bpf_printk: breaking skeleton generation

[mimullin-bbry]

The following patch breaks compilation of examples

diff --git a/examples/runqslower/src/bpf/runqslower.bpf.c b/examples/runqslower/src/bpf/runqslower.bpf.c
index cf821da..eca67ff 100644
--- a/examples/runqslower/src/bpf/runqslower.bpf.c
+++ b/examples/runqslower/src/bpf/runqslower.bpf.c
@@ -56,6 +56,7 @@ int handle__sched_wakeup(u64 *ctx)
        /* TP_PROTO(struct task_struct *p) */
        struct task_struct *p = (void *)ctx[0];

+       bpf_printk("this causes an error, but shouldn't");
        return trace_enqueue(p->tgid, p->pid);
 }

The following is observed during a cargo build

   Compiling capable v0.1.0 (/code/libbpf-rs-mainline/examples/capable)
error: failed to run custom build command for `runqslower v0.1.0 (/code/libbpf-rs-mainline/examples/runqslower)`

Caused by:
  process didn't exit successfully: `/code/libbpf-rs-mainline/target/debug/build/runqslower-b81ac9d4cca6e1bc/build-script-build` (exit status: 101)
  --- stderr
  Warning: unrecognized map: .maps
  Warning: unrecognized map: license
  error: expected `:`, found `.`
     --> <stdin>:167:29
      |
  167 |     pub handle__sched_wakeup.____fmt: [i8; 36],
      |                             ^ expected `:`

  thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Generate("Failed to generate skeleton for /tmp/.tmpizMHT9/runqslower.o: Failed to rustfmt: exit status: 1")', examples/runqslower/build.rs:19:47
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...
error: build failed

[mimullin-bbry]

Additional Info.

Adding the following patch solves the problem.

diff --git a/examples/runqslower/build.rs b/examples/runqslower/build.rs
index 59f3ddb..b164db3 100644
--- a/examples/runqslower/build.rs
+++ b/examples/runqslower/build.rs
@@ -16,6 +16,6 @@ fn main() {
     // all up.
     create_dir_all("./src/bpf/.output").unwrap();
     let skel = Path::new("./src/bpf/.output/runqslower.skel.rs");
-    SkeletonBuilder::new(SRC).generate(&skel).unwrap();
+    SkeletonBuilder::new(SRC).clang_args("-DBPF_NO_GLOBAL_DATA").generate(&skel).unwrap();
     println!("cargo:rerun-if-changed={}", SRC);
 }

Using BPF_NO_GLOBAL_DATA as default when doing a SkeletonBuilder, or clearly documenting that users must enable BPF_NO_GLOBAL_DATA when using bpf_printk (and limiting to printing three variables). Might be a temporary work around.

A naive solution is to apply the following diff.

diff --git a/libbpf-cargo/src/btf/btf.rs b/libbpf-cargo/src/btf/btf.rs
index ae86bff..2b783ce 100644
--- a/libbpf-cargo/src/btf/btf.rs
+++ b/libbpf-cargo/src/btf/btf.rs
@@ -604,7 +604,10 @@ impl<'a> Btf<'a> {
                     for datasec_var in &t.vars {
                         let var = match self.type_by_id(datasec_var.type_id)? {
                             BtfType::Var(v) => {
-                                if let Some(next_ty_id) = next_type(v.type_id)? {
+                                if v.name.contains(".____fmt") || v.name.contains(".___fmt") {
+                                    println!("skip the bpf_printk error");
+                                    continue;
+                                } else if let Some(next_ty_id) = next_type(v.type_id)? {
                                     dependent_types.push(next_ty_id);
                                 }

But this is super kludgy.

[anakryiko]

libbpf-rs should skip generating fields for static variables. That's what we did for libbpf skeletons a while ago and mandate that only global variables make it to the skeleton.