[ruby-core:111066] [Ruby master Bug#19156] ObjectSpace.dump_all segfault during string inspection

["byroot (Jean Boussier)" <noreply@ruby-lang.org>]

Issue #19156 has been updated by byroot (Jean Boussier).



Status changed from Open to Third Party's Issue



> It indeed does sound like it is not strictly a bug with MRI



I'll mark the ticket as 3rd party issue. I'd suggest reporting this to that project.



> I wonder if it should be more resilient to these kind of rogue objects and just filter them out?



I don't think it's possible. Ruby would need to somehow check if the memory region the string points to is still valid. 



> So somehow there is an object slot in MRI's ObjectSpace that shares an address with this empty mmap'ed space?



Not exactly. In Ruby string objects contain a `char *` pointer to an address in memory, and you can point it to whatever you want. In this case it's made to point to a mmaped region.



I'll have a quick look at that gem to see if I can find the source, but the fix might not be simple.



----------------------------------------

Bug #19156: ObjectSpace.dump_all segfault during string inspection

https://bugs.ruby-lang.org/issues/19156#change-100326



* Author: mk (Matthias Käppler)

* Status: Third Party's Issue

* Priority: Normal

* ruby -v: ruby 3.0.4p208 (2022-04-12 revision 3fa771dded) [x86_64-linux]

* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN

----------------------------------------

I am working on a feature that would allow our application to capture heap dumps during shutdown for later inspection.



These heap dumps are captured via `ObjectSpace.dump_all(output: io)`. While walking the object space, MRI occasionally segfaults while inspecting string objects in `search_nonascii` of `string.c`:



```

/usr/local/lib/ruby/3.0.0/objspace.rb:87: [BUG] Segmentation fault at 0x00007efee4201000

ruby 3.0.4p208 (2022-04-12 revision 3fa771dded) [x86_64-linux]

...



-- Control frame information -----------------------------------------------

c:0053 p:---- s:0312 e:000311 CFUNC  :_dump_all

c:0052 p:0130 s:0305 e:000304 METHOD /usr/local/lib/ruby/3.0.0/objspace.rb:87

c:0051 p:0023 s:0295 e:000294 METHOD /home/git/gitlab/lib/gitlab/memory/reports/heap_dump.rb:26

...



-- C level backtrace information -------------------------------------------

/usr/local/lib/libruby.so.3.0(rb_print_backtrace+0x11) [0x7efee4ad0c5e] vm_dump.c:758

/usr/local/lib/libruby.so.3.0(rb_vm_bugreport) vm_dump.c:998

/usr/local/lib/libruby.so.3.0(rb_bug_for_fatal_signal+0xf8) [0x7efee48d0b08] error.c:787

/usr/local/lib/libruby.so.3.0(sigsegv+0x55) [0x7efee4a23db5] signal.c:963

/lib/x86_64-linux-gnu/libpthread.so.0(__restore_rt+0x0) [0x7efee4f12140] ../sysdeps/pthread/funlockfile.c:28

/usr/local/lib/libruby.so.3.0(search_nonascii+0x30) [0x7efee4a3ca60] string.c:552

/usr/local/lib/libruby.so.3.0(coderange_scan) string.c:585

/usr/local/lib/libruby.so.3.0(enc_coderange_scan+0x1b) [0x7efee4a3e28a] string.c:709

/usr/local/lib/libruby.so.3.0(rb_enc_str_coderange) string.c:727

/usr/local/lib/ruby/3.0.0/x86_64-linux/objspace.so(is_broken_string+0x8) [0x7efeced9c304] ../../internal/string.h:116

/usr/local/lib/ruby/3.0.0/x86_64-linux/objspace.so(dump_object) objspace_dump.c:388

/usr/local/lib/ruby/3.0.0/x86_64-linux/objspace.so(heap_i+0x39) [0x7efeced9caa9] objspace_dump.c:521

/usr/local/lib/libruby.so.3.0(objspace_each_objects_without_setup+0xaf) [0x7efee48e878f] gc.c:3232

/usr/local/lib/libruby.so.3.0(objspace_each_objects_protected+0x14) [0x7efee48e87c4] gc.c:3242

/usr/local/lib/libruby.so.3.0(rb_ensure+0x12a) [0x7efee48d96aa] eval.c:1162

/usr/local/lib/libruby.so.3.0(objspace_each_objects+0x28) [0x7efee48fb458] gc.c:3310

/usr/local/lib/libruby.so.3.0(rb_objspace_each_objects) gc.c:3298

/usr/local/lib/ruby/3.0.0/x86_64-linux/objspace.so(objspace_dump_all+0x88) [0x7efeced9b068] objspace_dump.c:616

...

```



Unfortunately I couldn't get my hands on that memory region to see which strings are causing this since this doesn't always happen.



I suspect this is also a problem with MRI master since the code looks unchanged from 3.0.4.







-- 

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