[ruby-core:99183] [Ruby master Bug#17023] How to prevent String memory to be relocated in ruby-ffi

[larskanis@gmail.com]

Issue #17023 has been updated by larskanis (Lars Kanis).


Thank you Aaron for looking into this! The patch avoids that the particular string is relocated, but unfortunately the patch is not sufficient to avoid relocation of any strings given to FFI. A slightly modified version of the script fails on ruby-2.7 and on master:

```ruby
File.write "string-relocate.c", <<-EOC
  static char *g_str;

  void set(char* str) {
    g_str = str;
  }

  char* get() {
    return g_str;
  }
EOC
system "gcc -shared -fPIC string-relocate.c -o string-relocate.so"

require 'ffi'

class Foo
  extend FFI::Library
  ffi_lib File.expand_path('string-relocate.so')

  attach_function :set, [:string], :void
  attach_function :get, [], :string

  A = ""

  def initialize(count)
    A.replace "a" * count

    set(A)

    GC.verify_compaction_references(toward: :empty, double_heap: true)

    puts "get(#{count}): #{get} (should be: #{A})"
  end
end

Foo.new(23)
Foo.new(24)
```

The output is something like:
```
get(23): h8�U (should be: aaaaaaaaaaaaaaaaaaaaaaa)
get(24): aaaaaaaaaaaaaaaaaaaaaaaa (should be: aaaaaaaaaaaaaaaaaaaaaaaa)
```

In fact we recommend using a constant for this kind of usage since years: https://github.com/ffi/ffi/wiki/Core-Concepts#string-memory-allocation

I don't think there is a generic way to fix this issue other than pinning all strings on `RSTRING_PTR()` usage. However `RSTRING_PTR()` would be massive over-pinning, since the pointer is most often used only for a very short time frame and the string is movable again afterwards.

So actually I was looking for something like `rb_obj_mark_unmovable()` to pin the string. FFI could call this function on all string pointers passed to C. In FFI we don't know how long the string pointer is in use in the C library, so that marking all argument strings as unmovable is over-pinning as well. But for sure this would pin way less strings than `RSTRING_PTR()`.


----------------------------------------
Bug #17023: How to prevent String memory to be relocated in ruby-ffi
https://bugs.ruby-lang.org/issues/17023#change-86562

* Author: larskanis (Lars Kanis)
* Status: Closed
* Priority: Normal
* Assignee: tenderlovemaking (Aaron Patterson)
* ruby -v: ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-linux]
* Backport: 2.5: DONTNEED, 2.6: DONTNEED, 2.7: REQUIRED
----------------------------------------
[ruby-ffi](https://github.com/ffi/ffi) allows to pass String objects to C by using the `:string` argument type. This way the string memory returned by `RSTRING_PTR` is passed to the C function. The user has to ensure on Ruby level that the string isn't GC'ed - as long as it is used on C level. That's the contract and this worked with all past ruby versions, but ruby-2.7 introduced `GC.compact`, which can relocate strings to another memory location.

This example shows the situation and that the string is relocated although it is still referenced in ruby code:
```ruby
File.write "string-relocate.c", <<-EOC
  static char *g_str;

  void set(char* str) {
    g_str = str;
  }

  char* get() {
    return g_str;
  }
EOC
system "gcc -shared -fPIC string-relocate.c -o string-relocate.so"

require 'ffi'

class Foo
  extend FFI::Library
  ffi_lib File.expand_path('string-relocate.so')

  attach_function :set, [:string], :void
  attach_function :get, [], :string

  def initialize(count)
    proc {} # necessary to trigger relocation
    a = "a" * count
    set(a)

    GC.verify_compaction_references(toward: :empty, double_heap: true)

    puts "get(#{count}): #{get} (should be: #{a})"
  end
end

Foo.new(23)
Foo.new(24)
```

The output looks like so on ruby-2.7.1:
```
get(23):  (should be: aaaaaaaaaaaaaaaaaaaaaaa)
get(24): aaaaaaaaaaaaaaaaaaaaaaaa (should be: aaaaaaaaaaaaaaaaaaaaaaaa)
```

So using `GC.compact` while a string parameter is in use, both on Ruby and on C level, can cause invalid memory access. How can this prevented?

A C extension is expected to use `rb_gc_mark()` in order to pin the VALUE to a memory location. But I couldn't find a way to pin a `VALUE` at the time the argument is passed to the C function, which is the only point in time ruby-ffi has access to it.


---Files--------------------------------
string-relocate.rb (653 Bytes)
0001-Only-marked-objects-should-be-considered-movable.patch (1.23 KB)


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