libbpf-cargo: Introduce dedicated link step to BpfObjBuilder

Building a BPF object file already invokes the libbpf linker in order to
strip DWARF debug information from the resulting file. In the future we
would like to link multiple files instead of only a single one, so
refactor the code in such a way that we have a dedicated link step in
BpfObjBuilder::build().

Signed-off-by: Daniel Müller <deso@posteo.net>

libbpf-cargo/src/build.rs

@@ -153,19 +153,35 @@ impl BpfObjBuilder {
 
     /// Build a BPF object file.
     pub fn build(&mut self, src: &Path, dst: &Path) -> Result<CompilationOutput> {
+        let obj_dir = tempdir().context("failed to create temporary directory")?;
+        let mut linker = libbpf_rs::Linker::new(dst)
+            .context("failed to instantiate libbpf object file linker")?;
+
         let output = self.with_compiler_args(|compiler_args| {
-            let output = Self::compile_single(src, dst, &self.compiler, compiler_args)
+            let tmp_dst = obj_dir.path().join(src.file_name().with_context(|| {
+                format!(
+                    "input path `{}` does not have a proper file name",
+                    src.display()
+                )
+            })?);
+
+            let output = Self::compile_single(src, &tmp_dst, &self.compiler, compiler_args)
                 .with_context(|| format!("failed to compile `{}`", src.display()))?;
 
+            linker
+                .add_file(tmp_dst)
+                .context("failed to add object file to BPF linker")?;
+
             Ok(output)
         })?;
 
-        // Compilation with clang may contain DWARF information that references
-        // system specific and temporary paths. That can render our generated
-        // skeletons unstable, potentially rendering them unsuitable for inclusion
-        // in version control systems. So strip this information.
-        strip_dwarf_info(dst)
-            .with_context(|| format!("Failed to strip object file {}", dst.display()))?;
+        // The resulting object file may contain DWARF information
+        // that references system specific and temporary paths. That
+        // can render our generated skeletons unstable, potentially
+        // making them unsuitable for inclusion in version control
+        // systems. Linking has the side effect of stripping this
+        // information.
+        linker.link().context("failed to link object file")?;
 
         Ok(output)
     }
@@ -230,25 +246,6 @@ fn extract_libbpf_headers_to_disk(_target_dir: &Path) -> Result<Option<PathBuf>>
     Ok(None)
 }
 
-/// Strip DWARF information from the provided BPF object file.
-///
-/// We rely on the `libbpf` linker here, which removes debug information as a
-/// side-effect.
-fn strip_dwarf_info(file: &Path) -> Result<()> {
-    let mut temp_file = file.as_os_str().to_os_string();
-    temp_file.push(".tmp");
-
-    fs::rename(file, &temp_file).context("Failed to rename compiled BPF object file")?;
-
-    let mut linker =
-        libbpf_rs::Linker::new(file).context("Failed to instantiate libbpf object file linker")?;
-    linker
-        .add_file(temp_file)
-        .context("Failed to add object file to BPF linker")?;
-    linker.link().context("Failed to link object file")?;
-    Ok(())
-}
-
 /// Concatenate a command and its arguments into a single string.
 fn concat_command<C, A, S>(command: C, args: A) -> OsString
 where