-Zharden-sls flag (target modifier) added to enable mitigation against straight line speculation (SLS) by azhogin · Pull Request #136597 · rust-lang/rust

azhogin · 2025-02-05T18:23:38Z

Flag (target modifier) to mitigate against straight line speculation (SLS).
-Zharden-sls=[none|all|return|indirect-jmp].

The flag enables the related features: +harden-sls-ijmp,+harden-sls-ret.
The flag is tracked as a target modifier to be equal between linked crates.

Tracking issue: #116851

rustbot · 2025-02-05T18:23:47Z

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rustbot · 2025-02-25T17:35:40Z

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

klensy · 2025-02-25T18:19:58Z

First commit adds retpoline feature, but PR descriptions says about sls.

azhogin · 2025-02-25T18:24:29Z

First commit adds retpoline feature, but PR descriptions says about sls.

Yes, first commit is in another PR #135927.
This one depends on.

Dylan-DPC · 2025-03-22T17:37:13Z

Marking this as blocked on #135927

Darksonn · 2025-04-28T14:40:49Z

I filed an MCP for this flag: rust-lang/compiler-team#869

bors · 2025-05-18T20:18:00Z

☔ The latest upstream changes (presumably #141232) made this pull request unmergeable. Please resolve the merge conflicts.

tests/ui/target-feature/retpoline-target-feature-flag.rs

workingjubilee · 2025-06-14T01:33:22Z

tests/ui/target-feature/retpoline-target-feature-flag.by_feature.stderr

+warning: target feature `retpoline-external-thunk` cannot be enabled with `-Ctarget-feature`: use `x86-retpoline` target modifier flag instead
+   |
+   = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>
+
+warning: 1 warning emitted


Why is this possible at all?

workingjubilee · 2025-06-14T01:33:41Z

tests/ui/target-feature/harden-sls-target-feature-flag.rs

+//@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=lib
+//@ needs-llvm-components: x86
+//@ [by_flag]compile-flags: -Zharden-sls=all
+//@ [by_feature]compile-flags: -Ctarget-feature=+harden-sls-ijmp,+harden-sls-ret


These should not be set by -Ctarget-feature.

@azhogin it would be great if we could make these hard errors now instead of an FCW

Changed to be hard error. Also it requires to add non-aux-compiler-flags directive for tests (related issue described here #147881). To workaround error generated at minicore auxiliary build.

ojeda · 2025-07-16T19:02:02Z

@azhogin Since #135927 is merged now, should this be rebased and unblocked now?

Thanks!

rustbot · 2025-11-17T12:13:46Z

compiletest directives have been modified. Please add or update docs for the
new or modified directive in src/doc/rustc-dev-guide/.

Some changes occurred in src/tools/compiletest

cc @jieyouxu

rustbot · 2025-11-18T11:21:14Z

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol, @tshepang

azhogin · 2025-11-18T13:44:34Z

In the Rust <-> Rust for Linux sync call we were wondering about the status on this one: I think what is open is the review above: #136597 (comment).

Rebased to main, the review comment addressed.

azhogin · 2025-11-18T13:46:36Z

@rustbot review

wesleywiser · 2025-12-03T21:19:54Z

compiler/rustc_codegen_ssa/src/target_features.rs

                Some((_, stability, _)) => {
                    if let Err(reason) = stability.toggle_allowed() {
-                        sess.dcx().emit_warn(errors::ForbiddenCTargetFeature {
+                        sess.dcx().emit_err(errors::ForbiddenCTargetFeature {


Sorry, I think we were unclear. We don't want to change all instances of the "forbidden target feature" warning into hard errors, we just want this new instance of it to be a hard error.

The reason these are currently a warning is because of backwards compatibly where previous stable compilers allowed these features to be specified. However, harden-sls is a new feature and so there isn't a backwards compatibility requirement. As such, it makes sense to treat invalid uses as a hard error rather than an ignorable warning.

This probably means there needs to be a new level of Stability that generates an error instead of a warning and then you can use that instead of Stability::Forbidden.

I pushed a commit here that implements that, it just needs to be used in this PR

rustbot · 2025-12-03T21:20:04Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

bors · 2025-12-10T13:36:25Z

☔ The latest upstream changes (presumably #149836) made this pull request unmergeable. Please resolve the merge conflicts.

tomassedovic · 2026-01-27T17:38:02Z

@azhogin just making sure you've seen this, the PR is now waiting on you to address this comment from 2025-12-03: #136597 (comment)

…t straight line speculation (SLS)

rustbot · 2026-01-29T02:42:09Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

azhogin · 2026-01-29T02:55:30Z

Rebased on cherry-picked wesleywiser's commit "Allow forbidden target features to be hard errors".
Harden-sls produces hard error.

@rustbot ready

rustbot assigned estebank Feb 5, 2025

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

azhogin force-pushed the azhogin/sls branch from 821c2d7 to ee66db5 Compare February 6, 2025 19:37

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azhogin force-pushed the azhogin/sls branch from ee66db5 to b5a512c Compare February 8, 2025 11:56

azhogin mentioned this pull request Feb 13, 2025

[RFC] Target Modifiers rust-lang/rfcs#3716

Merged

Darksonn mentioned this pull request Feb 13, 2025

Tracking Issue for target modifiers #136966

Open

15 tasks

ojeda mentioned this pull request Feb 13, 2025

rustc wanted features & bugfixes Rust-for-Linux/linux#355

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azhogin marked this pull request as ready for review February 25, 2025 17:35

azhogin marked this pull request as draft February 25, 2025 18:23

azhogin mentioned this pull request Feb 25, 2025

retpoline and retpoline-external-thunk flags (target modifiers) to enable retpoline-related target features #135927

Merged

briansmith mentioned this pull request Mar 12, 2025

Support mitigations that are normally done automatically by compilers in assembly briansmith/ring#2479

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azhogin force-pushed the azhogin/sls branch from b5a512c to a6f8d63 Compare July 20, 2025 09:56

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rustbot added A-compiletest Area: The compiletest test runner A-testsuite Area: The testsuite used to check the correctness of rustc T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) labels Nov 17, 2025

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azhogin force-pushed the azhogin/sls branch from 13864fe to 0e89893 Compare November 18, 2025 07:07

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azhogin force-pushed the azhogin/sls branch from 0e89893 to 692e17d Compare November 18, 2025 11:21

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wesleywiser and others added 2 commits January 29, 2026 09:27

Allow forbidden target features to be hard errors

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-Zharden-sls flag (target modifier) added to enable mitigation agains…

99b405a

…t straight line speculation (SLS)

azhogin force-pushed the azhogin/sls branch from 692e17d to 99b405a Compare January 29, 2026 02:42

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 Jan 29, 2026

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