Generate debug info for BPF extern declarations by danielmellado · Pull Request #161390 · rust-lang/rust

danielmellado · 2026-08-20T08:29:18Z

Reworked version of #152899, rebased on current main with review feedback addressed.

Emit debug info metadata for extern statics and functions on BPF targets so that LLVM can generate BTF entries for ksym/kfunc resolution at load time.

Allow #[link_section] on ForeignStatic and ForeignFn
Add LLVMRustDIBuilderCreateGlobalVariableExpression wrapper exposing isDefinition
Emit DIGlobalVariable (isDefinition: false) and DISubprogram (no SPFlagDefinition) for BPF externs
Refactor static debug info into shared inner function to reduce duplication

Co-authored-by: Altug Bozkurt altugbozkurt09@gmail.com

r? @jdonszelmann

rustbot · 2026-08-20T08:29:22Z

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

rustbot · 2026-08-20T08:29:24Z

jdonszelmann is currently at their maximum review capacity.
They may take a while to respond.

rustbot · 2026-08-20T08:29:26Z

Thanks for the pull request, and welcome! The Rust Project has assigned @jdonszelmann (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

Please see the contribution instructions for more information.

danielmellado · 2026-08-20T08:31:17Z

@vadorovsky @tohojo @Gbell26 ^^ FYI

bjorn3 · 2026-08-20T09:22:42Z

+        Allow(Target::ForeignStatic),
+        Allow(Target::ForeignFn),


This should probably be gated to ebpf.

View changes since the review

Thanks! I've added a target gate in LinkSectionParser::convert() that rejects #[link_section] on ForeignStatic and ForeignFn when the target arch is not BPF following the same pattern as the import_name_type x86 gate a few lines above, let me know if you'd like me to follow a different approach!

Investigate feasibility of using open-coded iterators from Aya to walk BPF program bytecode at load time (issue #19). Pin all Aya crates to git main (commit 70e1983) which includes kfunc relocation support from aya-rs/aya#1372. Add BpfInsn struct and instruction constants to bpf-rbacd-common. Declare the bpf_iter_num_{new,next,destroy} kfuncs as extern functions and implement walk_bpf_instructions() in the bpf_prog_load LSM hook. The code compiles and produces correct ELF relocations and BTF sections. End-to-end kfunc loading is blocked on rustc not emitting DWARF for extern declarations on BPF targets. Without that, bpf-linker cannot generate the .ksyms BTF DATASEC that Aya needs to resolve kfuncs. A compiler fix is in review at rust-lang/rust#161390. See docs/OPEN_CODED_ITERATORS.md for the full feasibility report and workarounds (bounded loops, bpf_loop). Signed-off-by: Daniel Mellado <dmellado@fedoraproject.org>

Enable ksyms/kfunc support for BPF programs by emitting debug info metadata for extern statics and functions on BPF targets. - Allow #[link_section] on ForeignStatic and ForeignFn targets - Add LLVMRustDIBuilderCreateGlobalVariableExpression C++ wrapper to expose the isDefinition parameter (the LLVM-C API hard-codes it to true) - Emit DIGlobalVariable with isDefinition: false for extern statics and DISubprogram without SPFlagDefinition for extern functions, gated to Arch::Bpf - Refactor build_global_var_di_node into a shared inner function to avoid duplicating the static variable debug info logic The BTF entries generated from this metadata are required by the kernel verifier to resolve ksyms and kfuncs at BPF program load time. Co-authored-by: Altug Bozkurt <altugbozkurt09@gmail.com>

tamird

Did an agentic review pass; some of these facts are at significant distance so I didn't verify them all. I'm erring on the side of false positives, please let me know if this is not acceptable.

View changes since this review

tamird · 2026-08-23T15:10:44Z

+        let function_type_metadata = create_subroutine_type(self, &signature);
+
+        let mut name = String::with_capacity(64);
+        type_names::push_item_name(tcx, def_id, false, &mut name);


Quoted LLM-generated analysis (Codex):

For #[link_name = "bpf_task_acquire"] fn acquire(...), the new metadata names the function acquire, while the LLVM symbol is bpf_task_acquire.

LLVM's BTF generator uses SP->getName() and ignores linkageName. Libbpf matches the BTF name against the ELF symbol, so it cannot resolve this extern.

tamird · 2026-08-23T15:10:44Z

+        if self.tcx.is_foreign_item(def_id) {
+            base::set_link_section(g, fn_attrs);
+            if self.tcx.sess.target.arch == Arch::Bpf {
+                debuginfo::build_extern_static_di_node(self, def_id, g);


Quoted LLM-generated analysis (Codex):

With #[linkage = "extern_weak"], check_and_apply_linkage returns Rust's synthetic internal pointer global, not the external symbol. This block therefore attaches .ksyms and debug metadata to the wrapper, leaving the actual weak symbol without matching BTF.

LLVM skips the external global without metadata and classifies the initialized internal wrapper as VAR_STATIC, regardless of DIGlobalVariable.isDefinition. Nullable weak function imports have the same problem: their native declaration is an LLVM function, but only the pointer wrapper receives global-variable metadata.

tamird · 2026-08-23T15:10:44Z

        cx.assume_dso_local(llfn, true);

+        if tcx.is_foreign_item(instance_def_id) {
+            base::set_link_section(llfn, tcx.codegen_fn_attrs(instance_def_id));


Quoted LLM-generated analysis (Codex):

If two compatible extern declarations name the same function, and the unannotated declaration is referenced before the declaration carrying #[link_section = ".ksyms"], get_declared_value(sym) reuses the first LLVM symbol and skips this block for the second declaration.

The section attribute consequently depends on reference order. LLVM only adds the external function's DATASEC entry when the function has a section, so the annotated declaration can still produce a function without its required .ksyms entry.

tamird · 2026-08-23T15:10:44Z

@@ -0,0 +1,35 @@
+// Checks that BPF extern declarations are emitted as debug info declarations.
+//
+//@ only-bpf


Quoted LLM-generated analysis (Codex):

only-bpf is evaluated against compiletest's suite target, not this test's --target flag. Normal x86_64/aarch64 CI therefore skips this test and link-section-foreign.rs.

Removing the filter alone is insufficient: these fixtures require BPF core, which the host-target suite does not build. The existing bpf-alu32.rs uses add-minicore/no_core with a target override instead.

tamird · 2026-08-23T15:10:44Z

+
+extern "C" {
+    // CHECK: @EXTERN_STATIC = external {{.*}}global i32
+    // CHECK-NOT: !DIGlobalVariable(name: "EXTERN_STATIC"


Quoted LLM-generated analysis (Codex):

This CHECK-NOT only searches between the static declaration and the next positive check, the extern_fn declaration. LLVM prints the DIGlobalVariable metadata after the functions, outside that interval.

Consequently, removing the BPF gate around extern-static debug-info emission would still pass this check; it does not cover the regression it intends to catch.

rustbot assigned jdonszelmann Aug 20, 2026

danielmellado mentioned this pull request Aug 20, 2026

feat: add open-coded iterator support for BPF instruction walking danielmellado/bpf-rbacd#30

Open

This comment has been minimized.

Sign in to view

bjorn3 reviewed Aug 20, 2026

View reviewed changes

danielmellado force-pushed the bpf-extern-debuginfo branch from 08d2375 to 17704ab Compare August 20, 2026 09:52

This comment has been minimized.

Sign in to view

danielmellado force-pushed the bpf-extern-debuginfo branch from 17704ab to cc0f67c Compare August 20, 2026 10:39

danielmellado force-pushed the bpf-extern-debuginfo branch from cc0f67c to 19fd8d5 Compare August 20, 2026 10:43

tamird reviewed Aug 23, 2026

View reviewed changes

Uh oh!

Uh oh!

Conversation

danielmellado commented Aug 20, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Aug 20, 2026

Uh oh!

rustbot commented Aug 20, 2026

Uh oh!

rustbot commented Aug 20, 2026

Uh oh!

danielmellado commented Aug 20, 2026

Uh oh!

This comment has been minimized.

bjorn3 Aug 20, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

danielmellado Aug 20, 2026

Choose a reason for hiding this comment

Uh oh!

This comment has been minimized.

tamird left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

tamird Aug 23, 2026

Choose a reason for hiding this comment

Uh oh!

tamird Aug 23, 2026

Choose a reason for hiding this comment

Uh oh!

tamird Aug 23, 2026

Choose a reason for hiding this comment

Uh oh!

tamird Aug 23, 2026

Choose a reason for hiding this comment

Uh oh!

tamird Aug 23, 2026

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

6 participants

danielmellado commented Aug 20, 2026 •

edited by rustbot

Loading

bjorn3 Aug 20, 2026 •

edited by rustbot

Loading

tamird left a comment •

edited by rustbot

Loading