PR #20530 introduced a save stack overflow in regexec.c

[demerphq]

Description
In #20530 we switched to using SSCHECK() instead of SSGROW(). This introduced a regresssion and exposed that SSCHECK() does not actually do what it looks like, and does not guarantee that the stack will hold the desired elements after it executes. (Which is pretty strange really.)

Steps to Reproduce
from: #20530 (comment)

Run this under valgrind:

  $_ = "x" x 1000;
    my $pat = '(.)' x 200;
    $pat = qr/($pat)+/;
    m/$pat/;

Expected behavior
We should not overflow. See #20824

Perl configuration
This is the configuration from a built version of the patch in #20824 which fixes this problem.

$ ./perl -Ilib -V
Summary of my perl5 (revision 5 version 37 subversion 9) configuration:
  Local Commit: b98001c8430e0402997a363600e22bdc037f1a96
  Ancestor: 81d9b321946a5fdd8db818f5c8d138728d21423c
  Platform:
    osname=linux
    osvers=5.14.0-1057-oem
    archname=x86_64-linux-thread-multi
    uname='linux oncidium 5.14.0-1057-oem #64-ubuntu smp mon jan 23 17:02:19 utc 2023 x86_64 x86_64 x86_64 gnulinux '
    config_args='-Dusethreads -Doptimize=-g -d -Dusedevel -Dcc=ccache gcc -Dld=gcc -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 ='-D_REENTRANT -D_GNU_SOURCE -fwrapv -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    optimize='-g'
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fwrapv -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion=''
    gccversion='12.2.1 20230218'
    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='gcc'
    ldflags =' -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /home/yorton/install/gcc-12/lib /usr/lib/x86_64-linux-gnu /usr/lib /usr/lib64
    libs=-lpthread -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -ldl -lm -lcrypt -lutil -lc
    libc=libc-2.31.so
    so=so
    useshrplib=false
    libperl=libperl.a
    gnulibc_version='2.31'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E'
    cccdlflags='-fPIC'
    lddlflags='-shared -g -L/usr/local/lib -fstack-protector-strong'


Characteristics of this binary (from libperl): 
  Compile-time options:
    DEBUGGING
    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
    USE_THREAD_SAFE_LOCALE
  Locally applied patches:
    386638a2b4294c7af449dcfdf670df85dedbd92a
    725e4a6205afc87be9ae92412474d39b08624dc8
    0d357e63ea9b2b6cfc184c98e44b12ff1a9f3184
    6c2b27ece1fe862309ee16d66bddd388fb7af6ba
    828bae551a7fc07a99c6bf42fdae003f7929d080
    251efb6a81241db52213170f7c1d2ab003d91bb3
    b98001c8430e0402997a363600e22bdc037f1a96
  Built under linux
  Compiled at Feb 20 2023 04:44:25
  %ENV:
    PERLBREW_CONFIGURE_FLAGS="-de -Dcc=ccache\ gcc -Dld=gcc"
    PERLBREW_HOME="/home/yorton/.perlbrew"
    PERLBREW_MANPATH="/home/yorton/perl5/perlbrew/perls/perl-5.34.1/man"
    PERLBREW_PATH="/home/yorton/perl5/perlbrew/bin:/home/yorton/perl5/perlbrew/perls/perl-5.34.1/bin"
    PERLBREW_PERL="perl-5.34.1"
    PERLBREW_ROOT="/home/yorton/perl5/perlbrew"
    PERLBREW_SHELLRC_VERSION="0.88"
    PERLBREW_VERSION="0.88"
  @INC:
    lib
    /usr/local/lib/perl5/site_perl/5.37.9/x86_64-linux-thread-multi
    /usr/local/lib/perl5/site_perl/5.37.9
    /usr/local/lib/perl5/5.37.9/x86_64-linux-thread-multi
    /usr/local/lib/perl5/5.37.9

[demerphq]

Fixed by #20824