1BZERO(3) Linux Programmer's Manual BZERO(3)
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6 bzero, explicit_bzero - zero a byte string
7
9 #include <strings.h>
10 void bzero(void *s, size_t n);
12 #include <string.h>
14 void explicit_bzero(void *s, size_t n);
16
18 The bzero() function erases the data in the n bytes of the memory
19 starting at the location pointed to by s, by writing zeros (bytes con‐
20 taining '\0') to that area.
21 The explicit_bzero() function performs the same task as bzero(). It
23 differs from bzero() in that it guarantees that compiler optimizations
24 will not remove the erase operation if the compiler deduces that the
25 operation is "unnecessary".
26
28 None.
29
31 explicit_bzero() first appeared in glibc 2.25.
32
34 For an explanation of the terms used in this section, see at‐
35 tributes(7).
36 ┌─────────────────┬───────────────┬─────────┐
38 │Interface │ Attribute │ Value │
39 ├─────────────────┼───────────────┼─────────┤
40 │bzero(), │ Thread safety │ MT-Safe │
41 │explicit_bzero() │ │ │
42 └─────────────────┴───────────────┴─────────┘
44 The bzero() function is deprecated (marked as LEGACY in POSIX.1-2001);
45 use memset(3) in new programs. POSIX.1-2008 removes the specification
46 of bzero(). The bzero() function first appeared in 4.3BSD.
47 The explicit_bzero() function is a nonstandard extension that is also
49 present on some of the BSDs. Some other implementations have a similar
50 function, such as memset_explicit() or memset_s().
51
53 The explicit_bzero() function addresses a problem that security-con‐
54 scious applications may run into when using bzero(): if the compiler
55 can deduce that the location to zeroed will never again be touched by a
56 correct program, then it may remove the bzero() call altogether. This
57 is a problem if the intent of the bzero() call was to erase sensitive
58 data (e.g., passwords) to prevent the possibility that the data was
59 leaked by an incorrect or compromised program. Calls to ex‐
60 plicit_bzero() are never optimized away by the compiler.
61 The explicit_bzero() function does not solve all problems associated
63 with erasing sensitive data:
64 1. The explicit_bzero() function does not guarantee that sensitive data
66 is completely erased from memory. (The same is true of bzero().)
67 For example, there may be copies of the sensitive data in a register
68 and in "scratch" stack areas. The explicit_bzero() function is not
69 aware of these copies, and can't erase them.
70 2. In some circumstances, explicit_bzero() can decrease security. If
72 the compiler determined that the variable containing the sensitive
73 data could be optimized to be stored in a register (because it is
74 small enough to fit in a register, and no operation other than the
75 explicit_bzero() call would need to take the address of the vari‐
76 able), then the explicit_bzero() call will force the data to be
77 copied from the register to a location in RAM that is then immedi‐
78 ately erased (while the copy in the register remains unaffected).
79 The problem here is that data in RAM is more likely to be exposed by
80 a bug than data in a register, and thus the explicit_bzero() call
81 creates a brief time window where the sensitive data is more vulner‐
82 able than it would otherwise have been if no attempt had been made
83 to erase the data.
84 Note that declaring the sensitive variable with the volatile qualifier
86 does not eliminate the above problems. Indeed, it will make them
87 worse, since, for example, it may force a variable that would otherwise
88 have been optimized into a register to instead be maintained in (more
89 vulnerable) RAM for its entire lifetime.
90 Notwithstanding the above details, for security-conscious applications,
92 using explicit_bzero() is generally preferable to not using it. The
93 developers of explicit_bzero() anticipate that future compilers will
94 recognize calls to explicit_bzero() and take steps to ensure that all
95 copies of the sensitive data are erased, including copies in registers
96 or in "scratch" stack areas.
97
99 bstring(3), memset(3), swab(3)
100
102 This page is part of release 5.10 of the Linux man-pages project. A
103 description of the project, information about reporting bugs, and the
104 latest version of this page, can be found at
105 https://www.kernel.org/doc/man-pages/.
106Linux 2019-03-06 BZERO(3)