1Lexical::SealRequireHinUtsse(r3)Contributed Perl DocumenLteaxtiicoanl::SealRequireHints(3)
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NAME

6       Lexical::SealRequireHints - prevent leakage of lexical hints
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SYNOPSIS

9           use Lexical::SealRequireHints;
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DESCRIPTION

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/"do"ed from it.
15       This bug, [perl #68590], was present from Perl 5.6 up to Perl 5.10,
16       fixed in Perl 5.11.0.  The second bug causes lexical state (normally a
17       blank "%^H" once the first bug is fixed) to leak outwards from
18       "utf8.pm", if it is automatically loaded during Unicode regular
19       expression matching, into whatever source is compiling at the time of
20       the regexp match.  This bug, [perl #73174], was present from Perl 5.8.7
21       up to Perl 5.11.5, fixed in Perl 5.12.0.
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23       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,
29       and of "do", so that they no longer exhibit these bugs.
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31       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"/"do" statements that are compiled after
35       the workaround goes into effect.  For "use" statements, and "require"
36       and "do" statements that are executed immediately and only once, it
37       suffices to invoke the workaround when loading the first module that
38       will set up vulnerable lexical state.  Delayed-action "require" and
39       "do" statements, however, are more troublesome, and can require the
40       workaround to be loaded much earlier.  Ultimately, an affected Perl
41       program may need to load the workaround as very nearly its first
42       action.  Invoking this module multiple times, from multiple modules, is
43       not a problem: the workaround is only applied once, and applies to
44       everything subsequently compiled.
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46       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" or
49       "do" 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 $_).
54

PERL VERSION DIFFERENCES

56       The history of the "%^H" bugs is complex.  Here is a chronological
57       statement of the relevant changes.
58
59       Perl 5.6.0
60           "%^H" introduced.  It exists only as a hash at compile time.  It is
61           not localised by "require"/"do", so lexical hints leak into every
62           module loaded, which is bug [perl #68590].
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64           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.
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71       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
76       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.
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87       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"/"do" does not localise the
94           runtime form of "%^H" (and still doesn't localise the compile-time
95           form).
96
97           A couple of special "%^H" entries are erroneously written only to
98           the runtime form.
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100           Pure Perl code, although it can localise the compile-time "%^H" by
101           normal means, can't adequately localise the runtime "%^H", except
102           by using a string eval stack frame.  This makes a satisfactory
103           global fix for the leakage bug impossible in pure Perl.
104
105       Perl 5.10.1
106           "require"/"do" now properly localise the runtime form of "%^H", but
107           still not the compile-time form.
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109           A global fix is once again possible in pure Perl, because the fix
110           only needs to localise the compile-time form.
111
112       Perl 5.11.0
113           "require"/"do" now properly localise both forms of "%^H", fixing
114           [perl #68590].  This makes [perl #73174] apparent without any
115           workaround for [perl #68590].
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117           The special "%^H" entries are now correctly written to both forms
118           of the hash.
119
120       Perl 5.12.0
121           The automatic loading of "utf8.pm" during Unicode regular
122           expression matching now properly restores "%^H", fixing [perl
123           #73174].
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BUGS

126       The operation of this module depends on influencing the compilation of
127       "require" and "do".  As a result, it cannot prevent lexical state
128       leakage through a "require"/"do" statement that was compiled before
129       this module was invoked.  Where problems occur, this module must be
130       invoked earlier.
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132       On all Perl versions that need a fix for the lexical hint leakage bug,
133       the pure Perl implementation of this module unavoidably breaks the use
134       of "require" without an explicit parameter (implicitly using $_).  This
135       is due to another bug in the Perl core, fixed in Perl 5.15.5, and is
136       inherent to the mechanism by which pure Perl code can hook "require".
137       The use of implicit $_ with "require" is rare, so although this state
138       of affairs is faulty it will actually work for most programs.  Perl
139       versions 5.12.0 and greater, despite having the "require" hooking bug,
140       don't actually exhibit a problem with the pure Perl version of this
141       module, because with the lexical hint leakage bug fixed there is no
142       need for this module to hook "require".
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144       There is a bug on Perl versions 5.15.5 to 5.15.7 affecting "do" which,
145       among other effects, causes "%^H" to leak into "do"ed files.  It is not
146       the same bug that affected Perl 5.6 to 5.11.  This module currently
147       does not work around this bug at all, but its test suite does detect
148       it.  As a result, this module fails its test suite on those Perl
149       versions.  This could change in future versions of this module.
150

SEE ALSO

152       perlpragma
153

AUTHOR

155       Andrew Main (Zefram) <zefram@fysh.org>
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158       Copyright (C) 2009, 2010, 2011, 2012, 2015, 2016, 2017, 2023 Andrew
159       Main (Zefram) <zefram@fysh.org>
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LICENSE

162       This module is free software; you can redistribute it and/or modify it
163       under the same terms as Perl itself.
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167perl v5.36.0                      2023-03-11      Lexical::SealRequireHints(3)
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