elaborate on slice wide pointer metadata #1499

RalfJung · 2024-05-23T05:35:12Z

As decided in rust-lang/unsafe-code-guidelines#510

src/behavior-considered-undefined.md

tmandry · 2024-06-03T23:19:47Z

src/behavior-considered-undefined.md

    * `dyn Trait` metadata is invalid if it is not a pointer to a vtable for
-      `Trait` that matches the actual dynamic trait the pointer or reference points to.
+      `Trait` that matches the actual dynamic type the pointer or reference points to.


Preexisting but how can this be a requirement for wide pointers, whose data portion is allowed to dangle, I assume?

Hm... yeah the actually implemented requirement is that the trait matches the trait given in the pointer type, i.e. a *const dyn Debug that points to a vtable for Display is UB.

traviscross · 2024-06-18T21:53:29Z

@rust-lang/opsem / @RalfJung: In looking carefully at this on the rustdocs call, we realized it might be helpful, both to us and when the lang teams takes this up, if someone might be able annotate the changes here (e.g. using the GH review features), with some explanation for each of these (e.g. "this was incorrect because...", "this was already true because we had said...", "we're adding this guarantee here because...").

RalfJung · 2024-06-19T06:33:30Z

src/behavior-considered-undefined.md

-  * Invalid metadata in a wide reference, `Box<T>`, or raw pointer:
+  * A reference or `Box<T>` that is [dangling], misaligned, or points to an invalid value
+    (using the actual dynamic type of the pointee as determined by the vtable in
+    the metadata in case of dynamically sized types).


This does not change anything, it clarifies what the pointed-to value is in case of e.g. &dyn Trait.

RalfJung · 2024-06-19T06:35:12Z

src/behavior-considered-undefined.md

+    (using the actual dynamic type of the pointee as determined by the vtable in
+    the metadata in case of dynamically sized types).
+  * Invalid metadata in a wide reference, `Box<T>`, or raw pointer. The requirement
+    for the metadata is determined by the type of the unsized tail:


Another piece of clarification, previously we were not very clear on what the metadata requirements are for types like (u32, [u32]): we spoke about "slice metadata" and the fact that this covered all types whose unsized tail is a slice was left implicit. Now it is explicit.

RalfJung · 2024-06-19T06:38:03Z

src/behavior-considered-undefined.md

-  * A reference or `Box<T>` that is [dangling], misaligned, or points to an invalid value.
-  * Invalid metadata in a wide reference, `Box<T>`, or raw pointer:
-    * `dyn Trait` metadata is invalid if it is not a pointer to a vtable for
-      `Trait` that matches the actual dynamic trait the pointer or reference points to.


"that matches the actual dynamic trait" was clearly nonsense. Also this is now moved to the "points to an invalid value" point since it's not about the metadata, it's about using the metadata to keep going recursively through the reference.

RalfJung · 2024-06-19T06:38:39Z

src/behavior-considered-undefined.md

      (i.e., it must not be read from uninitialized memory).
+      Furthermore, for wide references and `Box<T>`, slice metadata is invalid
+      if it makes the total size of the pointed-to value bigger than `isize::MAX`.


This is the only actual change, decided in rust-lang/unsafe-code-guidelines#510.

RalfJung · 2024-06-19T06:41:05Z

Note that the PR has two commits; it may help to consider them separately.

nikomatsakis

+1

scottmcm · 2024-07-03T17:52:33Z

Starting a lang RFC so it's officially approved, and we can use that to update docs elsewhere following this.

@rfcbot fcp merge

rfcbot · 2024-07-03T17:52:36Z

Team member @scottmcm has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

traviscross · 2024-07-03T19:50:28Z

@rfcbot reviewed

rfcbot · 2024-07-03T19:50:46Z

🔔 This is now entering its final comment period, as per the review above. 🔔

psst @scottmcm, I wasn't able to add the final-comment-period label, please do so.

traviscross · 2024-07-03T19:51:24Z

@rustbot labels -I-lang-nominated +final-comment-period

This is now in FCP so we can unnominate.

rfcbot · 2024-07-13T19:56:01Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

psst @scottmcm, I wasn't able to add the finished-final-comment-period label, please do so.

ehuss

Thanks, looks like FCP has finished.

Update books ## rust-lang/book 3 commits in f1e49bf7a8ea6c31ce016a52b8a4f6e1ffcfbc64..67fa536768013d9d5a13f3a06790521d511ef711 2024-07-12 21:21:45 UTC to 2024-07-05 17:35:06 UTC - Use Rust 2021 Edition for mdBook (rust-lang/book#3974) - Backport changes to chapter 11 (rust-lang/book#3969) - Upgrade to Rust 1.79.0 (rust-lang/book#3968) ## rust-lang/edition-guide 2 commits in 941db8b3df45fd46cd87b50a5c86714b91dcde9c..5454de3d12b9ccc6375b629cf7ccda8264640aac 2024-07-14 07:06:34 UTC to 2024-07-12 06:05:29 UTC - Update timeline etc. for Rust 2024 - 2024: Add note about never_type_fallback_flowing_into_unsafe lint level. (rust-lang/edition-guide#311) ## rust-embedded/book 1 commits in b10c6acaf0f43481f6600e95d4b5013446e29f7a..019f3928d8b939ec71b63722dcc2e46330156441 2024-07-11 17:46:10 UTC to 2024-07-11 17:46:10 UTC - typo on tooling.md (rust-embedded/book#373) ## rust-lang/reference 9 commits in 1ae3deebc3ac16e276b6558e01420f8e605def08..e2f0bdc4031866734661dcdb548184bde1450baf 2024-06-29 16:59:51 +0000 to 2024-07-15 17:52:44 +0000 - Suppress type length limit test and note that it is not enforced (rust-lang/reference#1527) - elaborate on slice wide pointer metadata (rust-lang/reference#1499) - '.inst' in inline-assembly changed to '.insn' (rust-lang/reference#1453) - Clarify that `asm!` blocks can be duplicated or deduplicated by the compiler (rust-lang/reference#1441) - Add mdbook-spec (rust-lang/reference#1520) - Add note about static libraries not linking their dependencies (rust-lang/reference#1472) - more explicitly explain the UB around immutable extern statics (rust-lang/reference#1502) - Improvements to `items/functions.md` (rust-lang/reference#1458) - Enable mdbook smart-punctuation. (rust-lang/reference#1516) ## rust-lang/rust-by-example 1 commits in 658c6c27cb975b92227936024816986c2d3716fb..89aecb6951b77bc746da73df8c9f2b2ceaad494a 2024-07-11 12:33:43 UTC to 2024-07-11 12:33:43 UTC - Update option_result.md (rust-lang/rust-by-example#1864) ## rust-lang/rustc-dev-guide 6 commits in d6e3a32a557db5902e714604def8015d6bb7e0f7..0c4d55cb59fe440d1a630e4e5774d043968edb3f 2024-07-15 15:16:43 UTC to 2024-07-01 19:05:14 UTC - Improve documentation of MIR queries & passes (rust-lang/rustc-dev-guide#1434) - Bump dependencies for date-check tool (rust-lang/rustc-dev-guide#2012) - Fix typo: lists -> lints (rust-lang/rustc-dev-guide#2011) - use "bootstrap" instead of "rustbuild" (rust-lang/rustc-dev-guide#2010) - Fix grammar issue in optimize-build.md (rust-lang/rustc-dev-guide#2009) - Update name of Fuchsia builder (rust-lang/rustc-dev-guide#2008)

Rollup merge of rust-lang#127770 - rustbot:docs-update, r=ehuss Update books ## rust-lang/book 3 commits in f1e49bf7a8ea6c31ce016a52b8a4f6e1ffcfbc64..67fa536768013d9d5a13f3a06790521d511ef711 2024-07-12 21:21:45 UTC to 2024-07-05 17:35:06 UTC - Use Rust 2021 Edition for mdBook (rust-lang/book#3974) - Backport changes to chapter 11 (rust-lang/book#3969) - Upgrade to Rust 1.79.0 (rust-lang/book#3968) ## rust-lang/edition-guide 2 commits in 941db8b3df45fd46cd87b50a5c86714b91dcde9c..5454de3d12b9ccc6375b629cf7ccda8264640aac 2024-07-14 07:06:34 UTC to 2024-07-12 06:05:29 UTC - Update timeline etc. for Rust 2024 - 2024: Add note about never_type_fallback_flowing_into_unsafe lint level. (rust-lang/edition-guide#311) ## rust-embedded/book 1 commits in b10c6acaf0f43481f6600e95d4b5013446e29f7a..019f3928d8b939ec71b63722dcc2e46330156441 2024-07-11 17:46:10 UTC to 2024-07-11 17:46:10 UTC - typo on tooling.md (rust-embedded/book#373) ## rust-lang/reference 9 commits in 1ae3deebc3ac16e276b6558e01420f8e605def08..e2f0bdc4031866734661dcdb548184bde1450baf 2024-06-29 16:59:51 +0000 to 2024-07-15 17:52:44 +0000 - Suppress type length limit test and note that it is not enforced (rust-lang/reference#1527) - elaborate on slice wide pointer metadata (rust-lang/reference#1499) - '.inst' in inline-assembly changed to '.insn' (rust-lang/reference#1453) - Clarify that `asm!` blocks can be duplicated or deduplicated by the compiler (rust-lang/reference#1441) - Add mdbook-spec (rust-lang/reference#1520) - Add note about static libraries not linking their dependencies (rust-lang/reference#1472) - more explicitly explain the UB around immutable extern statics (rust-lang/reference#1502) - Improvements to `items/functions.md` (rust-lang/reference#1458) - Enable mdbook smart-punctuation. (rust-lang/reference#1516) ## rust-lang/rust-by-example 1 commits in 658c6c27cb975b92227936024816986c2d3716fb..89aecb6951b77bc746da73df8c9f2b2ceaad494a 2024-07-11 12:33:43 UTC to 2024-07-11 12:33:43 UTC - Update option_result.md (rust-lang/rust-by-example#1864) ## rust-lang/rustc-dev-guide 6 commits in d6e3a32a557db5902e714604def8015d6bb7e0f7..0c4d55cb59fe440d1a630e4e5774d043968edb3f 2024-07-15 15:16:43 UTC to 2024-07-01 19:05:14 UTC - Improve documentation of MIR queries & passes (rust-lang/rustc-dev-guide#1434) - Bump dependencies for date-check tool (rust-lang/rustc-dev-guide#2012) - Fix typo: lists -> lints (rust-lang/rustc-dev-guide#2011) - use "bootstrap" instead of "rustbuild" (rust-lang/rustc-dev-guide#2010) - Fix grammar issue in optimize-build.md (rust-lang/rustc-dev-guide#2009) - Update name of Fuchsia builder (rust-lang/rustc-dev-guide#2008)

RalfJung mentioned this pull request May 23, 2024

Decide on validity for metadata of wide pointer/reference with slice tail rust-lang/unsafe-code-guidelines#510

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joshlf reviewed May 23, 2024

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elaborate on slice wide pointer metadata

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RalfJung force-pushed the wide-ptr-meta branch from a05b11c to 5762212 Compare May 23, 2024 06:00

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RalfJung force-pushed the wide-ptr-meta branch 2 times, most recently from 46bcc6c to 091b2f2 Compare June 19, 2024 06:36

RalfJung commented Jun 19, 2024

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typo fix and clarifications

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RalfJung force-pushed the wide-ptr-meta branch from 091b2f2 to 4f5888f Compare June 19, 2024 06:40

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traviscross mentioned this pull request Jul 3, 2024

Elaborate on the invariants for references-to-slices rust-lang/rust#121965

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Merged via the queue into rust-lang:master with commit 2a79314 Jul 13, 2024
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Update books rust-lang/rust#127770

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Add range attribute to scalar function results and arguments rust-lang/rust#128371

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elaborate on slice wide pointer metadata #1499

elaborate on slice wide pointer metadata #1499

RalfJung commented May 23, 2024

tmandry Jun 3, 2024

RalfJung Jun 4, 2024

traviscross commented Jun 18, 2024

RalfJung Jun 19, 2024

RalfJung Jun 19, 2024

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elaborate on slice wide pointer metadata #1499

elaborate on slice wide pointer metadata #1499

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