1Lexical::SealRequireHinUtsse(r3)Contributed Perl DocumenLteaxtiicoanl::SealRequireHints(3)
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6 Lexical::SealRequireHints - prevent leakage of lexical hints
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9 use Lexical::SealRequireHints;
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12 This module works around two historical bugs in Perl's handling of the
13 "%^H" (lexical hints) variable. One bug causes lexical state in one
14 file to leak into another that is "require"d/"use"d from it. This bug,
15 [perl #68590], was present from Perl 5.6 up to Perl 5.10, fixed in Perl
16 5.11.0. The second bug causes lexical state (normally a blank "%^H"
17 once the first bug is fixed) to leak outwards from "utf8.pm", if it is
18 automatically loaded during Unicode regular expression matching, into
19 whatever source is compiling at the time of the regexp match. This
20 bug, [perl #73174], was present from Perl 5.8.7 up to Perl 5.11.5,
21 fixed in Perl 5.12.0.
22 Both of these bugs seriously damage the usability of any module relying
24 on "%^H" for lexical scoping, on the affected Perl versions. It is in
25 practice essential to work around these bugs when using such modules.
26 On versions of Perl that require such a workaround, this module
27 globally changes the behaviour of "require", including "use" and the
28 implicit "require" performed in Unicode regular expression matching, so
29 that it no longer exhibits these bugs.
30 The workaround supplied by this module takes effect the first time its
32 "import" method is called. Typically this will be done by means of a
33 "use" statement. This should be done as early as possible, because it
34 only affects "require"/"use" statements that are compiled after the
35 workaround goes into effect. For "use" statements, and "require"
36 statements that are executed immediately and only once, it suffices to
37 invoke the workaround when loading the first module that will set up
38 vulnerable lexical state. Delayed-action "require" statements,
39 however, are more troublesome, and can require the workaround to be
40 loaded much earlier. Ultimately, an affected Perl program may need to
41 load the workaround as very nearly its first action. Invoking this
42 module multiple times, from multiple modules, is not a problem: the
43 workaround is only applied once, and applies to everything subsequently
44 compiled.
45 This module is implemented in XS, with a pure Perl backup version for
47 systems that can't handle XS modules. The XS version has a better
48 chance of playing nicely with other modules that modify "require"
49 handling. The pure Perl version can't work at all on some Perl
50 versions; users of those versions must use the XS. On all Perl
51 versions suffering the underlying hint leakage bug, pure Perl hooking
52 of "require" breaks the use of "require" without an explicit parameter
53 (implicitly using $_).
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56 The history of the "%^H" bugs is complex. Here is a chronological
57 statement of the relevant changes.
58 Perl 5.6.0
60 "%^H" introduced. It exists only as a hash at compile time. It is
61 not localised by "require", so lexical hints leak into every module
62 loaded, which is bug [perl #68590].
63 The "CORE::GLOBAL" mechanism doesn't work cleanly for "require",
65 because overriding "require" loses the necessary special parsing of
66 bareword arguments to it. As a result, pure Perl code can't
67 properly globally affect the behaviour of "require". Pure Perl
68 code can localise "%^H" itself for any particular "require"
69 invocation, but a global fix is only possible through XS.
70 Perl 5.7.2
72 The "CORE::GLOBAL" mechanism now works cleanly for "require", so
73 pure Perl code can globally affect the behaviour of "require" to
74 achieve a global fix for the bug.
75 Perl 5.8.7
77 When "utf8.pm" is automatically loaded during Unicode regular
78 expression matching, "%^H" now leaks outward from it into whatever
79 source is compiling at the time of the regexp match, which is bug
80 [perl #73174]. It often goes unnoticed, because [perl #68590]
81 makes "%^H" leak into "utf8.pm" which then doesn't modify it, so
82 what leaks out tends to be identical to what leaked in. If [perl
83 #68590] is worked around, however, "%^H" tends to be (correctly)
84 blank inside "utf8.pm", and this bug therefore blanks it for the
85 outer module.
86 Perl 5.9.4
88 "%^H" now exists in two forms. In addition to the relatively
89 ordinary hash that is modified during compilation, the value that
90 it had at each point in compilation is recorded in the compiled op
91 tree, for later examination at runtime. It is in a special
92 representation-sharing format, and writes to "%^H" are meant to be
93 performed on both forms. "require" does not localise the runtime
94 form of "%^H" (and still doesn't localise the compile-time form).
95 A couple of special "%^H" entries are erroneously written only to
97 the runtime form.
98 Pure Perl code, although it can localise the compile-time "%^H" by
100 normal means, can't adequately localise the runtime "%^H", except
101 by using a string eval stack frame. This makes a satisfactory
102 global fix for the leakage bug impossible in pure Perl.
103 Perl 5.10.1
105 "require" now properly localises the runtime form of "%^H", but
106 still not the compile-time form.
107 A global fix is once again possible in pure Perl, because the fix
109 only needs to localise the compile-time form.
110 Perl 5.11.0
112 "require" now properly localises both forms of "%^H", fixing [perl
113 #68590]. This makes [perl #73174] apparent without any workaround
114 for [perl #68590].
115 The special "%^H" entries are now correctly written to both forms
117 of the hash.
118 Perl 5.12.0
120 The automatic loading of "utf8.pm" during Unicode regular
121 expression matching now properly restores "%^H", fixing [perl
122 #73174].
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125 The operation of this module depends on influencing the compilation of
126 "require". As a result, it cannot prevent lexical state leakage
127 through a "require" statement that was compiled before this module was
128 invoked. Where problems occur, this module must be invoked earlier.
129 On all Perl versions that need a fix for the lexical hint leakage bug,
131 the pure Perl implementation of this module unavoidably breaks the use
132 of "require" without an explicit parameter (implicitly using $_). This
133 is due to another bug in the Perl core, fixed in Perl 5.15.5, and is
134 inherent to the mechanism by which pure Perl code can hook "require".
135 The use of implicit $_ with "require" is rare, so although this state
136 of affairs is faulty it will actually work for most programs. Perl
137 versions 5.12.0 and greater, despite having the "require" hooking bug,
138 don't actually exhibit a problem with the pure Perl version of this
139 module, because with the lexical hint leakage bug fixed there is no
140 need for this module to hook "require".
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143 perlpragma
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146 Andrew Main (Zefram) <zefram@fysh.org>
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149 Copyright (C) 2009, 2010, 2011, 2012, 2015, 2016, 2017 Andrew Main
150 (Zefram) <zefram@fysh.org>
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153 This module is free software; you can redistribute it and/or modify it
154 under the same terms as Perl itself.
155perl v5.34.0 2022-01-21 Lexical::SealRequireHints(3)