Retag as FnEntry on `drop_in_place` #103957

JakobDegen · 2022-11-04T07:07:53Z

This commit changes the mir drop shim to always retag its argument as if it were a &mut.

cc rust-lang/unsafe-code-guidelines#373

rustbot · 2022-11-04T07:07:58Z

r? @nagisa

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JakobDegen · 2022-11-04T07:08:19Z

Eh, sorry, forgot to r? ghost, feel free to unassign yourself

rustbot · 2022-11-04T07:08:21Z

Failed to set assignee to ghost: invalid assignee

Note: Only org members, users with write permissions, or people who have commented on the PR may be assigned.

compiler/rustc_mir_transform/src/shim.rs

RalfJung · 2022-11-05T09:44:08Z

Yeah assigning ghost isn't a thing any more with triagebot, I think...

JakobDegen · 2022-11-12T01:34:07Z

@saethlin had tested this and found no regressions in the ecosystem, so seems reasonable to have. Marking as ready:

r? @RalfJung

The MIR gen for this is covered by an existing test. I'd also like to add a test to ensure that we find the UB in Miri - should I be adding that to the miri subtree in this PR or submitting a separate PR to rust-lang/miri?

rustbot · 2022-11-12T01:34:13Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

RalfJung · 2022-11-13T15:53:06Z

Please add the miri test in the same PR. It's great that we can finally do this. :)

RalfJung

LGTM, apart from the missing test.

compiler/rustc_mir_transform/src/shim.rs

RalfJung · 2022-11-19T10:52:49Z

In fact we probably want at least 2 tests -- one for the aliasing concerns and one for the validity (aligned and non-null).

RalfJung · 2022-11-26T09:12:20Z

@rustbot author
@JakobDegen let me know if you need help adding the test. :)

rustbot · 2022-11-27T00:11:32Z

The Miri subtree was changed

cc @rust-lang/miri

JakobDegen · 2022-11-27T00:11:53Z

Sorry for the delay, was traveling. Tests added. @rustbot ready

rustbot · 2022-12-21T23:23:05Z

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @celinval, @vakaras

Some changes occurred to the CTFE / Miri engine

cc @rust-lang/miri

JakobDegen · 2022-12-21T23:24:33Z

Rebased and attached a commit to the end. Drop terminators are now a complete nop if the type has no drop glue and are documented as such.

@rustbot ready

…onst eval

RalfJung · 2022-12-22T15:15:05Z

compiler/rustc_middle/src/mir/syntax.rs

+    /// After drop elaboration: `Drop` terminators are a complete nop for types that have no drop
+    /// glue. For other types, `Drop` terminators behave exactly like a call to
+    /// `core::mem::drop_in_place` with a pointer to the given place.


This is the key change in the MIR spec here. It reflects what the codegen backends already do.

RalfJung · 2022-12-22T15:15:30Z

@bors r+

bors · 2022-12-22T15:15:32Z

📌 Commit 0229281 has been approved by RalfJung

It is now in the queue for this repository.

bors · 2022-12-22T17:48:46Z

⌛ Testing commit 0229281 with merge cca80b9...

bors · 2022-12-22T20:48:27Z

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing cca80b9 to master...

rust-timer · 2022-12-22T23:12:44Z

Finished benchmarking commit (cca80b9): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

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This benchmark run did not return any relevant results for this metric.

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	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-3.4%, -3.4%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.0%	[2.0%, 2.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.0%	[2.0%, 2.0%]	1

fix vec::IntoIter::drop on high-alignment ZST This fixes a soundness bug: IntoIter would call `drop_in_place` on an insufficiently aligned pointer. So if a ZST with alignment greater 1 had drop glue, that would be called with an unaligned reference. Since rust-lang#103957, Miri checks alignment even if the type does not have drop glue, which is how this bug was found. r? `@thomcc`

fix vec::IntoIter::drop on high-alignment ZST This fixes a soundness bug: IntoIter would call `drop_in_place` on an insufficiently aligned pointer. So if a ZST with alignment greater 1 had drop glue, that would be called with an unaligned reference. Since rust-lang#103957, Miri checks alignment even if the type does not have drop glue, which is how this bug was found. r? ``@thomcc``

fix vec::IntoIter::drop on high-alignment ZST This fixes a soundness bug: IntoIter would call `drop_in_place` on an insufficiently aligned pointer. So if a ZST with alignment greater 1 had drop glue, that would be called with an unaligned reference. Since rust-lang/rust#103957, Miri checks alignment even if the type does not have drop glue, which is how this bug was found. r? ``@thomcc``

Retag as FnEntry on `drop_in_place` This commit changes the mir drop shim to always retag its argument as if it were a `&mut`. cc rust-lang/unsafe-code-guidelines#373

[rustc_ty_utils] Treat `drop_in_place`'s *mut argument like &mut when adding LLVM attributes This resurrects PR rust-lang#103614, which has sat idle for a while. This could probably use a new perf run, since we're on a new LLVM version now. r? `@oli-obk` cc `@RalfJung` --- LLVM can make use of the `noalias` parameter attribute on the parameter to `drop_in_place` in areas like argument promotion. Because the Rust compiler fully controls the code for `drop_in_place`, it can soundly deduce parameter attributes on it. In rust-lang#103957, Miri was changed to retag `drop_in_place`'s argument as if it was `&mut`, matching this change.

rustbot assigned nagisa Nov 4, 2022

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JakobDegen force-pushed the drop-retag branch 2 times, most recently from 9d269f6 to c4b5c4d Compare November 12, 2022 01:32

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[rustc_ty_utils] Add the LLVM noalias parameter attribute to drop_in_place in certain cases. #103614

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JakobDegen force-pushed the drop-retag branch from c4b5c4d to 8c9d9df Compare November 27, 2022 00:11

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JakobDegen force-pushed the drop-retag branch from 8c9d9df to 143475e Compare November 27, 2022 03:04

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Dec 21, 2022

JakobDegen force-pushed the drop-retag branch from 0fa3a32 to 8f3584f Compare December 21, 2022 23:28

Don't run Drop terminators on types that do not have drop glue in c…

0229281

…onst eval

JakobDegen force-pushed the drop-retag branch from 8f3584f to 0229281 Compare December 22, 2022 00:16

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Dec 22, 2022

RalfJung mentioned this pull request Dec 22, 2022

Add test ensuring that drop terminator of no-drop type is a NOP rust-lang/miri#2737

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bors added the merged-by-bors This PR was explicitly merged by bors. label Dec 22, 2022

bors merged commit cca80b9 into rust-lang:master Dec 22, 2022

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RalfJung mentioned this pull request Dec 23, 2022

fix vec::IntoIter::drop on high-alignment ZST #106084

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JakobDegen deleted the drop-retag branch January 1, 2023 21:27

JakobDegen mentioned this pull request Jan 27, 2023

Treat Drop as a rmw operation #107271

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erikdesjardins mentioned this pull request May 20, 2023

[rustc_ty_utils] Treat drop_in_place's *mut argument like &mut when adding LLVM attributes #111807

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Retag as FnEntry on `drop_in_place` #103957

Retag as FnEntry on `drop_in_place` #103957

JakobDegen commented Nov 4, 2022 •

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Retag as FnEntry on drop_in_place #103957

Retag as FnEntry on drop_in_place #103957

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Retag as FnEntry on `drop_in_place` #103957

Retag as FnEntry on `drop_in_place` #103957

JakobDegen commented Nov 4, 2022 •

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