man3/bzero.3

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  25 .TH BZERO 3  2017-03-13 "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 .sp
  32 .BI "void bzero(void *" s ", size_t " n );
  33
  34 .BI "void explicit_bzero(void *" s ", size_t " n );
  35 .fi
  36 .SH DESCRIPTION
  37 The
  38 .BR bzero ()
  39 function erases the data in the
  40 .I n
  41 bytes of the memory starting at the location pointed to by
  42 .IR s ,
  43 by writing zeroes (bytes containing \(aq\\0\(aq) to that area.
  44
  45 The
  46 .BR explicit_bzero ()
  47 function performs the same task as
  48 .BR bzero ().
  49 It differs from
  50 .BR bzero ()
  51 in that it guarantees that compiler optimizations will not remove the
  52 erase operation if the compiler deduces that the operation is "unnecessary".
  53 .SH RETURN VALUE
  54 None.
  55 .SH VERSIONS
  56 .BR explicit_bzero ()
  57 first appeared in glibc 2.25.
  58 .SH ATTRIBUTES
  59 For an explanation of the terms used in this section, see
  60 .BR attributes (7).
  61 .TS
  62 allbox;
  63 lb lb lb
  64 l l l.
  65 Interface       Attribute       Value
  66 T{
  67 .BR bzero (),
  68 .br
  69 .BR explicit_bzero ()
  70 T}      Thread safety   MT-Safe
  71 .TE
  72 .SH CONFORMING TO
  73 The
  74 .BR bzero ()
  75 function is deprecated (marked as LEGACY in POSIX.1-2001); use
  76 .BR memset (3)
  77 in new programs.
  78 POSIX.1-2008 removes the specification of
  79 .BR bzero ().
  80 The
  81 .BR bzero ()
  82 function first appeared in 4.3BSD.
  83
  84 The
  85 .BR explicit_bzero ()
  86 function is a nonstandard extension that is also present on some of the BSDs.
  87 Some other implementations have a similar function, such as
  88 .BR memset_explicit ()
  89 or
  90 .BR memset_s ().
  91 .SH NOTES
  92 The
  93 .BR explicit_bzero ()
  94 function addresses a problem that security-conscious applications
  95 may run into when using
  96 .BR bzero ():
  97 if the compiler can deduce that the location to zeroed will
  98 never again be touched by a
  99 .I correct
 100 program, then it may remove the
 101 .BR bzero ()
 102 call altogether.
 103 This is a problem if the intent of the
 104 .BR bzero ()
 105 call was to erase sensitive data (e.g., passwords)
 106 to prevent the possibility that the data was leaked
 107 by an incorrect or compromised program.
 108 Calls to
 109 .BR explicit_bzero ()
 110 are never optimized away by the compiler.
 111
 112 The
 113 .BR explicit_bzero ()
 114 function does not solve all problems associated with erasing sensitive data:
 115 .IP 1. 3
 116 The
 117 .BR explicit_bzero ()
 118 function does
 119 .I not
 120 guarantee that sensitive data is completely erased from memory.
 121 (The same is true of
 122 .BR bzero ().)
 123 For example, there may be copies of the sensitive data in
 124 a register and in "scratch" stack areas.
 125 The
 126 .BR explicit_bzero ()
 127 function is not aware of these copies, and can't erase them.
 128 .IP 2.
 129 In some circumstances,
 130 .BR explicit_bzero ()
 131 can
 132 .I decrease
 133 security.
 134 If the compiler determined that the variable containing the
 135 sensitive data could be optimized to be stored in a register
 136 (because it is small enough to fit in a register,
 137 and no operation other than the
 138 .BR explicit_bzero ()
 139 call would need to take the address of the variable), then the
 140 .BR explicit_bzero ()
 141 call will force the data to be copied from the register
 142 to a location in RAM that is then immediately erased
 143 (while the copy in the register remains unaffected).
 144 The problem here is that data in RAM is more likely to be exposed
 145 by a bug than data in a register, and thus the
 146 .BR explicit_bzero ()
 147 call creates a brief time window where the sensitive data is more
 148 vulnerable than it would otherwise have been
 149 if no attempt had been made to erase the data.
 150 .PP
 151 Note that declaring the sensitive variable with the
 152 .B volatile
 153 qualifier does
 154 .I not
 155 eliminate the above problems.
 156 Indeed, it will make them worse, since, for example,
 157 it may force a variable that would otherwise have been optimized
 158 into a register to instead be maintained in (more vulnerable)
 159 RAM for its entire lifetime.
 160
 161 Notwithstanding the above details, for security-conscious applications, using
 162 .BR explicit_bzero ()
 163 is generally preferable to not using it.
 164 The developers of
 165 .BR explicit_bzero ()
 166 anticipate that future compilers will recognize calls to
 167 .BR explicit_bzero ()
 168 and take steps to ensure that all copies of the sensitive data are erased,
 169 including copies in registers or in "scratch" stack areas.
 170 .SH SEE ALSO
 171 .BR bstring (3),
 172 .BR memset (3),
 173 .BR swab (3)