Add `-Zfunction-return={keep,thunk-extern}` option #116892

ojeda · 2023-10-18T15:10:49Z

This is intended to be used for Linux kernel RETHUNK builds.

With this commit (optionally backported to Rust 1.73.0), plus a patched Linux kernel to pass the flag, I get a RETHUNK build with Rust enabled that is objtool-warning-free and is able to boot in QEMU and load a sample Rust kernel module.

Issue: #116853.

rustbot · 2023-10-18T15:11:00Z

r? @wesleywiser

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ojeda · 2023-10-18T15:24:26Z

Note that LLVM ignores the attribute in non-x86, but I have not replicated Clang's check that this only works in x86 for the CLI flag for the moment (should it be part of the target spec?).

Also, I linked the feature request issue as the tracking one in the docs -- we can reuse it changing the OP message, I assume, but please let me know if it is best to create a new one.

compiler/rustc_interface/src/tests.rs

compiler/rustc_session/src/config.rs

compiler/rustc_codegen_llvm/src/attributes.rs

src/doc/unstable-book/src/compiler-flags/function-return.md

ojeda · 2023-10-19T22:25:43Z

compiler/rustc_session/src/session.rs

+            // FIXME: In principle, the inherited base LLVM target code model could be large,
+            // but this only checks whether we were passed one explicitly (like Clang does).


If there is a way to get the effective code model, it would be nice to use it.

Maybe check sess.target.options.code_model ?

That covers even less, no? i.e. code_model() covers codegen and target options.

ojeda · 2023-10-19T22:43:51Z

I added a folder but that still counts against the tidy limit in tests/ui. Should I move the folder to e.g. tests/ui/invalid-compile-flags or increase the limit by 1?

ojeda · 2023-10-20T11:51:01Z

One more note: GCC and Clang give an error if one tries to use function-return at all, i.e. even with keep, on non-x86. However, in this PR keep is allowed for all architectures.

I did it like this because it seemed simpler (no Option) and more flexible, especially since (in the case of this flag) other architectures could support particular modes, e.g. s390 expolines (https://github.com/llvm/llvm-project/blob/49af6502c6dcb4a7f7520178bd14df396f78240c/clang/lib/Frontend/CompilerInvocation.cpp#L1942).

But perhaps consistency with GCC/Clang is better?

ojeda · 2023-10-20T13:00:57Z

I added a folder but that still counts against the tidy limit in tests/ui. Should I move the folder to e.g. tests/ui/invalid-compile-flags or increase the limit by 1?

I moved them inside the other folder, and noticed one in invalid/ should be in that invalid-compile-flags/ folder: #116975.

The Rust compiler does not support the equivalent of `-mfunction-return=thunk-extern` yet [1]. Thus, currently, `objtool` warns about it, e.g.: samples/rust/rust_print.o: warning: objtool: _R...init+0xa5c: 'naked' return found in RETHUNK build The support in `rustc` for `-Zfunction-return` has been submitted and is being reviewed [2]. It adds the needed LLVM function attributes and, with it, I got a RETHUNK kernel build with Rust enabled that does not print the `objtool` related warnings, boots in QEMU and can load a kernel loadable module. In any case, until proper/complete support is added to `rustc`, make it a hard restriction until the mitigation is in place. This may have an impact for developers that may not need/care about the mitigation in the Rust side (e.g. Ubuntu offers Rust as a "technology preview" [3]), but given we are getting closer to having the first actual in-tree Rust kernel users, it seems like the right time to disallow it. This should also avoid confusion [4]. Link: rust-lang/rust#116853 [1] Link: rust-lang/rust#116892 [2] Link: https://lore.kernel.org/rust-for-linux/ZSQXqX2%2Flhf5ICZP@gpd/ [3] Link: https://lore.kernel.org/rust-for-linux/CANiq72n6DMeXQrgOzS_+3VdgNYAmpcnneAHJnZERUQhMExg+0A@mail.gmail.com/ [4] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

When support for `-Zfunction-return` lands in Rust [1], this patch may be used to enable RETHUNK support on top of the previous patch. Link: rust-lang/rust#116892 [1] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

The Rust compiler does not support the equivalent of `-mfunction-return=thunk-extern` yet [1]. Thus, currently, `objtool` warns about it, e.g.: samples/rust/rust_print.o: warning: objtool: _R...init+0xa5c: 'naked' return found in RETHUNK build The support in `rustc` for `-Zfunction-return` has been submitted and is being reviewed [2]. It adds the needed LLVM function attributes and, with it, I got a RETHUNK kernel build with Rust enabled that does not print the `objtool` related warnings, boots in QEMU and can load a kernel loadable module. In any case, until proper/complete support is added to `rustc`, make it a hard restriction until the mitigation is in place. This may have an impact for developers that may not need/care about the mitigation in the Rust side (e.g. Ubuntu offers Rust as a "technology preview" [3]), but given we are getting closer to having the first actual in-tree Rust kernel users, it seems like the right time to disallow it. This should also avoid confusion [4]. Link: rust-lang/rust#116853 [1] Link: rust-lang/rust#116892 [2] Link: https://lore.kernel.org/rust-for-linux/ZSQXqX2%2Flhf5ICZP@gpd/ [3] Link: https://lore.kernel.org/rust-for-linux/CANiq72n6DMeXQrgOzS_+3VdgNYAmpcnneAHJnZERUQhMExg+0A@mail.gmail.com/ [4] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

When support for `-Zfunction-return` lands in Rust [1], this patch may be used to enable RETHUNK support on top of the previous patch. Link: rust-lang/rust#116892 [1] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

bors · 2023-10-30T21:39:27Z

☔ The latest upstream changes (presumably #117405) made this pull request unmergeable. Please resolve the merge conflicts.

ojeda · 2023-10-31T07:51:57Z

Resolved trivial formatting conflict.

wesleywiser · 2023-11-01T21:18:31Z

Thanks for already getting the tracking issue set up @ojeda!

@bors r+

ojeda · 2023-11-30T12:23:23Z

Rebased, added // ignore-dawrin and // ignore-sgx.

For Darwin, if there is a "standard" way to check for the extra underscore, please let me know (relaxing the regex felt wrong, but perhaps duplicating the cases is OK for this sort of test). Otherwise, LLVM's own test use linux-gnu for these tests, so I guess it is OK.

ojeda · 2023-11-30T14:09:40Z

Re-blessed tests (previous error -> 1 previous error).

This is intended to be used for Linux kernel RETHUNK builds. With this commit (optionally backported to Rust 1.73.0), plus a patched Linux kernel to pass the flag, I get a RETHUNK build with Rust enabled that is `objtool`-warning-free and is able to boot in QEMU and load a sample Rust kernel module. Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

ojeda · 2023-11-30T20:08:18Z

A bunch of trivial fixes above.

@wesleywiser @Dylan-DPC This is ready again -- let's see if other targets fail in the full CI.

wesleywiser · 2023-11-30T20:30:56Z

Thanks for the rebase @ojeda!

@bors r+ rollup=iffy

bors · 2023-11-30T20:30:59Z

📌 Commit 2d47622 has been approved by wesleywiser

It is now in the queue for this repository.

bors · 2023-11-30T22:10:33Z

⌛ Testing commit 2d47622 with merge f45631b...

bors · 2023-12-01T00:06:56Z

☀️ Test successful - checks-actions
Approved by: wesleywiser
Pushing f45631b to master...

rust-timer · 2023-12-01T02:23:33Z

Finished benchmarking commit (f45631b): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.8%	[-1.8%, -1.8%]	1
Improvements ✅ (secondary)	-1.3%	[-1.3%, -1.3%]	1
All ❌✅ (primary)	-1.8%	[-1.8%, -1.8%]	1

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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-14.8%	[-21.4%, -9.2%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-14.8%	[-21.4%, -9.2%]	7

Binary size

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

Bootstrap: 674.44s -> 674.191s (-0.04%)
Artifact size: 313.39 MiB -> 313.41 MiB (0.01%)

ojeda · 2023-12-01T12:01:54Z

@wesleywiser Thanks for the very quick review and have a nice holiday (I saw the other PR by chance when looking for this one :)

wesleywiser · 2023-12-01T12:12:04Z

Thanks! ☺️

The Rust compiler does not support the equivalent of `-mfunction-return=thunk-extern` yet [1]. Thus, currently, `objtool` warns about it, e.g.: samples/rust/rust_print.o: warning: objtool: _R...init+0xa5c: 'naked' return found in RETHUNK build The support in `rustc` for `-Zfunction-return` has been submitted and is being reviewed [2]. It adds the needed LLVM function attributes and, with it, I got a RETHUNK kernel build with Rust enabled that does not print the `objtool` related warnings, boots in QEMU and can load a kernel loadable module. In any case, until proper/complete support is added to `rustc`, make it a hard restriction until the mitigation is in place. This may have an impact for developers that may not need/care about the mitigation in the Rust side (e.g. Ubuntu offers Rust as a "technology preview" [3]), but given we are getting closer to having the first actual in-tree Rust kernel users, it seems like the right time to disallow it. This should also avoid confusion [4]. Link: rust-lang/rust#116853 [1] Link: rust-lang/rust#116892 [2] Link: https://lore.kernel.org/rust-for-linux/ZSQXqX2%2Flhf5ICZP@gpd/ [3] Link: https://lore.kernel.org/rust-for-linux/CANiq72n6DMeXQrgOzS_+3VdgNYAmpcnneAHJnZERUQhMExg+0A@mail.gmail.com/ [4] Signed-off-by: Miguel Ojeda <ojeda@kernel.org> Acked-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>

When support for `-Zfunction-return` lands in Rust [1], this patch may be used to enable RETHUNK support on top of the previous patch. Link: rust-lang/rust#116892 [1] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

The Rust compiler does not support the equivalent of `-mfunction-return=thunk-extern` yet [1]. Thus, currently, `objtool` warns about it, e.g.: samples/rust/rust_print.o: warning: objtool: _R...init+0xa5c: 'naked' return found in RETHUNK build The support in `rustc` for `-Zfunction-return` has been submitted and is being reviewed [2]. It adds the needed LLVM function attributes and, with it, I got a RETHUNK kernel build with Rust enabled that does not print the `objtool` related warnings, boots in QEMU and can load a kernel loadable module. In any case, until proper/complete support is added to `rustc`, make it a hard restriction until the mitigation is in place. This may have an impact for developers that may not need/care about the mitigation in the Rust side (e.g. Ubuntu offers Rust as a "technology preview" [3]), but given we are getting closer to having the first actual in-tree Rust kernel users, it seems like the right time to disallow it. This should also avoid confusion [4]. Link: rust-lang/rust#116853 [1] Link: rust-lang/rust#116892 [2] Link: https://lore.kernel.org/rust-for-linux/ZSQXqX2%2Flhf5ICZP@gpd/ [3] Link: https://lore.kernel.org/rust-for-linux/CANiq72n6DMeXQrgOzS_+3VdgNYAmpcnneAHJnZERUQhMExg+0A@mail.gmail.com/ [4] Signed-off-by: Miguel Ojeda <ojeda@kernel.org> Acked-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>

When support for `-Zfunction-return` lands in Rust [1], this patch may be used to enable RETHUNK support on top of the previous patch. Link: rust-lang/rust#116892 [1] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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

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Introduce perma-unstable wasm-c-abi flag #117919

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Add `-Zfunction-return={keep,thunk-extern}` option #116892

Add `-Zfunction-return={keep,thunk-extern}` option #116892

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ojeda commented Dec 1, 2023 •

edited

wesleywiser commented Dec 1, 2023

		// FIXME: In principle, the inherited base LLVM target code model could be large,
		// but this only checks whether we were passed one explicitly (like Clang does).

Add -Zfunction-return={keep,thunk-extern} option #116892

Add -Zfunction-return={keep,thunk-extern} option #116892

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ojeda commented Dec 1, 2023 • edited

wesleywiser commented Dec 1, 2023

Add `-Zfunction-return={keep,thunk-extern}` option #116892

Add `-Zfunction-return={keep,thunk-extern}` option #116892

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edited