Lexical::SealRequireHints(3pm) User Contributed Perl Documentation
NAME
Lexical::SealRequireHints - prevent leakage of lexical hints
SYNOPSIS
use Lexical::SealRequireHints;
DESCRIPTION
This module works around two historical bugs in Perl's handling of the
"%^H" (lexical hints) variable. One bug causes lexical state in one
file to leak into another that is "require"d/"use"d/"do"ed from it.
This bug, [perl #68590], was present from Perl 5.6 up to Perl 5.10,
fixed in Perl 5.11.0. The second bug causes lexical state (normally a
blank "%^H" once the first bug is fixed) to leak outwards from
"utf8.pm", if it is automatically loaded during Unicode regular
expression matching, into whatever source is compiling at the time of
the regexp match. This bug, [perl #73174], was present from Perl 5.8.7
up to Perl 5.11.5, fixed in Perl 5.12.0.
Both of these bugs seriously damage the usability of any module relying
on "%^H" for lexical scoping, on the affected Perl versions. It is in
practice essential to work around these bugs when using such modules.
On versions of Perl that require such a workaround, this module
globally changes the behaviour of "require", including "use" and the
implicit "require" performed in Unicode regular expression matching,
and of "do", so that they no longer exhibit these bugs.
The workaround supplied by this module takes effect the first time its
"import" method is called. Typically this will be done by means of a
"use" statement. This should be done as early as possible, because it
only affects "require"/"use"/"do" statements that are compiled after
the workaround goes into effect. For "use" statements, and "require"
and "do" statements that are executed immediately and only once, it
suffices to invoke the workaround when loading the first module that
will set up vulnerable lexical state. Delayed-action "require" and
"do" statements, however, are more troublesome, and can require the
workaround to be loaded much earlier. Ultimately, an affected Perl
program may need to load the workaround as very nearly its first
action. Invoking this module multiple times, from multiple modules, is
not a problem: the workaround is only applied once, and applies to
everything subsequently compiled.
This module is implemented in XS, with a pure Perl backup version for
systems that can't handle XS modules. The XS version has a better
chance of playing nicely with other modules that modify "require" or
"do" handling. The pure Perl version can't work at all on some Perl
versions; users of those versions must use the XS. On all Perl
versions suffering the underlying hint leakage bug, pure Perl hooking
of "require" breaks the use of "require" without an explicit parameter
(implicitly using $_).
PERL VERSION DIFFERENCES
The history of the "%^H" bugs is complex. Here is a chronological
statement of the relevant changes.
Perl 5.6.0
"%^H" introduced. It exists only as a hash at compile time. It is
not localised by "require"/"do", so lexical hints leak into every
module loaded, which is bug [perl #68590].
The "CORE::GLOBAL" mechanism doesn't work cleanly for "require",
because overriding "require" loses the necessary special parsing of
bareword arguments to it. As a result, pure Perl code can't
properly globally affect the behaviour of "require". Pure Perl
code can localise "%^H" itself for any particular "require"
invocation, but a global fix is only possible through XS.
Perl 5.7.2
The "CORE::GLOBAL" mechanism now works cleanly for "require", so
pure Perl code can globally affect the behaviour of "require" to
achieve a global fix for the bug.
Perl 5.8.7
When "utf8.pm" is automatically loaded during Unicode regular
expression matching, "%^H" now leaks outward from it into whatever
source is compiling at the time of the regexp match, which is bug
[perl #73174]. It often goes unnoticed, because [perl #68590]
makes "%^H" leak into "utf8.pm" which then doesn't modify it, so
what leaks out tends to be identical to what leaked in. If [perl
#68590] is worked around, however, "%^H" tends to be (correctly)
blank inside "utf8.pm", and this bug therefore blanks it for the
outer module.
Perl 5.9.4
"%^H" now exists in two forms. In addition to the relatively
ordinary hash that is modified during compilation, the value that
it had at each point in compilation is recorded in the compiled op
tree, for later examination at runtime. It is in a special
representation-sharing format, and writes to "%^H" are meant to be
performed on both forms. "require"/"do" does not localise the
runtime form of "%^H" (and still doesn't localise the compile-time
form).
A couple of special "%^H" entries are erroneously written only to
the runtime form.
Pure Perl code, although it can localise the compile-time "%^H" by
normal means, can't adequately localise the runtime "%^H", except
by using a string eval stack frame. This makes a satisfactory
global fix for the leakage bug impossible in pure Perl.
Perl 5.10.1
"require"/"do" now properly localise the runtime form of "%^H", but
still not the compile-time form.
A global fix is once again possible in pure Perl, because the fix
only needs to localise the compile-time form.
Perl 5.11.0
"require"/"do" now properly localise both forms of "%^H", fixing
[perl #68590]. This makes [perl #73174] apparent without any
workaround for [perl #68590].
The special "%^H" entries are now correctly written to both forms
of the hash.
Perl 5.12.0
The automatic loading of "utf8.pm" during Unicode regular
expression matching now properly restores "%^H", fixing [perl
#73174].
BUGS
The operation of this module depends on influencing the compilation of
"require" and "do". As a result, it cannot prevent lexical state
leakage through a "require"/"do" statement that was compiled before
this module was invoked. Where problems occur, this module must be
invoked earlier.
On all Perl versions that need a fix for the lexical hint leakage bug,
the pure Perl implementation of this module unavoidably breaks the use
of "require" without an explicit parameter (implicitly using $_). This
is due to another bug in the Perl core, fixed in Perl 5.15.5, and is
inherent to the mechanism by which pure Perl code can hook "require".
The use of implicit $_ with "require" is rare, so although this state
of affairs is faulty it will actually work for most programs. Perl
versions 5.12.0 and greater, despite having the "require" hooking bug,
don't actually exhibit a problem with the pure Perl version of this
module, because with the lexical hint leakage bug fixed there is no
need for this module to hook "require".
There is a bug on Perl versions 5.15.5 to 5.15.7 affecting "do" which,
among other effects, causes "%^H" to leak into "do"ed files. It is not
the same bug that affected Perl 5.6 to 5.11. This module currently
does not work around this bug at all, but its test suite does detect
it. As a result, this module fails its test suite on those Perl
versions. This could change in future versions of this module.
SEE ALSO
perlpragma(1)
AUTHOR
Andrew Main (Zefram) <zefram@fysh.org>
COPYRIGHT
Copyright (C) 2009, 2010, 2011, 2012, 2015, 2016, 2017, 2023 Andrew
Main (Zefram) <zefram@fysh.org>
LICENSE
This module is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.34.3 2024-03-12 Lexical::SealRequireHints(3pm)
lexical-sealrequirehints 0.12.0 - Generated Mon Aug 4 16:29:15 CDT 2025