Allow section override when using patchable-function-entries by pmur · Pull Request #157445 · rust-lang/rust

pmur · 2026-06-04T20:12:24Z

Sometimes it is necessary to group patchable function entrypoint records in distinct linker sections. This is the case for some bpf functions within the linux kernel which shouldn't be visible to ftrace.

Extend -Zpatchable-function-entry to accept an argument of the form prefix_nops,total_nops,record_section, which places all entry record into a user specified section.

Likewise, extend the patchable_function_entry attribute to accept an optional section="name" option to place a function into a specific section.

This is made possible by llvm attribute patchable-function-entry-section added in llvm 21.

rustbot · 2026-06-04T21:52:32Z

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer

rustbot · 2026-06-04T21:52:34Z

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pmur · 2026-06-04T22:01:21Z

I think this is an important feature which we weren't aware of during the initial implementation of #123115.

Adding this should allow Rust to reduce the need to implement mcount or fentry (essentially x86 only) for compatibility within the linux kernel, and likely reduces the need for rust-lang/rfcs#3917.

pmur · 2026-06-17T14:56:53Z

@rustbot reroll

chenyukang · 2026-06-22T04:07:40Z

@rustbot reroll

mejrs

What are the rules for valid section names? This implementation accepts every string. Is #[patchable_function_entry(section = "")] valid? Can I use names with null bytes in them?

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mejrs · 2026-06-24T11:18:13Z

    /// Nops after the entry
    entry: u8,
+    /// An optional section name to record the entry location
+    section: Option<String>,


Suggested change

section: Option<String>,

section: Option<Symbol>,

This will be simpler if you pass around the symbol (not String/&str) everywhere, just use as_str before calling llvm api.

I think this is initialized before string interning is available. Or, so the runtime crash leads me to believe. Is there somewhere to insert a late initialization?

Ah, I wasn't aware of that, makes sense. I'd leave it as is then.

rustbot · 2026-06-24T11:21:29Z

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

mejrs · 2026-06-24T11:24:28Z

+                sym::section => {
+                    // Duplicate entries are not allowed
+                    if section.is_some() {
+                        errored = true;


nit: usually we use Option<ErrorGuaranteed> for "has errored" variables. Feel free to implement it or not, at your option.

View changes since the review

Can that be done within the attribute parsing framework? I think I am constrained to the SingleAttributeParser trait.

Functions like duplicate_key return ErrorGuaranteed for this reason.

pmur · 2026-06-24T20:56:39Z

What are the rules for valid section names?

I think that gets punted to the assembler, and the targets object file format. For ELF, I think that is any nul terminated string.

mejrs · 2026-06-25T12:54:31Z

What are the rules for valid section names?

I think that gets punted to the assembler, and the targets object file format. For ELF, I think that is any nul terminated string.

So this would fail at the codegen stage? It makes for a better user experience to reject it earlier, similar to #155817

rustbot · 2026-06-25T15:57:19Z

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.

pmur · 2026-06-25T16:03:46Z

Updated to handle nul's more gracefully. llvm doesn't handle the inline nul gracefully.

Sometimes it is necessary to group patchable function entrypoint records in distinct linker sections. This is the case for some bpf functions within the linux kernel which shouldn't be visible to ftrace. Extend `-Zpatchable-function-entry` to accept an argument of the form `prefix_nops,total_nops,record_section`, which places all entry record into a user specified section. Likewise, extend the `patchable_function_entry` attribute to accept an optional `section="name"` option to place a function into a specific section. This is made possible by llvm attribute `patchable-function-entry-section` added in llvm 21.

rust-log-analyzer · 2026-06-25T21:18:30Z

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

  SKIP_SUBMODULES: src/gcc
  IMAGE: tidy
##[endgroup]
    Updating crates.io index
error: failed to get `askama_derive` as a dependency of package `askama_macros v0.16.0`
    ... which satisfies dependency `askama_macros = "=0.16.0"` of package `askama v0.16.0`
    ... which satisfies dependency `askama = "^0.16.0"` of package `citool v0.1.0 (/home/runner/work/rust/rust/src/ci/citool)`

Caused by:
  failed to load source for dependency `askama_derive`

Caused by:
  unable to update registry `crates-io`

Caused by:
  download of as/ka/askama_derive failed

Caused by:
  curl failed

Caused by:

mejrs · 2026-06-25T23:25:41Z

Updated to handle nul's more gracefully. llvm doesn't handle the inline nul gracefully.

Thanks, this looks good. What about empty strings? IIRC with link_name, llvm would just make up something.

pmur marked this pull request as ready for review June 4, 2026 21:52

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 4, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jun 4, 2026

rustbot assigned jackh726 Jun 4, 2026

ojeda mentioned this pull request Jun 5, 2026

Rust unstable features needed for the kernel Rust-for-Linux/linux#2

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rustbot assigned chenyukang and unassigned jackh726 Jun 17, 2026

rustbot assigned mejrs and unassigned chenyukang Jun 22, 2026

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rustbot 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 Jun 24, 2026

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pmur force-pushed the murp/extend-patchable branch from 8f7613b to c729ed3 Compare June 25, 2026 15:57

pmur force-pushed the murp/extend-patchable branch from c729ed3 to 55b62aa Compare June 25, 2026 21:16

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