[ruby-core:95129] [Ruby master Bug#16184] Entry persists in catch table even though its labels were removed, which may cause [BUG]

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

File crush.log added

> Could you post a reproducer that reliably crashes Ruby?

I guess this is not really possible to crash Ruby reliably via this issue. Here is a reproducer that crashes on my MIPS32 LE machine one out of 20 times or so:

```ruby
puts "BEGIN"

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

throw "foo"
```

I'm attaching the interpreter output from one of the crashes.

As far as I understood, here's what happens. When `"foo"` is thrown from the last line of the reproducer, exception table is considered. The exception table contains only one entry. The `start` and `end` fields of the entry come from labels which position were never initialized, since the rescue block was removed before the compilation process reached `iseq_set_sequence`. So, the `start` and `end` fields of the cache table entry contain (somewhat processed) garbage from unitialized memory. Here are a few examples from machine where crush sometimes happens:

```
$ cat disasm.rb

is = RubyVM::InstructionSequence.compile_file(ARGV[0])
puts is.disasm()
$ for x in `seq 1 10`; do ruby disasm.rb test3.rb | grep ' rescue'; done
| catch type: rescue st: 0000 ed: 0001 sp: -002 cont: 0000
| catch type: rescue st: 7168352 ed: 7168492 sp: -002 cont: 0000
| catch type: rescue st: 0000 ed: 0001 sp: -002 cont: 0000
| catch type: rescue st: 9085280 ed: 9085420 sp: -002 cont: 0000
| catch type: rescue st: 0000 ed: 0001 sp: -002 cont: 0000
| catch type: rescue st: 0000 ed: 0001 sp: -002 cont: 0000
| catch type: rescue st: 0000 ed: 0001 sp: -002 cont: 0000
| catch type: rescue st: 0000 ed: 0001 sp: -002 cont: 0000
| catch type: rescue st: 7070048 ed: 7070188 sp: -002 cont: 0000
| catch type: rescue st: 10264928 ed: 10265068 sp: -002 cont: 0000
```

To actually crash Ruby, `start` should very close to 0, and `end` field should be large enough. In this case, Ruby considers the entry applicable and goes to the  position `cont` field provides (which also comes from uninitialized memory), and sets `sp` to some very large value ( `(usigned int) -1` or `(usigned int) -2`), and segfaults.

So whether Ruby crashes or not depends on the contents of the memory where labels are allocated by `compile_rescue` function. On my x86_64 machine it always has zeroes for some reason, so crashing Ruby on it is probably impossible. I'm just "lucky" to have a machine where this memory actually contains some garbage.

----------------------------------------
Bug #16184: Entry persists in catch table even though its labels were removed, which may cause [BUG]
https://bugs.ruby-lang.org/issues/16184#change-81764

* Author: iv-m (Ivan Melnikov)
* Status: Feedback
* 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. 




---Files--------------------------------
ruby-2.5.5-alt-fix-crash-on-mipsel.patch (372 Bytes)
crush.log (8.14 KB)


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