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Detect (non-raw) borrows of null ZST pointers in CheckNull #136601

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Merged
bors merged 2 commits into from
Feb 9, 2025
Merged

Detect (non-raw) borrows of null ZST pointers in CheckNull #136601

bors merged 2 commits into from
Feb 9, 2025

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@compiler-errors compiler-errors commented Feb 5, 2025 •
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Fixes #136568. Ensures that we check that borrows of derefs are non-null in the CheckNull pass even if it's a ZST pointee.

I'm actually surprised that this is UB in Miri, but if it's certainly UB, then this PR modifies the null check to be stricter. I couldn't find anywhere in https://doc.rust-lang.org/reference/behavior-considered-undefined.html that discusses this case specifically, but I didn't read it too closely, or perhaps it's just missing a bullet point.

On the contrary, if this is actually erroneous UB in Miri, then I'm happy to close this (and perhaps fix the null check in Miri to exclude ZSTs?)

On the double contrary, if this is still an "open question", I'm also happy to close this and wait for a decision to be made.

r? @saethlin cc @RalfJung (perhaps you feel strongly about this change)

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Feb 5, 2025
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rustbot commented Feb 5, 2025

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

@compiler-errors compiler-errors mentioned this pull request Feb 5, 2025
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theemathas commented Feb 5, 2025 •
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I think this is definitely UB, because null references, even of ZSTs, already currently cause "unexpected" behavior.

For example, this code prints None, even in debug mode:

fn main() {
    let null_ref = unsafe { &*std::ptr::null::<()>() };
    println!("{:?}", Some(null_ref));
}

This code panics in debug mode, but prints None () in release mode.

fn main() {
    let null_ref = unsafe { &*std::ptr::null::<()>() };
    let x = Some(std::hint::black_box(null_ref));
    let y = x.unwrap();
    println!("{x:?} {y:?}");
}

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compiler-errors commented Feb 5, 2025

Oh, that is a good point. I forgot that null references of ZSTs are definitely UB bc of the niche.

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RalfJung commented Feb 6, 2025

References are non-null (validity/language invariant), and "creating an invalid value" is insta-UB. So yes, &*ptr::null::<T>() is UB for every T.

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RalfJung reviewed Feb 6, 2025
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saethlin commented Feb 7, 2025

r=me with comments tweaked ^

@saethlin saethlin added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 7, 2025
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rustbot commented Feb 8, 2025

This PR changes Stable MIR

cc @oli-obk, @celinval, @ouz-a

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compiler-errors commented Feb 8, 2025

I've also fixed the typo of occured -> occurred.

@bors r=saethlin

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bors commented Feb 8, 2025

📌 Commit a61537f has been approved by saethlin

It is now in the queue for this repository.

@bors 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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 8, 2025
Urgau added a commit to Urgau/rust that referenced this pull request Feb 8, 2025
@Urgau
Rollup merge of rust-lang#136601 - compiler-errors:borrow-null-zst, r…
fdaeabd 
…=saethlin

Detect (non-raw) borrows of null ZST pointers in CheckNull

Fixes rust-lang#136568. Ensures that we check that borrows of derefs are non-null in the `CheckNull` pass **even if** it's a ZST pointee.

I'm actually surprised that this is UB in Miri, but if it's certainly UB, then this PR modifies the null check to be stricter. I couldn't find anywhere in https://doc.rust-lang.org/reference/behavior-considered-undefined.html that discusses this case specifically, but I didn't read it too closely, or perhaps it's just missing a bullet point.

On the contrary, if this is actually erroneous UB in Miri, then I'm happy to close this (and perhaps fix the null check in Miri to exclude ZSTs?)

On the double contrary, if this is still an "open question", I'm also happy to close this and wait for a decision to be made.

r? `@saethlin` cc `@RalfJung` (perhaps you feel strongly about this change)
This was referenced Feb 8, 2025
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bors added a commit to rust-lang-ci/rust that referenced this pull request Feb 9, 2025
@bors
Auto merge of rust-lang#136754 - Urgau:rollup-qlkhjqr, r=Urgau
0edbadb 
Rollup of 5 pull requests

Successful merges:

 - rust-lang#134679 (Windows: remove readonly files)
 - rust-lang#136213 (Allow Rust to use a number of libc filesystem calls)
 - rust-lang#136530 (Implement `x perf` directly in bootstrap)
 - rust-lang#136601 (Detect (non-raw) borrows of null ZST pointers in CheckNull)
 - rust-lang#136659 (Pick the max DWARF version when LTO'ing modules with different versions )

r? `@ghost`
`@rustbot` modify labels: rollup
bors added a commit to rust-lang-ci/rust that referenced this pull request Feb 9, 2025
@bors
Auto merge of rust-lang#136754 - Urgau:rollup-qlkhjqr, r=Urgau
cd6ca8e 
Rollup of 5 pull requests

Successful merges:

 - rust-lang#134679 (Windows: remove readonly files)
 - rust-lang#136213 (Allow Rust to use a number of libc filesystem calls)
 - rust-lang#136530 (Implement `x perf` directly in bootstrap)
 - rust-lang#136601 (Detect (non-raw) borrows of null ZST pointers in CheckNull)
 - rust-lang#136659 (Pick the max DWARF version when LTO'ing modules with different versions )

r? `@ghost`
`@rustbot` modify labels: rollup
bors added a commit to rust-lang-ci/rust that referenced this pull request Feb 9, 2025
@bors
Auto merge of rust-lang#136754 - Urgau:rollup-qlkhjqr, r=Urgau
a26e97b 
Rollup of 5 pull requests

Successful merges:

 - rust-lang#134679 (Windows: remove readonly files)
 - rust-lang#136213 (Allow Rust to use a number of libc filesystem calls)
 - rust-lang#136530 (Implement `x perf` directly in bootstrap)
 - rust-lang#136601 (Detect (non-raw) borrows of null ZST pointers in CheckNull)
 - rust-lang#136659 (Pick the max DWARF version when LTO'ing modules with different versions )

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors merged commit e5bc12e into rust-lang:master Feb 9, 2025
6 checks passed
@rustbot rustbot added this to the 1.86.0 milestone Feb 9, 2025
rust-timer added a commit to rust-lang-ci/rust that referenced this pull request Feb 9, 2025
@rust-timer
Unrolled build for rust-lang#136601
3747e70 
Rollup merge of rust-lang#136601 - compiler-errors:borrow-null-zst, r=saethlin

Detect (non-raw) borrows of null ZST pointers in CheckNull

Fixes rust-lang#136568. Ensures that we check that borrows of derefs are non-null in the `CheckNull` pass **even if** it's a ZST pointee.

I'm actually surprised that this is UB in Miri, but if it's certainly UB, then this PR modifies the null check to be stricter. I couldn't find anywhere in https://doc.rust-lang.org/reference/behavior-considered-undefined.html that discusses this case specifically, but I didn't read it too closely, or perhaps it's just missing a bullet point.

On the contrary, if this is actually erroneous UB in Miri, then I'm happy to close this (and perhaps fix the null check in Miri to exclude ZSTs?)

On the double contrary, if this is still an "open question", I'm also happy to close this and wait for a decision to be made.

r? ``@saethlin`` cc ``@RalfJung`` (perhaps you feel strongly about this change)
github-actions bot pushed a commit to tautschnig/verify-rust-std that referenced this pull request Mar 11, 2025