Perl
cygwin: as of the 5.39.10 version bump perl fails to fork with mro loaded
Module:
Description
Since the 5.39.10 version bump CI has been failing on Cygwin with errors like:
0 [main] perl 11614 child_info_fork::abort: address space needed by 'mro.dll' (0x190000) is already occupied
Can't fork, trying again in 5 seconds at t/lib/MakeMaker/Test/Utils.pm line 325.
I've managed to reproduce this locally and consistently with current cygwin, though with a different address:
tony@enceladus ~/dev/perl/git/perl
$ ./perl -Ilib -Mmro -efork
0 [main] perl 34054 child_info_fork::abort: address space needed by 'mro.dll' (0x400000) is already occupied
This is likely caused by a conflict between cygperl5._39_10.dll and mro.dll:
$ rebase -i `find . -name '*.dll'` ./perl.exe | grep -F '*'
/home/tony/dev/perl/git/perl/cygperl5_39_10.dll base 0x00041db50000 size 0x00b4c000 *
/home/tony/dev/perl/git/perl/lib/auto/mro/mro.dll base 0x00041e510000 size 0x0002c000 *
Building without -DDEBUGGING does not fail to work, probably because the DLL uses less address space and hence there's no conflict:
# without -DDEBUGGING
/home/tony/dev/perl/git/perl/cygperl5_39_10.dll base 0x00041db50000 size 0x0081e000
/home/tony/dev/perl/git/perl/lib/auto/mro/mro.dll base 0x00041e510000 size 0x00029000
I think this was caused by the name change for the cygperl DLL introduced by the version bump.
We use --enable-auto-image-base in the perl and module makefiles to generate the base addresses of DLLs in the perl build in cygwin. This generates the DLL bases addresses based on a hash of the DLL name, resulting in the conflict here.
Steps to Reproduce
- On cygwin, build perl with:
./Configure -des -Dusedevel -DDEBUGGING -Doptimize=-g
make test-prep
- fork with mro loaded:
./perl -Ilib -Mmro -efork
Expected behavior
fork() is successful.
Perl configuration
# perl -V output goes here
Summary of my perl5 (revision 5 version 39 subversion 10) configuration:
Commit id: e37d0248e80396cf27564f6205584d2024df704d
Platform:
osname=cygwin
osvers=3.5.1-1.x86_64
archname=cygwin-thread-multi
uname='cygwin_nt-10.0-19045 enceladus 3.5.1-1.x86_64 2024-02-27 11:54 utc x86_64 cygwin '
config_args='-des -Dusedevel -Doptimize=-g -DDEBUGGING'
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='gcc'
ccflags ='-U__STRICT_ANSI__ -D_GNU_SOURCE -fwrapv -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector-strong'
optimize='-g'
cppflags='-U__STRICT_ANSI__ -D_GNU_SOURCE -fwrapv -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector-strong'
ccversion=''
gccversion='11.4.0'
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='g++'
ldflags =' -Wl,--enable-auto-import -Wl,--export-all-symbols -Wl,--enable-auto-image-base -fstack-protector-strong -L/usr/local/lib'
libpth=/usr/lib /usr/lib/w32api /usr/local/lib /lib
libs=-lpthread -ldl -lcrypt
perllibs=-lpthread -ldl -lcrypt
libc=/usr/lib/libcygwin.a
so=dll
useshrplib=true
libperl=cygperl5_39_10.dll
gnulibc_version=''
Dynamic Linking:
dlsrc=dl_dlopen.xs
dlext=dll
d_dlsymun=undef
ccdlflags=' '
cccdlflags=' '
lddlflags=' --shared -Wl,--enable-auto-import -Wl,--export-all-symbols -Wl,--enable-auto-image-base -L/usr/local/lib -fstack-protector-strong'
Characteristics of this binary (from libperl):
Compile-time options:
DEBUGGING
HAS_LONG_DOUBLE
HAS_STRTOLD
HAS_TIMES
MULTIPLICITY
PERLIO_LAYERS
PERL_COPY_ON_WRITE
PERL_DONT_CREATE_GVSV
PERL_HASH_FUNC_SIPHASH13
PERL_HASH_USE_SBOX32
PERL_MALLOC_WRAP
PERL_OP_PARENT
PERL_PRESERVE_IVUV
PERL_TRACK_MEMPOOL
PERL_USE_DEVEL
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
Built under cygwin
Compiled at Mar 27 2024 15:03:04
%ENV:
CYGWIN="detect_bloda"
@INC:
lib
/usr/local/lib/perl5/site_perl/5.39.10/cygwin-thread-multi
/usr/local/lib/perl5/site_perl/5.39.10
/usr/local/lib/perl5/5.39.10/cygwin-thread-multi
/usr/local/lib/perl5/5.39.10
Do we know what kind of hashing is used for the base address, and how likely conflicts are in general? It sounds like if we waited until we bumped the version again, the precise failure we're seeing here would disappear. But if the hashing is meant to prevent this issue, it seems odd that we're seeing it now. Did we just get incredibly unlucky, or has something else changed that would make this type of failure more likely?
The hash doesn't look like anything complex.
I think we just ran into a hash collision, with 64-bits it's unlikely to happen, but it has here. I expect it's even worse for 32-bit cygwin, but we don't test that, and cygwin no longer support it.
The base calculation uses 17 bits of the hash (vs 10 for 32-bit), so a simple direct collision I think it's about 1 in 89 chance of a direct conflict.
I'm working on a workaround fix for the CI failure (adding -Dstatic_ext) and a note for perldelta.