_bzero ()
 170 and take steps to ensure that all copies of the sensitive data are erased,
 171 including copies in registers or in "scratch" stack areas.
 172 .SH SEE ALSO
 173 .BR bstring (3),
 174 .BR memset (3),
 175 .BR swab (3)