Only use file_operations::setfl on AUFS-patched kernels (closes #167) (#182)

Debian and Ubuntu, which we're using for all our CI and local dev, have
a patched kernel which adds a setfl method to struct file_operations, so
our builds don't work on unpatched (e.g., upstream) kernels. The
patchset also does an EXPORT_SYMBOL_GPL(setfl), so we can key on that to
determine whether we need to fill in the setfl method or not.

Current patches are at
https://salsa.debian.org/kernel-team/linux/tree/debian/5.2.9-2/debian/patches/features/all/aufs5

Author: geofft

build.rs

@@ -85,8 +85,28 @@ fn handle_kernel_version_cfg(bindings_path: &PathBuf) {
     }
 }
 
+fn handle_kernel_symbols_cfg(symvers_path: &PathBuf) {
+    let f = BufReader::new(fs::File::open(symvers_path).unwrap());
+    for line in f.lines() {
+        let line = line.unwrap();
+        if let Some(symbol) = line.split_ascii_whitespace().nth(1) {
+            if symbol == "setfl" {
+                println!("cargo:rustc-cfg=kernel_aufs_setfl");
+                break;
+            }
+        }
+    }
+}
+
 fn main() {
     println!("cargo:rerun-if-env-changed=KDIR");
+    let kdir = env::var("KDIR").unwrap_or(format!(
+        "/lib/modules/{}/build",
+        std::str::from_utf8(&(Command::new("uname").arg("-r").output().unwrap().stdout))
+            .unwrap()
+            .trim()
+    ));
+
     println!("cargo:rerun-if-env-changed=CLANG");
     println!("cargo:rerun-if-changed=kernel-cflags-finder/Makefile");
     let output = Command::new("make")
@@ -136,6 +156,7 @@ fn main() {
         .expect("Couldn't write bindings!");
 
     handle_kernel_version_cfg(&out_path.join("bindings.rs"));
+    handle_kernel_symbols_cfg(&PathBuf::from(&kdir).join("Module.symvers"));
 
     let mut builder = cc::Build::new();
     println!("cargo:rerun-if-env-changed=CLANG");

src/chrdev.rs

@@ -225,6 +225,7 @@ impl FileOperationsVtable {
             #[cfg(kernel_4_20_0_or_greater)]
             remap_file_range: None,
             sendpage: None,
+            #[cfg(kernel_aufs_setfl)]
             setfl: None,
             setlease: None,
             show_fdinfo: None,