fcntl failure after eval

flipper has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks,

I recently came across a very strange problem where fcntl() would fail if I had previously used a string eval - I worked around it by doing a require instead, but I'd be interested to hear an explanation of what's going on...

Without the eval, the program below produces output:

root@geedorah:~# perl foo.pl 
eval error - ''
lock says: 0 but true
unlock says: 0 but true
[download]

If I do the string eval, I get:

included by eval at (eval 1) line 2.
eval error - ''
lock says: UNDEFINED - Invalid argument
unlock says: 0 but true
[download]

Any suggestions or explanations welcome...

#!/usr/bin/perl -w
use strict;
use Fcntl qw(:DEFAULT :seek);
use 5.10.0;

our @extraoptions = ();
my %fh;

my $str = "#push \@extraoptions, '--bvtonly';\npush \@extraoptions, '-
+-debug';warn 'included by eval'";

1 && eval($str);
print "eval error - '$@'\n";

doit();
exit;

sub doit {
        my $file = '/tmp/z';
        say "lock says: ".(lockf($file) // "UNDEFINED - $!");
        say "unlock says: ".(unlockf($file) // "UNDEFINED - $!");
}
sub lockf {
        my $version = shift;
        open $fh{$version}, ">>$version" or die;
        my $flags = pack('sslli', F_WRLCK, SEEK_SET, 0, 0);# struct fl
+ock from fcntl - flock doesn't work on NFS
        return fcntl($fh{$version}, F_SETLK, $flags);
}

sub unlockf {
        my $version = shift;
        if($fh{$version}){
                my $flags = pack('sslli', F_UNLCK, SEEK_SET, 0, 0);# s
+truct flock from fcntl - flock doesn't work on NFS
                my $ret= fcntl($fh{$version}, F_SETLK, $flags);
                delete $fh{$version};
                return $ret;
        }else{
                die "Cannot release lock I don't hold for $version!";
        }
}
[download]

root@geedorah:~# perl -V

Summary of my perl5 (revision 5 version 10 subversion 1) configuration
+:
   
  Platform:
    osname=linux, osvers=2.6.32-5-amd64, archname=x86_64-linux-gnu-thr
+ead-multi
    uname='linux brahms 2.6.32-5-amd64 #1 smp tue jun 14 09:42:28 utc 
+2011 x86_64 gnulinux '
    config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN -Dccc
+dlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/us
+r/share/perl/5.10 -Darchlib=/usr/lib/perl/5.10 -Dvendorprefix=/usr -D
+vendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/
+usr/local -Dsitelib=/usr/local/share/perl/5.10.1 -Dsitearch=/usr/loca
+l/lib/perl/5.10.1 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/
+man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/m
+an/man3 -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -
+Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -DDEBUGGING=-g -Doptimize=-
+O2 -Duseshrplib -Dlibperl=libperl.so.5.10.1 -Dd_dosuid -des'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=und
+ef
    use64bitint=define, use64bitall=define, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fno-strict
+-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_S
+OURCE -D_FILE_OFFSET_BITS=64',
    optimize='-O2 -g',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fno-strict-aliasing
+ -pipe -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.4.5', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=1
+6
    ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t',
+ lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib /lib64 /usr/lib64
    libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
    perllibs=-ldl -lm -lpthread -lc -lcrypt
    libc=/lib/libc-2.11.2.so, so=so, useshrplib=true, libperl=libperl.
+so.5.10.1
    gnulibc_version='2.11.2'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -g -L/usr/local/lib -fs
+tack-protector'


Characteristics of this binary (from libperl): 
  Compile-time options: MULTIPLICITY PERL_DONT_CREATE_GVSV
                        PERL_IMPLICIT_CONTEXT PERL_MALLOC_WRAP USE_64_
+BIT_ALL
                        USE_64_BIT_INT USE_ITHREADS USE_LARGE_FILES
                        USE_PERLIO USE_REENTRANT_API
  Locally applied patches:
        DEBPKG:debian/arm_thread_stress_timeout - http://bugs.debian.o
+rg/501970 Raise the timeout of ext/threads/shared/t/stress.t to accom
+modate slower build hosts
        DEBPKG:debian/cpan_config_path - Set location of CPAN::Config 
+to /etc/perl as /usr may not be writable.
        DEBPKG:debian/cpan_definstalldirs - Provide a sensible INSTALL
+DIRS default for modules installed from CPAN.
        DEBPKG:debian/db_file_ver - http://bugs.debian.org/340047 Remo
+ve overly restrictive DB_File version check.
        DEBPKG:debian/doc_info - Replace generic man(1) instructions w
+ith Debian-specific information.
        DEBPKG:debian/enc2xs_inc - http://bugs.debian.org/290336 Tweak
+ enc2xs to follow symlinks and ignore missing @INC directories.
        DEBPKG:debian/errno_ver - http://bugs.debian.org/343351 Remove
+ Errno version check due to upgrade problems with long-running proces
+ses.
        DEBPKG:debian/extutils_hacks - Various debian-specific ExtUtil
+s changes
        DEBPKG:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation t
+o the binary targets.
        DEBPKG:debian/instmodsh_doc - Debian policy doesn't install .p
+acklist files for core or vendor.
        DEBPKG:debian/ld_run_path - Remove standard libs from LD_RUN_P
+ATH 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/m68k_thread_stress - http://bugs.debian.org/4958
+26 Disable some threads tests on m68k for now due to missing TLS.
        DEBPKG:debian/mod_paths - Tweak @INC ordering for Debian
        DEBPKG:debian/module_build_man_extensions - http://bugs.debian
+.org/479460 Adjust Module::Build manual page extensions for the Debia
+n Perl policy
        DEBPKG:debian/perl_synopsis - http://bugs.debian.org/278323 Re
+arrange perl.pod
        DEBPKG:debian/prune_libs - http://bugs.debian.org/128355 Prune
+ the list of libraries wanted to what we actually need.
        DEBPKG:debian/use_gdbm - Explicitly link against -lgdbm_compat
+ in ODBM_File/NDBM_File. 
        DEBPKG:fixes/assorted_docs - http://bugs.debian.org/443733 [38
+4f06a] Math::BigInt::CalcEmu documentation grammar fix
        DEBPKG:fixes/net_smtp_docs - http://bugs.debian.org/100195 [rt
+.cpan.org #36038] Document the Net::SMTP 'Port' option
        DEBPKG:fixes/processPL - http://bugs.debian.org/357264 [rt.cpa
+n.org #17224] Always use PERLRUNINST when building perl modules.
        DEBPKG:debian/perlivp - http://bugs.debian.org/510895 Make per
+livp skip include directories in /usr/local
        DEBPKG:fixes/pod2man-index-backslash - http://bugs.debian.org/
+521256 Escape backslashes in .IX entries
        DEBPKG:debian/disable-zlib-bundling - Disable zlib bundling in
+ Compress::Raw::Zlib
        DEBPKG:fixes/kfreebsd_cppsymbols - http://bugs.debian.org/5330
+98 [3b910a0] Add gcc predefined macros to $Config{cppsymbols} on GNU/
+kFreeBSD.
        DEBPKG:debian/cpanplus_definstalldirs - http://bugs.debian.org
+/533707 Configure CPANPLUS to use the site directories by default.
        DEBPKG:debian/cpanplus_config_path - Save local versions of CP
+ANPLUS::Config::System into /etc/perl.
        DEBPKG:fixes/kfreebsd-filecopy-pipes - http://bugs.debian.org/
+537555 [16f708c] Fix File::Copy::copy with pipes on GNU/kFreeBSD
        DEBPKG:fixes/anon-tmpfile-dir - http://bugs.debian.org/528544 
+[perl #66452] Honor TMPDIR when open()ing an anonymous temporary file
        DEBPKG:fixes/abstract-sockets - http://bugs.debian.org/329291 
+[89904c0] Add support for Abstract namespace sockets.
        DEBPKG:fixes/hurd_cppsymbols - http://bugs.debian.org/544307 [
+eeb92b7] Add gcc predefined macros to $Config{cppsymbols} on GNU/Hurd
+.
        DEBPKG:fixes/autodie-flock - http://bugs.debian.org/543731 All
+ow for flock returning EAGAIN instead of EWOULDBLOCK on linux/parisc
        DEBPKG:fixes/archive-tar-instance-error - http://bugs.debian.o
+rg/539355 [rt.cpan.org #48879] Separate Archive::Tar instance error s
+trings from each other
        DEBPKG:fixes/positive-gpos - http://bugs.debian.org/545234 [pe
+rl #69056] [c584a96] Fix \G crash on first match
        DEBPKG:debian/devel-ppport-ia64-optim - http://bugs.debian.org
+/548943 Work around an ICE on ia64
        DEBPKG:fixes/trie-logic-match - http://bugs.debian.org/552291 
+[perl #69973] [0abd0d7] Fix a DoS in Unicode processing [CVE-2009-362
+6]
        DEBPKG:fixes/hppa-thread-eagain - http://bugs.debian.org/55421
+8 make the threads-shared test suite more robust, fixing failures on 
+hppa
        DEBPKG:fixes/crash-on-undefined-destroy - http://bugs.debian.o
+rg/564074 [perl #71952] [1f15e67] Fix a NULL pointer dereference when
+ looking for a DESTROY method
        DEBPKG:fixes/tainted-errno - http://bugs.debian.org/574129 [pe
+rl #61976] [be1cf43] fix an errno stringification bug in taint mode
        DEBPKG:fixes/safe-upgrade - http://bugs.debian.org/582978 Upgr
+ade Safe.pm to 2.25, fixing CVE-2010-1974
        DEBPKG:fixes/tell-crash - http://bugs.debian.org/578577 [f4817
+f3] Fix a tell() crash on bad arguments.
        DEBPKG:fixes/format-write-crash - http://bugs.debian.org/57953
+7 [perl #22977] [421f30e] Fix a crash in format/write
        DEBPKG:fixes/arm-alignment - http://bugs.debian.org/289884 [f1
+c7503] Prevent gcc from optimizing the alignment test away on armel
        DEBPKG:fixes/fcgi-test - Fix a failure in CGI/t/fast.t when FC
+GI is installed
        DEBPKG:fixes/hurd-ccflags - http://bugs.debian.org/587901 Make
+ hints/gnu.sh append to $ccflags rather than overriding them
        DEBPKG:debian/squelch-locale-warnings - http://bugs.debian.org
+/508764 Squelch locale warnings in Debian package maintainer scripts
        DEBPKG:fixes/lc-numeric-docs - http://bugs.debian.org/379329 [
+perl #78452] [903eb63] LC_NUMERIC documentation fixes
        DEBPKG:fixes/lc-numeric-sprintf - http://bugs.debian.org/60154
+9 [perl #78632] [b3fd614] Fix sprintf not to ignore LC_NUMERIC with c
+onstants
        DEBPKG:fixes/concat-stack-corruption - http://bugs.debian.org/
+596105 [perl #78674] [e3393f5] Fix stack pointer corruption in pp_con
+cat() with 'use encoding'
        DEBPKG:fixes/cgi-multiline-header - http://bugs.debian.org/606
+995 [CVE-2010-2761 CVE-2010-4410 CVE-2010-4411] CGI.pm MIME boundary 
+and multiline header vulnerabilities
        DEBPKG:fixes/casing-taint-cve-2011-1487 - http://bugs.debian.o
+rg/622817 [perl #87336] fix unwanted taint laundering in lc(), uc() e
+t al.
        DEBPKG:fixes/safe-reval-rdo-cve-2010-1447 - [PATCH] Wrap by de
+fault coderefs returned by rdo and reval
        DEBPKG:patchlevel - http://bugs.debian.org/567489 List package
+d patches for 5.10.1-17squeeze2 in patchlevel.h
  Built under linux
  Compiled at Jun 30 2011 22:28:00
  @INC:
    /etc/perl
    /usr/local/lib/perl/5.10.1
    /usr/local/share/perl/5.10.1
    /usr/lib/perl5
    /usr/share/perl5
    /usr/lib/perl/5.10
    /usr/share/perl/5.10
    /usr/local/lib/site_perl
    .
root@geedorah:~#
[download]

Comment on fcntl failure after eval Select or Download Code

Replies are listed 'Best First'.
Re: fcntl failure after eval by Animator (Hermit) on Aug 11, 2011 at 15:04 UTC
I ran your code on my system with perl-5.10.0, perl-5.11.0, perl-5.11.3, perl-5.11.4, perl-5.12.0 and perl-5.13.8 and all behave as expected. I also tested it on another debian machine with perl-5.10.0 and that behaves as expected as well. This of course does not tell that much since you are using perl-5.10.1 with different ./Configure options and with different patches... What I suggest: install strace and strace the program to see what the system fcntl call really returns. A second suggestion, maybe more complex, is to install a stock perl-5.10.1 (without patches that is) and attempt to reproduce it with that. Update (2011-08-22 07:09 UTC): I ran my tests on 32 bit systems; running it on a 64 bit system confirms the same problem	[reply]
Re^2: fcntl failure after eval by flipper (Beadle) on Aug 11, 2011 at 19:26 UTC
Something utterly bizarre is going on - if I add a line `warn "flags '$flags'";` in lockf() just above the return statement, it works, even with the eval! It happens on different machines, all debian stable 64bit . I will try a vanilla perl tomorrow. From strace, the length parameter which I'm trying to pass through pack as 0 is (sometimes) corrupted. strace when it goes wrong: `root@geedorah:~# strace perl foo.pl 2>&1 \|grep fcntl fcntl(3, F_SETFD, FD_CLOEXEC) = 0 fcntl(3, F_SETFD, FD_CLOEXEC) = 0 fcntl(3, F_SETLK, {type=F_WRLCK, whence=SEEK_SET, start=0, len=-858311 +8848}) = -1 EINVAL (Invalid argument) fcntl(3, F_SETLK, {type=F_UNLCK, whence=SEEK_SET, start=0, len=0}) = 0 root@geedorah:~#` [download] And when it goes right: `root@geedorah:~# strace perl foo.pl 2>&1 \|grep fcntl fcntl(3, F_SETFD, FD_CLOEXEC) = 0 fcntl(3, F_SETFD, FD_CLOEXEC) = 0 fcntl(3, F_SETLK, {type=F_WRLCK, whence=SEEK_SET, start=0, len=0}) = 0 fcntl(3, F_SETLK, {type=F_UNLCK, whence=SEEK_SET, start=0, len=0}) = 0` [download]	[reply] [d/l] [select]
Re: fcntl failure after eval by ikegami (Patriarch) on Aug 11, 2011 at 21:58 UTC
This ain't right. `pack('sslli', F_WRLCK, SEEK_SET, 0, 0); 12345 1 2 3 4` [download] So I gotta wonder if the string is at all right.	[reply] [d/l]
Re^2: fcntl failure after eval by ikegami (Patriarch) on Aug 11, 2011 at 22:26 UTC
Ok, omitting the last input appears to be acceptable. `$ perl -wE'say length pack "sslli", 0, 0, 0, 0;' 16 $ perl -wE'say length pack "sslli", 0, 0, 0, 0, 0;' 16` [download] However, that's far shorter than expected, at least on the 64-bit system I used. $ cat a.c #include <fcntl.h> #include <stddef.h> #include <stdio.h> int main() { struct flock lock; printf("flock: %lu\n", sizeof(lock)); printf("flock.l_type: %lu @ %2lu\n", sizeof(lock.l_type ), offse +tof(struct flock, l_type )); printf("flock.l_whence: %lu @ %2lu\n", sizeof(lock.l_whence), offse +tof(struct flock, l_whence)); printf("flock.l_start: %lu @ %2lu\n", sizeof(lock.l_start ), offse +tof(struct flock, l_start )); printf("flock.l_len: %lu @ %2lu\n", sizeof(lock.l_len ), offse +tof(struct flock, l_len )); printf("flock.l_pid: %lu @ %2lu\n", sizeof(lock.l_pid ), offse +tof(struct flock, l_pid )); return 0; } $ gcc -Wall a.c -o a && a flock: 32 flock.l_type: 2 @ 0 flock.l_whence: 2 @ 2 flock.l_start: 8 @ 8 flock.l_len: 8 @ 16 flock.l_pid: 4 @ 24 [download] On that system, one would need `$ perl -wE'say length pack "s s x4 q q L x4", 0, 0, 0, 0, 0;' 32` [download]	[reply] [d/l] [select]
Re^3: fcntl failure after eval by Animator (Hermit) on Aug 12, 2011 at 08:09 UTC
This does look like the right answer. There are 3 different cases to consider: a 32 bit system with large file support (-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64) a 32 bit system without large file support a 64 bit system ikegami's c program for each of the three: 32 bit system with LFS: $ gcc -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -Wall a.c -o a && a flock: 24 flock.l_type: 2 @ 0 flock.l_whence: 2 @ 2 flock.l_start: 8 @ 4 flock.l_len: 8 @ 12 flock.l_pid: 4 @ 20 32 bit system without LFS: $ gcc -Wall a.c -o a && a flock: 16 flock.l_type: 2 @ 0 flock.l_whence: 2 @ 2 flock.l_start: 4 @ 4 flock.l_len: 4 @ 8 flock.l_pid: 4 @ 12 64 bit system: $ gcc -Wall a.c -o a && a flock: 32 flock.l_type: 2 @ 0 flock.l_whence: 2 @ 2 flock.l_start: 8 @ 8 flock.l_len: 8 @ 16 flock.l_pid: 4 @ 24 The test perls: stock perl-5.10.0, running on a 32 bit system with large file support debian perl-5.10.0, running on a 32 bit system with large file support debian perl-5.10.1, running on a 64 bit system Changing the op's code to: `my $flags = pack('sslli', F_WRLCK, SEEK_SET, 5, 0);` perl 1: length of flags: 16 bits, expected length: 24 bits fcntl64(3, F_SETLK64, {type=F_WRLCK, whence=SEEK_SET, start=5, len=0}, 0x9d827d0) = 0 => this probably works by accident perl 2: length of flags: 16 bits, expected length: 24 bits (running as a normal user) fcntl64(3, F_SETLK64, {type=F_WRLCK, whence=SEEK_SET, start=5, len=0}, 0x819b4a8) = 0 => this probably works by accident perl 2: length of flags: 16 bits, expected length: 24 bits (running as root) fcntl64(3, F_SETLK64, {type=F_WRLCK, whence=SEEK_SET, start=5, len=2314885392840523776}, 0x81ed670) = 0 => the length parameter does not contain the expected value perl 3: length of flags: 16 bits, expected length: 32 bits fcntl(3, F_SETLK, {type=F_WRLCK, whence=SEEK_SET, start=0, len=0}) = 0 => not correct, start is 0 but 'expected' value is 5 `my $flags = pack('sslllli', F_WRLCK, SEEK_SET, 5, 0, 2, 0, 0);` perl 1: length of flags: 24 bits, expected length: 24 bits fcntl64(3, F_SETLK64, {type=F_WRLCK, whence=SEEK_SET, start=5, len=2}, 0x970c7d0) = 0 perl 2: length of flags: 24 bits, expected length: 24 bits (running as a normal user and as root) fcntl64(3, F_SETLK64, {type=F_WRLCK, whence=SEEK_SET, start=5, len=2}, 0x819b4a8) = 0 perl 3: length of flags: 24 bits, expected length: 32 bits fcntl(3, F_SETLK, {type=F_WRLCK, whence=SEEK_SET, start=8589934592, len=0}) = 0 `my $flags = pack("s s x4 q q L x4", F_WRLCK, SEEK_SET, 5, 2, 0);` perl 3: length of flags: 32 bits, expected length: 32 bits fcntl(3, F_SETLK, {type=F_WRLCK, whence=SEEK_SET, start=5, len=2}) = 0 What this all means is that the value you need for `$flags` depends highly on the system it is running.. Which makes me wonder wheter or not there is a better way to accomplish what the op wants..	[reply] [d/l] [select]
Re^4: fcntl failure after eval by ikegami (Patriarch) on Aug 12, 2011 at 08:30 UTC
Re^5: fcntl failure after eval by flipper (Beadle) on Aug 12, 2011 at 10:13 UTC
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