From: Ralf Jung <post@ralfj.de>
Date: Wed, 14 Sep 2022 07:55:15 +0000 (+0200)
Subject: tweak the update
X-Git-Url: https://git.ralfj.de/web.git/commitdiff_plain/9ef7135bd7a925723cf2577b11d02bfd13cfc6d9?hp=45f7eb108dbf4258d7bd962fe15b64773fa633ae

tweak the update
---

diff --git a/ralf/_posts/2022-04-11-provenance-exposed.md b/ralf/_posts/2022-04-11-provenance-exposed.md
index 39237ae..709dbc6 100644
--- a/ralf/_posts/2022-04-11-provenance-exposed.md
+++ b/ralf/_posts/2022-04-11-provenance-exposed.md
@@ -280,8 +280,10 @@ A [first careful step](https://github.com/rust-lang/rust/pull/95547) has recentl
 **Update (2022-09-14):** After a lot more discussion, the current model pursued by the Unsafe Code Guidelines WG is to say that pointer-to-integer transmutation is permitted, but just strips provenance without exposing it.
 That means the program with the casts replaced by transmutation is UB, because the `ptr` it ends up dereferencing has invalid provenance.
 However, the transmutation itself is not UB.
-Basically, pointer-to-integer transmutation is equivalent to [the `addr` method](https://doc.rust-lang.org/nightly/std/primitive.pointer.html#method.addr), with all its caveats -- in particular, transmuting a pointer to an integer and back is like calling `addr` and then calling [`ptr::invalid`](https://doc.rust-lang.org/nightly/std/ptr/fn.invalid.html); that is not a lossless round-trip.
-This model has some nice properties that help compiler optimizations (such as removing unnecessary store-load round-trips). **/Update**
+Basically, pointer-to-integer transmutation is equivalent to [the `addr` method](https://doc.rust-lang.org/nightly/std/primitive.pointer.html#method.addr), with all its caveats -- in particular, transmuting a pointer to an integer and back is like calling `addr` and then calling [`ptr::invalid`](https://doc.rust-lang.org/nightly/std/ptr/fn.invalid.html).
+That is a *lossy* round-trip: it loses provenance information, making the resulting pointer invalid to dereference.
+It is lossy even if we use a regular integer-to-pointer cast (or `from_exposed_addr`) for the conversion back to a pointer, since the original provenance might never have been exposed.
+Compared to declaring the transmutation itself UB, this model has some nice properties that help compiler optimizations (such as removing unnecessary store-load round-trips). **/Update**
 
 ## A new hope for Rust
 
@@ -289,7 +291,7 @@ All in all, while the situation may be very complicated, I am actually more hope
 The three core pillars of this approach are:
 - making pointer-integer casts "expose" the pointer's provenance,
 - offering `ptr.addr()` to learn a pointer's address *without* exposing its provenance,
-- and disallowing pointer-integer transmutation round-trips.
+- and making pointer-integer transmutation round-trips lossy (such that the resulting pointer cannot be dereferenced).
 
 Together, they imply that we can optimize "nice" code (that follows Strict Provenance, and does not "expose" or use integer-pointer casts) perfectly, without any risk of breaking code that does use pointer-integer round-trips.
 In the easiest possible approach, the compiler can simply treat pointer-integer and integer-pointer casts as calls to some opaque external function.