Use of uninitialized value $_[0] in defined operator

[jdv]

This is a bug report for perl from [email protected],
generated with the help of perlbug 1.42 running under perl 5.36.0.

---

[Please describe your issue here]

Here's the issue illustrated:

foo@b6846e6dc18c:/opt/bar$ cat test.pl
#!/usr/bin/env perl

use strict;
use warnings;

sub f { defined $_[0]; }
my %h;
my $s;
f($h{$s});

foo@b6846e6dc18c:/opt/bar$ perl test.pl
Use of uninitialized value $s in hash element at test.pl line 9.
Use of uninitialized value $_[0] in defined operator at test.pl line 6.
foo@b6846e6dc18c:/opt/bar$

And here's the irc convo about it on libera #perl:

14:52 < jdv> "Use of uninitialized value $[0] in defined operator at" and?...
14:52 < jdv> did I not have enough coffee or is that strange?
14:53 < LeoNerd> Uhmm... that's not supposed to happen
14:54 < huf> this could be a whole new genre of useless warnings
14:56 < huf> Useful use of private variable in non-void context at ...
14:56 < huf> things like that
...
15:22 < jdv> https://gist.github.com/jdv/6e466dccc2e0599921328612eb656b19
15:22 < Paperbot> Link title(s): [ gist:6e466dccc2e0599921328612eb656b19 · GitHub ]
15:22 < jdv> is probably as good as i can do
15:25 < ether> %h is empty. so $h{ anything } is going to be undef. but you're also using an undefined hash key. so of course you're getting "uninitialized" warnings. what about this is
strange?
15:26 < jdv> the fact that the defined call is spitting a warning on an undef value
15:28 < mauke> ah, magic
15:28 < mauke> eval: use warnings; sub { defined $[0]; defined $[0] }->( {}->{+undef} )
15:28 <+perlbot> mauke: Useless use of defined operator in void context at (IRC) line 1. Use of uninitialized value in hash element at (IRC) line 1. Use of uninitialized value $[0] in
defined operator at (IRC) line 1. Use of uninitialized value $[0] in defined operator at (IRC) line 1.
15:28 < jdv> not a big deal to me. just a little surprising when taken out of context.
15:29 < mauke> somehow it's preserving the undefness of the key in the hash value aliased to $[0]
15:29 < jdv> hopefully that other warning is always accompanying
15:29 < mauke> so every time $[0] is accessed, it triggers another warning
15:29 < jdv> oh, args magic?
15:29 < mauke> which is misattributed to $[0] itself
15:30 < mauke> arguments are in a "potential lvalue" context
15:30 < mauke> because if the sub assigns to $_[0], the hash element has to spring into existence
15:30 < jdv> :)
15:32 < mauke> you should report this as a bug in perl
15:33 < monkfish> affects signatures too, btw
15:33 < jdv> eh, i guess. ok, thanks:)
15:33 < ether> oh that's really cute
15:34 < monkfish> yes, please do. ghastly would be my adjective.
15:36 < jdv> pfft. its just a warning;)
15:36 < jdv> i will. in a bit.
15:36 < monkfish> the warning can be fatalised, however.
15:37 < monkfish> anyway, thanks.

[Please do not change anything below this line]

---

Flags:
category=core
severity=low

Site configuration information for perl 5.36.0:

Configured by Debian at Sat Nov 25 20:59:54 UTC 2023.

Summary of my perl5 (revision 5 version 36 subversion 0) configuration:

Platform:
osname=linux
osvers=4.19.0
archname=x86_64-linux-gnu-thread-multi
uname='linux localhost 4.19.0 #1 smp debian 4.19.0 x86_64 gnulinux '
config_args='-Dmksymlinks -Dusethreads -Duselargefiles -Dcc=x86_64-linux-gnu-gcc -Dcpp=x86_64-linux-gnu-cpp -Dld=x86_64-linux-gnu-gcc -Dccflags=-DDEBIAN -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -ffile-prefix-map=/dummy/build/dir=. -fstack-protector-strong -Wformat -Werror=format-security -Dldflags= -Wl,-z,relro -Dlddlflags=-shared -Wl,-z,relro -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.36 -Darchlib=/usr/lib/x86_64-linux-gnu/perl/5.36 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.36 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.36.0 -Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.36.0 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Duse64bitint -Dman1ext=1 -Dman3ext=3perl
-Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil -Ui_xlocale -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -dEs -Duseshrplib -Dlibperl=libperl.so.5.36.0'
hint=recommended
useposix=true
d_sigaction=define
useithreads=define
usemultiplicity=define
use64bitint=define
use64bitall=define
uselongdouble=undef
usemymalloc=n
default_inc_excludes_dot=define
Compiler:
cc='x86_64-linux-gnu-gcc'
ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
optimize='-O2 -g'
cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include'
ccversion=''
gccversion='12.2.0'
gccosandvers=''
intsize=4
longsize=8
ptrsize=8
doublesize=8
byteorder=12345678
doublekind=3
d_longlong=define
longlongsize=8
d_longdbl=define
longdblsize=16
longdblkind=3
ivtype='long'
ivsize=8
nvtype='double'
nvsize=8
Off_t='off_t'
lseeksize=8
alignbytes=8
prototype=define
Linker and Libraries:
ld='x86_64-linux-gnu-gcc'
ldflags =' -fstack-protector-strong -L/usr/local/lib'
libpth=/usr/local/lib /usr/lib/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib
libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
perllibs=-ldl -lm -lpthread -lc -lcrypt
libc=/lib/x86_64-linux-gnu/libc.so.6
so=so
useshrplib=true
libperl=libperl.so.5.36
gnulibc_version='2.36'
Dynamic Linking:
dlsrc=dl_dlopen.xs
dlext=so
d_dlsymun=undef
ccdlflags='-Wl,-E'
cccdlflags='-fPIC'
lddlflags='-shared -L/usr/local/lib -fstack-protector-strong'

Locally applied patches:
DEBPKG:debian/cpan_definstalldirs - Provide a sensible INSTALLDIRS default for modules installed from CPAN.
DEBPKG:debian/db_file_ver - https://bugs.debian.org/340047 Remove overly restrictive DB_File version check.
DEBPKG:debian/doc_info - Replace generic man(1) instructions with Debian-specific information.
DEBPKG:debian/enc2xs_inc - https://bugs.debian.org/290336 Tweak enc2xs to follow symlinks and ignore missing @inc directories.
DEBPKG:debian/errno_ver - https://bugs.debian.org/343351 Remove Errno version check due to upgrade problems with long-running processes.
DEBPKG:debian/libperl_embed_doc - https://bugs.debian.org/186778 Note that libperl-dev package is required for embedded linking
DEBPKG:fixes/respect_umask - Respect umask during installation
DEBPKG:debian/writable_site_dirs - Set umask approproately for site install directories
DEBPKG:debian/extutils_set_libperl_path - EU:MM: set location of libperl.a under /usr/lib
DEBPKG:debian/no_packlist_perllocal - Don't install .packlist or perllocal.pod for perl or vendor
DEBPKG:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation to the binary targets.
DEBPKG:debian/instmodsh_doc - Debian policy doesn't install .packlist files for core or vendor.
DEBPKG:debian/ld_run_path - Remove standard libs from LD_RUN_PATH as per Debian policy.
DEBPKG:debian/libnet_config_path - Set location of libnet.cfg to /etc/perl/Net as /usr may not be writable.
DEBPKG:debian/perlivp - https://bugs.debian.org/510895 Make perlivp skip include directories in /usr/local
DEBPKG:debian/squelch-locale-warnings - https://bugs.debian.org/508764 Squelch locale warnings in Debian package maintainer scripts
DEBPKG:debian/patchlevel - https://bugs.debian.org/567489 List packaged patches for 5.36.0-7+deb12u1 in patchlevel.h
DEBPKG:fixes/document_makemaker_ccflags - https://bugs.debian.org/628522 [rt.cpan.org #68613] Document that CCFLAGS should include $Config{ccflags}
DEBPKG:debian/find_html2text - https://bugs.debian.org/640479 Configure CPAN::Distribution with correct name of html2text
DEBPKG:debian/perl5db-x-terminal-emulator.patch - https://bugs.debian.org/668490 Invoke x-terminal-emulator rather than xterm in perl5db.pl
DEBPKG:debian/cpan-missing-site-dirs - https://bugs.debian.org/688842 Fix CPAN::FirstTime defaults with nonexisting site dirs if a parent is writable
DEBPKG:fixes/memoize_storable_nstore - [rt.cpan.org #77790] https://bugs.debian.org/587650 Memoize::Storable: respect 'nstore' option not respected
DEBPKG:debian/makemaker-pasthru - https://bugs.debian.org/758471 Pass LD settings through to subdirectories
DEBPKG:debian/makemaker-manext - https://bugs.debian.org/247370 Make EU::MakeMaker honour MANnEXT settings in generated manpage headers
DEBPKG:debian/kfreebsd-softupdates - https://bugs.debian.org/796798 Work around Debian Bug#796798
DEBPKG:fixes/memoize-pod - [rt.cpan.org #89441] Fix POD errors in Memoize
DEBPKG:debian/hurd-softupdates - https://bugs.debian.org/822735 Fix t/op/stat.t failures on hurd
DEBPKG:fixes/math_complex_doc_great_circle - https://bugs.debian.org/697567 [rt.cpan.org #114104] Math::Trig: clarify definition of great_circle_midpoint
DEBPKG:fixes/math_complex_doc_see_also - https://bugs.debian.org/697568 [rt.cpan.org #114105] Math::Trig: add missing SEE ALSO
DEBPKG:fixes/math_complex_doc_angle_units - https://bugs.debian.org/731505 [rt.cpan.org #114106] Math::Trig: document angle units
DEBPKG:fixes/cpan_web_link - https://bugs.debian.org/367291 CPAN: Add link to main CPAN web site
DEBPKG:debian/hppa_op_optimize_workaround - https://bugs.debian.org/838613 Temporarily lower the optimization of op.c on hppa due to gcc-6 problems
DEBPKG:debian/installman-utf8 - https://bugs.debian.org/840211 Generate man pages with UTF-8 characters
DEBPKG:debian/hppa_opmini_optimize_workaround - https://bugs.debian.org/869122 Lower the optimization level of opmini.c on hppa
DEBPKG:debian/sh4_op_optimize_workaround - https://bugs.debian.org/869373 Also lower the optimization level of op.c and opmini.c on sh4
DEBPKG:debian/perldoc-pager - https://bugs.debian.org/870340 [rt.cpan.org #120229] Fix perldoc terminal escapes when sensible-pager is less
DEBPKG:debian/prune_libs - https://bugs.debian.org/128355 Prune the list of libraries wanted to what we actually need.
DEBPKG:debian/mod_paths - Tweak @inc ordering for Debian
DEBPKG:debian/deprecate-with-apt - https://bugs.debian.org/747628 Point users to Debian packages of deprecated core modules
DEBPKG:debian/disable-stack-check - https://bugs.debian.org/902779 [GH #16607] Disable debugperl stack extension checks for binary compatibility with perl
DEBPKG:debian/perlbug-editor - https://bugs.debian.org/922609 Use "editor" as the default perlbug editor, as per Debian policy
DEBPKG:debian/eu-mm-perl-base - https://bugs.debian.org/962138 Suppress an ExtUtils::MakeMaker warning about our non-default @inc
DEBPKG:fixes/io_socket_ip_ipv6 - Disable getaddrinfo(3) AI_ADDRCONFIG for localhost and IPv4 numeric addresses
DEBPKG:debian/usrmerge-lib64 - https://bugs.debian.org/914128 Configure / libpth.U: Do not adjust glibpth when /usr/lib64 is present.
DEBPKG:debian/usrmerge-realpath - https://bugs.debian.org/914128 Configure / libpth.U: use realpath --no-symlinks on Debian
DEBPKG:debian/configure-regen - https://bugs.debian.org/762638 Regenerate Configure et al. after probe unit changes
DEBPKG:fixes/x32-io-msg-skip - https://bugs.debian.org/922609 Skip io/msg.t on x32 due to broken System V message queues
DEBPKG:debian/hurd-eumm-workaround - https://bugs.debian.org/1018289 Work around a MakeMaker regression breaking GNU/Hurd hint files
DEBPKG:fixes/json-pp-warnings - https://bugs.debian.org/1019757 Call unimport first to silence warnings
DEBPKG:fixes/readline-stream-errors - [80c1f1e] [GH #6799] https://bugs.debian.org/1016369 only clear the stream error state in readline() for glob()
DEBPKG:fixes/readline-stream-errors-test - [0b60216] [GH #6799] https://bugs.debian.org/1016369 test that <> doesn't clear the stream error state
DEBPKG:fixes/lto-test-fix - [69b4fa3] [GH #20518] https://bugs.debian.org/1015579 skip checking categorization of libperl symbols for LTO builds
DEBPKG:fixes/CVE-2023-47038 - [7047915] https://bugs.debian.org/1056746 Fix read/write past buffer end: perl-security#140

---

@inc for perl 5.36.0:
/etc/perl
/usr/local/lib/x86_64-linux-gnu/perl/5.36.0
/usr/local/share/perl/5.36.0
/usr/lib/x86_64-linux-gnu/perl5/5.36
/usr/share/perl5
/usr/lib/x86_64-linux-gnu/perl-base
/usr/lib/x86_64-linux-gnu/perl/5.36
/usr/share/perl/5.36
/usr/local/lib/site_perl

---

Environment for perl 5.36.0:
HOME=/home/foo
LANG (unset)
LANGUAGE (unset)
LD_LIBRARY_PATH (unset)
LOGDIR (unset)
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PERL5OPT=
PERL_BADLANG (unset)
SHELL (unset)

[tonycoz]

The problem warning here is produced within defelem_target() (mg.c), and is caused because a copy of the index SV $s is stored in the magic for $_[0] and that copy is also undef.

defelem_target() calls hv_fetch_ent() with that SV and so produces the warning when hv_common() eventually calls SvPV_const() on the key.

One option might be to store an undef key as "" in the defelem magic, which will work for built-in hashes, but I'm not sure how that will interact with tied hashes.