Issue #16694 has been updated by k0kubun (Takashi Kokubun).


> But how about performance? It has to lead to slower compilation (at least the second time). I worry about usability of JIT with disabled PCH. At least it would be cool if we can document somewhere the impact

First of all, I don't think JIT is useful for scripting purposes as most of the code may be called only a few times which leaves few optimization chances for JIT. Thus I think the main use case is a long-running server process, and performance of compiled code is more important than time taken for compilation which should be way shorter than the process's uptime.
As you said, the usability depends on how it could be slow. I agree that the impact should be documented.

> Any program creating and reading a file which somehow affects program behaviour has the same security risk. For an attacker it would be easier to corrupt some ruby source (or byte) code file loaded during CRuby work

While I didn't talk about it as I was not sure what part of PCH Vit intended is a security risk, I agree with the point for CRuby's usage if it's about a risk caused by modifying PCH. An attacker who has permission to modify Ruby's prefix should be able to modify Ruby's standard libraries too, without waiting for JIT-ing it.

> Unfortunately PCH for PIE GCC can not work with page randominazation. 
> I don't see that somebody in GCC community will re-implement PCH in the same way as it is done in Clang.
> * use only clang for such environments

Good to know that Clang can be used as a workaround for an environment enabling page randomization.

> * header minimization (I used it originally but it does not improve JIT speed compilation when PCH is used)

Ah right... we may need it again for RHEL.

> * use one more approach based on non-fat LTO object file generated from the header as LTO works for GCC (and Clang) when page randomization (ASLR) is used.

Interesting idea. And yes, the effectiveness of inlining should be investigated before adopting this approach.

> Unfortunately, besides the advices I can not help solving this problem in the near future as I am currently busy with GCC and the light-weight JIT compiler project.

Thank you for your advice, and for all your hard work :)

----------------------------------------
Bug #16694: JIT vs hardened GCC with PCH
https://bugs.ruby-lang.org/issues/16694#change-84726

* Author: vo.x (Vit Ondruch)
* Status: Assigned
* Priority: Normal
* Assignee: k0kubun (Takashi Kokubun)
* ruby -v: ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
Preparing Ruby package for RHEL 8, I observe the following error:

~~~
$ cd /builddir/build/BUILD/ruby-2.6.3/

$ make test-all TESTS=test/ruby/test_rubyvm_mjit.rb 
Run options: "--ruby=./miniruby -I./lib -I. -I.ext/common  ./tool/runruby.rb --extout=.ext  -- --disable-gems" --excludes-dir=./test/excludes --name=!/memory_leak/

# Running tests:

[1/4] TestRubyVMMJIT#test_pause = 0.24 s
  1) Failure:
TestRubyVMMJIT#test_pause [/builddir/build/BUILD/ruby-2.6.3/test/ruby/test_rubyvm_mjit.rb:32]:
unexpected stdout:
'''
truefalsefalse```

stderr:
'''
/tmp/_ruby_mjit_p712u0.c:1:37: error: one or more PCH files were found, but they were invalid
 #include "/tmp/_ruby_mjit_hp712u0.h"
                                     ^
compilation terminated due to -Wfatal-errors.
/tmp/_ruby_mjit_p712u1.c:1:37: error: one or more PCH files were found, but they were invalid
 #include "/tmp/_ruby_mjit_hp712u0.h"
                                     ^
compilation terminated due to -Wfatal-errors.
/tmp/_ruby_mjit_p712u2.c:1:37: error: one or more PCH files were found, but they were invalid
 #include "/tmp/_ruby_mjit_hp712u0.h"
                                     ^
compilation terminated due to -Wfatal-errors.
/tmp/_ruby_mjit_p712u3.c:1:37: error: one or more PCH files were found, but they were invalid
 #include "/tmp/_ruby_mjit_hp712u0.h"
                                     ^
compilation terminated due to -Wfatal-errors.
/tmp/_ruby_mjit_p712u4.c:1:37: error: one or more PCH files were found, but they were invalid
 #include "/tmp/_ruby_mjit_hp712u0.h"
                                     ^
compilation terminated due to -Wfatal-errors.
Successful MJIT finish
```.
<5> expected but was
<0>.

Finished tests in 0.937667s, 4.2659 tests/s, 24.5290 assertions/s.  
4 tests, 23 assertions, 1 failures, 0 errors, 0 skips

ruby -v: ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-linux]
make: *** [uncommon.mk:761: yes-test-all] Error 1
~~~

As it turns out, this is because GCC in RHEL is fully hardened. Unfortunately, due to GCC design, when GCC is fully hardened, it cannot properly handle PCH due to memory address relocation. Moreover, PCH are also security risk, so it seems they are going to be disabled entirely on RHEL.

Now I wonder what is the impact on Ruby JIT. I worry that with disabled PCH, the Ruby performance with JIT will be even worser without JIT. May be it is not good idea to use GCC for JIT. What are your thoughts?

The original ticket with all the details is here [1].

[1]: https://bugzilla.redhat.com/show_bug.cgi?id=1721553



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