[ruby-core:95105] [Ruby master Bug#16184] Entry persists in catch table even though its labels were removed, which may cause [BUG]

Issue #16184 has been reported by iv-m (Ivan Melnikov).

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Bug #16184: Entry persists in catch table even though its labels were removed, which may cause [BUG]
https://bugs.ruby-lang.org/issues/16184

* Author: iv-m (Ivan Melnikov)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.5.5p157 (2019-03-15) [mipsel-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
When `remove_unreachable_chunk` removes the code from within a rescue block, the catch table entry corresponding the block is not removed. Here is a simple reproducer (tested with ruby 2.5.5):


``` ruby
puts "BEGIN"

if false
  begin
    require "some_mad_stuff"
  rescue LoadError
    puts "no mad stuff loaded"
  end
end

puts "END"
```

Here is the corresponding disasm:

```
== disasm: #<ISeq:<main>@test2.rb:1 (1,0)-(12,10)>======================
== catch table
| catch type: rescue st: 11022376 ed: 11022516 sp: -002 cont: 0000
== disasm: #<ISeq:rescue in <main>@test2.rb:7 (7,2)-(9,2)>==============
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] "\#$!"
0000 getlocal_OP__WC__0 "\#$!"                                        (   7)
0002 getinlinecache   9, <is:0>
0005 getconstant      :LoadError
0007 setinlinecache   <is:0>
0009 checkmatch       3
0011 branchunless     20
0013 putself                                                          (   8)[Li]
0014 putstring        "no mad stuff loaded"
0016 opt_send_without_block <callinfo!mid:puts, argc:1, FCALL|ARGS_SIMPLE>, <callcache>
0019 leave                                                            (   7)
0020 getlocal_OP__WC__0 "\#$!"
0022 throw            0
| catch type: retry  st: 11022516 ed: 0000 sp: -001 cont: 11022376
|------------------------------------------------------------------------
0000 putself                                                          (   2)[Li]
0001 putstring        "BEGIN"
0003 opt_send_without_block <callinfo!mid:puts, argc:1, FCALL|ARGS_SIMPLE>, <callcache>
0006 pop
0007 putself                                                          (  12)[Li]
0008 putstring        "END"
0010 opt_send_without_block <callinfo!mid:puts, argc:1, FCALL|ARGS_SIMPLE>, <callcache>
0013 leave
```

The interesting line here is:

```
catch type: rescue st: 11022376 ed: 11022516 sp: -002 cont: 0000
```

As the instruction corresponding the `begin..rescue` block were optimized away, the `sp` filed of the continue label was still -1 (or -2) and `position`s of start, end and continue labels are never initialized. When any exception happens in the remaining code (requires a bit more complex reproducer, happens in real life), this may cause an interpreter to segfault.

I've discovered this issue when building net-scp gem, which has this `if false; require` pattern in its Rakefile: https://github.com/net-ssh/net-scp/blob/v2.0.0/Rakefile . Interpreter reproducibly segfaults when trying to run it on my MIPS32 LE machine. 






-- 
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