Setting $0 on macOS results in environment variables being included in the command name

[grr]

Description

Setting $0 on macOS results in the command as returned by ps including environment variables. The below script will produce output like:

...
/tmp/a.pl 10/10
COMMAND
/tmp/a.pl 10/1 PERLBREW_SHELLRC_VERSION=0.94

Note the modified command name is truncated and the environment variable is added. That is the first listed variable when running env. Running with any number of command-line arguments > 0 (ex. /tmp/a.pl a b c) prevents
the truncation of the command and doesn't print the environment variable.
However, using the system perl (threaded 5.30.2) works fine with only 0 command-line arguments. With
each additional command-line argument, the command name will contain an additional environment
variable.

https://perldoc.perl.org/perlvar#$0 documents many limitations in setting $0, but this is not mentioned. And running ps auxw doesn't show any other process that includes an environment variable. Perl >= 5.8 exhibit this issue.

Steps to Reproduce

Script:

#!/usr/bin/env perl
use strict;
use warnings;

my @range = (1..10);
for my $i (@range) {
    local $0 = "$0 $i/" . scalar @range;
    print "$0\n";
    print `ps -p $$ -o command`;
    sleep 1;
}

Expected behavior

Setting $0 should not result in including environment variables in the command name.

Perl configuration

Summary of my perl5 (revision 5 version 36 subversion 0) configuration:
   
  Platform:
    osname=darwin
    osvers=20.6.0
    archname=darwin-2level
    uname='darwin iphone 20.6.0 darwin kernel version 20.6.0: tue feb 22 21:10:42 pst 2022; root:xnu-7195.141.26~1release_arm64_t8101 arm64 '
    config_args='-de -Dprefix=/Users/grr/local/perlbrew/perls/perl-5.36 -Aeval:scriptdir=/Users/grr/local/perlbrew/perls/perl-5.36/bin'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=undef
    usemultiplicity=undef
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
  Compiler:
    cc='cc'
    ccflags ='-fno-common -DPERL_DARWIN -mmacosx-version-min=11.6 -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -I/opt/local/include -DPERL_USE_SAFE_PUTENV'
    optimize='-O3'
    cppflags='-fno-common -DPERL_DARWIN -mmacosx-version-min=11.6 -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -I/opt/local/include'
    ccversion=''
    gccversion='Apple LLVM 13.0.0 (clang-1300.0.29.30)'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=8
    longdblkind=0
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='cc'
    ldflags =' -mmacosx-version-min=11.6 -fstack-protector-strong -L/usr/local/lib -L/opt/local/lib'
    libpth=/usr/local/lib /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/13.0.0/lib /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib /opt/local/lib /usr/lib
    libs=-lgdbm
    perllibs=
    libc=
    so=dylib
    useshrplib=false
    libperl=libperl.a
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=bundle
    d_dlsymun=undef
    ccdlflags=' '
    cccdlflags=' '
    lddlflags=' -mmacosx-version-min=11.6 -bundle -undefined dynamic_lookup -L/usr/local/lib -L/opt/local/lib -fstack-protector-strong'


Characteristics of this binary (from libperl): 
  Compile-time options:
    HAS_TIMES
    PERLIO_LAYERS
    PERL_COPY_ON_WRITE
    PERL_DONT_CREATE_GVSV
    PERL_MALLOC_WRAP
    PERL_OP_PARENT
    PERL_PRESERVE_IVUV
    PERL_USE_SAFE_PUTENV
    USE_64_BIT_ALL
    USE_64_BIT_INT
    USE_LARGE_FILES
    USE_LOCALE
    USE_LOCALE_COLLATE
    USE_LOCALE_CTYPE
    USE_LOCALE_NUMERIC
    USE_LOCALE_TIME
    USE_PERLIO
    USE_PERL_ATOF
  Built under darwin
  Compiled at May 27 2022 21:14:58
  %ENV:
    PERL5LIB="/Users/grr/local/perl5"
    PERLBREW_HOME="/Users/grr/.perlbrew"
    PERLBREW_MANPATH="/Users/grr/local/perlbrew/perls/current/man"
    PERLBREW_PATH="/Users/grr/local/perlbrew/bin:/Users/grr/local/perlbrew/perls/current/bin"
    PERLBREW_PERL="current"
    PERLBREW_ROOT="/Users/grr/local/perlbrew"
    PERLBREW_SHELLRC_VERSION="0.94"
    PERLBREW_VERSION="0.94"
    PERL_CPANM_OPT="-q --mirror https://www.cpan.org --mirror https://cpan.metacpan.org"
  @INC:
    /Users/grr/local/perl5
    /Users/grr/local/perlbrew/perls/perl-5.36/lib/site_perl/5.36.0/darwin-2level
    /Users/grr/local/perlbrew/perls/perl-5.36/lib/site_perl/5.36.0
    /Users/grr/local/perlbrew/perls/perl-5.36/lib/5.36.0/darwin-2level
    /Users/grr/local/perlbrew/perls/perl-5.36/lib/5.36.0

[jkeenan]

Description

Setting $0 on macOS results in the command as returned by ps including environment variables. The below script will produce output like:

...
/tmp/a.pl 10/10
COMMAND
/tmp/a.pl 10/1 PERLBREW_SHELLRC_VERSION=0.94

Note the modified command name is truncated and the environment variable is added. ....

Can this ticket be reviewed by someone on a Mac?

[FGasper]

Doesn’t happen for me w/ MacPorts-provided 5.36:

> pbpaste| perl5.36
- 1/10
COMMAND
- 1/10
- 2/10
COMMAND
- 2/10
- 3/10
COMMAND
- 3/10
- 4/10
COMMAND
- 4/10
- 5/10
COMMAND
- 5/10

> perl5.36 -V
Summary of my perl5 (revision 5 version 36 subversion 0) configuration:

  Platform:
    osname=darwin
    osvers=21.3.0
    archname=darwin-thread-multi-2level
    uname='darwin montereyx.internal.macports.net 21.3.0 darwin kernel version 21.3.0: wed jan 5 21:37:58 pst 2022; root:xnu-8019.80.24~20release_x86_64 x86_64 '
    config_args='-des -Dprefix=/opt/local -Dscriptdir=/opt/local/bin -Dvendorprefix=/opt/local -Dusemultiplicity=y -Dusethreads -Duseshrplib -Dcc=/usr/bin/cc -Dman1ext=1pm -Dman3ext=3pm -Dinstallstyle=lib/perl5 -Dman1dir=/opt/local/share/man/man1p -Dman3dir=/opt/local/share/man/man3p -Dsitebin=/opt/local/libexec/perl5.36/sitebin -Dsiteman1dir=/opt/local/share/perl5.36/siteman/man1 -Dsiteman3dir=/opt/local/share/perl5.36/siteman/man3 -Dvendorbin=/opt/local/libexec/perl5.36 -Dvendorman1dir=/opt/local/share/perl5.36/man/man1 -Dvendorman3dir=/opt/local/share/perl5.36/man/man3 -Dpager=/usr/bin/less -sR -Dperlpath=/opt/local/bin/perl5.36 -Dstartperl=#!/opt/local/bin/perl5.36 -Acppflags=-I/opt/local/include  -Accflags=-pipe -Os   -Alddlflags=-L/opt/local/lib -Wl,-headerpad_max_install_names   -Aldflags=-L/opt/local/lib -Wl,-headerpad_max_install_names  '
    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='/usr/bin/cc'
    ccflags ='-fno-common -DPERL_DARWIN  -pipe -Os   -fno-strict-aliasing -fstack-protector-strong -I/opt/local/include -DPERL_USE_SAFE_PUTENV'
    optimize='-O3'
    cppflags='-I/opt/local/include  -fno-common -DPERL_DARWIN  -pipe -Os   -fno-strict-aliasing -fstack-protector-strong -I/opt/local/include'
    ccversion=''
    gccversion='Apple LLVM 13.0.0 (clang-1300.0.29.30)'
    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='/usr/bin/cc'
    ldflags ='  -L/opt/local/lib -Wl,-headerpad_max_install_names   -fstack-protector-strong'
    libpth=/opt/local/lib /Library/Developer/CommandLineTools/usr/lib/clang/13.0.0/lib /Library/Developer/CommandLineTools/SDKs/MacOSX12.sdk/usr/lib /Library/Developer/CommandLineTools/usr/lib /usr/lib
    libs=-lgdbm
    perllibs=
    libc=
    so=dylib
    useshrplib=true
    libperl=libperl.dylib
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=bundle
    d_dlsymun=undef
    ccdlflags=' '
    cccdlflags=' '
    lddlflags='  -bundle -undefined dynamic_lookup -L/opt/local/lib -Wl,-headerpad_max_install_names   -fstack-protector-strong'


Characteristics of this binary (from libperl):
  Compile-time options:
    HAS_TIMES
    MULTIPLICITY
    PERLIO_LAYERS
    PERL_COPY_ON_WRITE
    PERL_DONT_CREATE_GVSV
    PERL_MALLOC_WRAP
    PERL_OP_PARENT
    PERL_PRESERVE_IVUV
    PERL_USE_SAFE_PUTENV
    USE_64_BIT_ALL
    USE_64_BIT_INT
    USE_ITHREADS
    USE_LARGE_FILES
    USE_LOCALE
    USE_LOCALE_COLLATE
    USE_LOCALE_CTYPE
    USE_LOCALE_NUMERIC
    USE_LOCALE_TIME
    USE_PERLIO
    USE_PERL_ATOF
    USE_REENTRANT_API
    USE_THREAD_SAFE_LOCALE
  Built under darwin
  Compiled at Jul 11 2022 09:54:26
  @INC:
    /opt/local/lib/perl5/site_perl/5.36/darwin-thread-multi-2level
    /opt/local/lib/perl5/site_perl/5.36
    /opt/local/lib/perl5/vendor_perl/5.36/darwin-thread-multi-2level
    /opt/local/lib/perl5/vendor_perl/5.36
    /opt/local/lib/perl5/5.36/darwin-thread-multi-2level
    /opt/local/lib/perl5/5.36

[grr]

Doesn’t happen for me w/ MacPorts-provided 5.36:

Confirmed- I also have no problems with macports version. Should be simple to also test with system perl: /usr/bin/perl ./a.pl 1.

[xenu]

The bug still exists in blead but it's now much harder to trigger (you need to use a very long string) due to semi-related changes from this PR: #19514

This is the memory layout of argv and environ:

argv[0]<NULL>argv[1]<NULL>argv[2]<NULL>environ[0]<NULL>environ[1]<NULL>...

To change the process name we overwrite that memory block with our string. It turns out that macOS requires that string to be terminated with as many null-terminators as there were argv elements. So for example, in the above scenario, the correct way to change the process name to "foo" would be to set argv[0] to foo<NULL><NULL><NULL>.

Our code currently uses the following (incorrect) algorithm:

1. PL_origalen is set to the combined size of argv and environ (before the mentioned PR, it was just argv).
2. If length($0) == PL_origalen - 1, copy $0 to argv[0].
3. If length($0) > PL_origalen - 1, copy the first PL_origalen - 1 bytes of $0 to argv[0] .
4. Otherwise, copy $0 to argv[0] and pad it with PL_origalen - length($0) zeros.

As you can see, in many scenarios we will end up with fewer null terminators than required.