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Consting the core magic vtables breaks mod_perl #14450

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p5pRT opened this issue Jan 28, 2015 · 6 comments

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commented Jan 28, 2015

Migrated from rt.perl.org#123687 (status was 'rejected')

Searchable as RT123687$

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commented Jan 28, 2015

From @steve-m-hay

The following commit causes mod_perl (svn trunk) to fail on startup (on Windows, at least) using 5.21.8​:

http​://perl5.git.perl.org/perl.git/commit/c910fead78

It works fine on the same system (VC++ 2010, with httpd-2.4.10) using 5.21.7.

I get an access violation in modperl_env_init() when trying to replace the now const PL_vtbls with its own versions​:

void modperl_env_init(void)
{
  /* save originals */
  StructCopy(&PL_vtbl_env, &MP_PERL_vtbl_env, MGVTBL);
  StructCopy(&PL_vtbl_envelem, &MP_PERL_vtbl_envelem, MGVTBL);

  /* replace with our versions */
  StructCopy(&MP_vtbl_env, &PL_vtbl_env, MGVTBL);
  StructCopy(&MP_vtbl_envelem, &PL_vtbl_envelem, MGVTBL);
}

Is this diddling with perl's globals an unreasonable thing for mod_perl to be trying to do? I fear the answer may be yes...

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commented Jan 28, 2015

From @Leont

On Wed, Jan 28, 2015 at 2​:30 AM, Steve Hay <perlbug-followup@​perl.org>
wrote​:

The following commit causes mod_perl (svn trunk) to fail on startup (on
Windows, at least) using 5.21.8​:

http​://perl5.git.perl.org/perl.git/commit/c910fead78

It works fine on the same system (VC++ 2010, with httpd-2.4.10) using
5.21.7.

I get an access violation in modperl_env_init() when trying to replace the
now const PL_vtbls with its own versions​:

void modperl_env_init(void)
{
/* save originals */
StructCopy(&PL_vtbl_env, &MP_PERL_vtbl_env, MGVTBL);
StructCopy(&PL_vtbl_envelem, &MP_PERL_vtbl_envelem, MGVTBL);

/\* replace with our versions \*/
StructCopy\(&MP\_vtbl\_env\, &PL\_vtbl\_env\, MGVTBL\);
StructCopy\(&MP\_vtbl\_envelem\, &PL\_vtbl\_envelem\, MGVTBL\);

}

Is this diddling with perl's globals an unreasonable thing for mod_perl to
be trying to do? I fear the answer may be yes...

Yes, I think overwriting the vtables is unreasonable. What I think is more
reasonable (but still not completely reasonable or sane) is to lookup
%ENV's magic, and then replace the pointer to PL_vtbl_env with a pointer to
MP_vtbl_env. You may have to add some svt_copy magic to make it cast
MP_vtbl_envelem instead of PL_vtbl_envelem on the elements.

Leon

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commented Jan 28, 2015

The RT System itself - Status changed from 'new' to 'open'

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commented Jan 29, 2015

From @bulk88

Leon Timmermans wrote​:

Yes, I think overwriting the vtables is unreasonable. What I think is
more reasonable (but still not completely reasonable or sane) is to
lookup %ENV's magic, and then replace the pointer to PL_vtbl_env with a
pointer to MP_vtbl_env. You may have to add some svt_copy magic to make
it cast MP_vtbl_envelem instead of PL_vtbl_envelem on the elements.

Leon

All of that plus.

Perl mg vtable ptrs are basically GUIDs. Even if all members in the
vtable are 0, it serves to separate classes/modules from other
classes/modules (unless the CC deduped things, but MSVC only dedups
functions and string literals, not const named global vars). Perl
originally had just the U8/I8 "how"/"mg_type" to separate MGs. 2 were
reserved for extensions, 'U' and '~'. At some point someone invented to
use the vtable ptrs to separate all the ~ magic. My (MSVC) C debugger
shows symbol names of pointers if they are global/static allocated C
vars, if I inspect a struct. In examining a crash dump, to see a foreign
pointer, means it is time to track down that foreign pointer. A perl
core/interp C global var ptr will always be between
0x2800,0000-0x2820,0000 on Win32 Perl (Sarathy picked 0x28000000 a very
long time ago with no comments in git history why). If the pointer isn't
in that range, then it is malloced or from a XS DLL. So if a module
wants its own tied/magic %ENV, replace the existing PL_vtbl* ptr with
its own MP_vtbl* symbol (possibly with copied over fnc ptr from the
previous PL_vtbl* struct), this way in examining a crash dump, you know
this magic hash/SV is owned/maintained by modperl and not p5p since it
will have a MP_vtbl* prefix in the C debugger instead of a PL_vtbl*
symbol. Checking for patching of the core vtbl will require 1 or 2 more
clicks, to look inside PL_vtbl* struct, and see the C function name
symbols of those pointers, which is a tiny bit more work.

Further commenting, since MS is closed source and has1 developer, Win32
Kernel uses 4 byte integers, that happen to always be printable ASCII,
to identify memory allocation types. "Allocation types" in other words
are classes/objects, but Win32 kernel is plain C, so "classes" is a
fuzzy word. http​://alter.org.ua/docs/win/pooltag/ . But to pull off pool
tags requires exactly 1 person to hand them out and maintain a registry
like domain names have, which is difficult with FOSS, but easy inside 1
company (MS). Perl's mg vtable ptrs seem to be unique enough, and
expanding mg_type from char to U32 seems like a waste at this point in
the history of Perl.

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commented Feb 6, 2015

From @steve-m-hay

Thanks for the replies. I've raised a mod_perl ticket for this instead now​:

https://rt.cpan.org/Ticket/Display.html?id=101962

so am closing this perl ticket.

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commented Feb 6, 2015

@steve-m-hay - Status changed from 'open' to 'rejected'

@p5pRT p5pRT closed this Feb 6, 2015
@p5pRT p5pRT added the Severity Low label Oct 19, 2019
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