Issue #13245 has been updated by Benoit Daloze.


I strongly agree.

Since the API is Thread#[key] and not <pre>Thread[key]</pre> (meaning it should only be possible to get TLS from the current thread), this is a kind of bad API in the first place (same for thread_variable_*).


One of the consequences is that parallel implementations of Ruby must then synchronize when accessing these fake thread-locals,
as they might be modified concurrently by another thread.
Just disallowing writes from another thread does not fully help for this,
reads from another thread should also be prohibited to be truly thread-local and need no synchronization.

There must be many failures though with this change, could you post the output of test-all?

I know a few places in ruby/spec that use TLS wrong,
even though it does look "convenient" as it's a way to store inter-thread information per thread.
I'll be happy to fix those if we go ahead with this.

----------------------------------------
Feature #13245: [PATCH] reject inter-thread TLS modification
https://bugs.ruby-lang.org/issues/13245#change-63173

* Author: Shyouhei Urabe
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
Thread local and fiber local storages are by nature expected to be
visible from within the thread / fiber and not from anywhere else. OK?

That's a hoax.

The truth is they are visible from _anywhere_.  Thread local variable
for instance is just a set of key-value pair that is stored inside of
a thread instance variable.  No access control is ever attempted since
the beginning.  So you can touch any thread's any local storages at
will at any time without any synchronization.

This embarrasing "feature" has been there for a long time.  I thinnk
it's too late to remove the TLS accessor methods because they are used
everywhere.  BUT, even with that assumption, I strongly believe that
tampering other thread's local storage is something you must not.

Preventing such access should harm no one.

Signed-off-by: Urabe, Shyouhei <shyouhei / ruby-lang.org>

```diff
---
 test/ruby/test_fiber.rb  | 7 +++++++
 test/ruby/test_thread.rb | 7 +++++++
 thread.c                 | 8 ++++++++
 3 files changed, 22 insertions(+)

diff --git a/test/ruby/test_fiber.rb b/test/ruby/test_fiber.rb
index d1d15828fc..319de9e7b2 100644
--- a/test/ruby/test_fiber.rb
+++ b/test/ruby/test_fiber.rb
@@ -169,6 +169,13 @@ def tvar(var, val)
     assert_equal(nil, Thread.current[:v]);
   end
 
+  def test_inter_thread_tls
+    t = Thread.new { }
+    assert_raise(ThreadError) do
+      t[:foo] = "bar"
+    end
+  end
+
   def test_alive
     fib = Fiber.new{Fiber.yield}
     assert_equal(true, fib.alive?)
diff --git a/test/ruby/test_thread.rb b/test/ruby/test_thread.rb
index c82018dc8e..5ad663becf 100644
--- a/test/ruby/test_thread.rb
+++ b/test/ruby/test_thread.rb
@@ -95,6 +95,13 @@ def test_thread_variable_frozen
     end
   end
 
+  def test_inter_thread_variable
+    t = Thread.new { }
+    assert_raise(ThreadError) do
+      t.thread_variable_set(:foo, "bar")
+    end
+  end
+
   def test_mutex_synchronize
     m = Thread::Mutex.new
     r = 0
diff --git a/thread.c b/thread.c
index 597fd293d6..03f7777aae 100644
--- a/thread.c
+++ b/thread.c
@@ -3154,6 +3154,10 @@ rb_thread_local_aset(VALUE thread, ID id, VALUE val)
 	rb_error_frozen("thread locals");
     }
 
+    if (th != GET_THREAD()) {
+        rb_raise(rb_eThreadError, "inter-thread access of TLS prohibited");
+    }
+
     return threadptr_local_aset(th, id, val);
 }
 
@@ -3231,6 +3235,10 @@ rb_thread_variable_set(VALUE thread, VALUE id, VALUE val)
 	rb_error_frozen("thread locals");
     }
 
+    if (thread != GET_THREAD()->self) {
+        rb_raise(rb_eThreadError, "inter-thread access of TLS prohibited");
+    }
+
     locals = rb_ivar_get(thread, id_locals);
     return rb_hash_aset(locals, rb_to_symbol(id), val);
 }
-- 
2.11.1
```



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