Remove kernel-cflags-finder (fixes #191)

The bulk of this is the changes from #185 that didn't make it into #188.

We also upgrade Travis's Clang/LLVM version to 9 to pick up some sort
of bugfix in relocation processing. With older versions, we get

[ 1827.072410] module: x86/modules: Skipping invalid relocation target, existing value is nonzero for type 1, loc 00000000197e607d, val ffffffffc0703520

(where that "loc" seems to be random/garbage data, disconcertingly).
Clang 9 is already required by kernels 5.0+ so this shouldn't be too
much of a problem.

Co-authored-by: Alex Gaynor <alex.gaynor@gmail.com>

Author: geofft

.travis.yml

@@ -1,6 +1,5 @@
 matrix:
     include:
-        - dist: xenial
         - dist: bionic
 
 language: rust
@@ -13,10 +12,11 @@ branches:
 
 install:
     - sudo apt-get install -y "linux-headers-$(uname -r)" coreutils
+    - sudo apt-get install clang-9
     - rustup component add rust-src rustfmt clippy
 
 script:
-    - ./tests/run_tests.py
+    - CLANG=clang-9 ./tests/run_tests.py
     - |
       for p in . hello-world tests/*; do
         if [ -d "$p" ]; then

README.md

@@ -19,9 +19,8 @@ various other examples in the tests/ directory.
 We run [bindgen](https://github.com/rust-lang/rust-bindgen) on the
 kernel headers to generate automatic Rust FFI bindings. bindgen is
 powered by [Clang](https://clang.llvm.org), so we use the kernel's
-own build system to determine the appropriate CFLAGS (see
-`kernel-cflags-finder`) and pass them to bindgen (see `build.rs`). Then we
-write safe bindings to these types (see the various files inside `src/`).
+own build system to determine the appropriate CFLAGS. Then we write safe
+bindings to these types (see the various files inside `src/`).
 
 Each kernel module in Rust lives in a `staticlib` crate, which generates
 a `.a` file. We pass this object to the Linux kernel's own module build
@@ -50,14 +49,15 @@ for expected status. They'll need src/c_types.rs ported, too.
 You'll need to have [Rust](https://www.rust-lang.org) - in particular
 Rust nightly, as we use [some unstable
 features](https://github.com/fishinabarrel/linux-kernel-module-rust/issues/41) -
-and [Clang](https://clang.llvm.org) installed. You need LLVM/Clang 3.9
-or higher [to bind constants
-properly](https://github.com/rust-lang/rust-bindgen/issues/1316). If
-you're running kernel 5.0 or newer, [you'll need Clang
-9](https://github.com/fishinabarrel/linux-kernel-module-rust/issues/123)
-(released September 2019), which adds support for `asm goto`.
-You may need to set the `CLANG` environment variable appropriately,
-e.g., `CLANG=clang-9`.
+and [Clang](https://clang.llvm.org) installed. You need LLVM/Clang 9
+(released September 2019) or higher for multiple reasons, primarily
+[support for `asm goto`]
+(https://github.com/fishinabarrel/linux-kernel-module-rust/issues/123).
+If you're on Debian, Ubuntu, or a derivative, https://apt.llvm.org is
+great.
+
+If the binary named `clang` is too old, make sure to set the `CC` or
+`CLANG` environment variable appropriately, e.g., `CC=clang-9`.
 
 Very recent kernels may require newer versions of Clang - try Clang 11
 if older versions don't work for you.

build.rs

@@ -1,6 +1,5 @@
 use std::io::{BufRead, BufReader};
 use std::path::PathBuf;
-use std::process::Command;
 use std::{env, fs};
 
 const INCLUDED_TYPES: &[&str] = &["file_system_type", "mode_t", "umode_t", "ctl_table"];
@@ -112,38 +111,39 @@ fn handle_kernel_symbols_cfg(symvers_path: &PathBuf) {
     }
 }
 
-fn add_env_if_present(cmd: &mut Command, var: &str) {
-    if let Ok(val) = env::var(var) {
-        cmd.env(var, val);
+// Takes the CFLAGS from the kernel Makefile and changes all the include paths to be absolute
+// instead of relative.
+fn prepare_cflags(cflags: &str, kernel_dir: &str) -> Vec<String> {
+    let cflag_parts = shlex::split(&cflags).unwrap();
+    let mut cflag_iter = cflag_parts.iter();
+    let mut kernel_args = vec![];
+    while let Some(arg) = cflag_iter.next() {
+        if arg.starts_with("-I") && !arg.starts_with("-I/") {
+            kernel_args.push(format!("-I{}/{}", kernel_dir, &arg[2..]));
+        } else if arg == "-include" {
+            kernel_args.push(arg.to_string());
+            let include_path = cflag_iter.next().unwrap();
+            if include_path.starts_with('/') {
+                kernel_args.push(include_path.to_string());
+            } else {
+                kernel_args.push(format!("{}/{}", kernel_dir, include_path));
+            }
+        } else {
+            kernel_args.push(arg.to_string());
+        }
     }
+    kernel_args
 }
 
 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 mut cmd = Command::new("make");
-    cmd.arg("-C")
-        .arg("kernel-cflags-finder")
-        .arg("-s")
-        .env_clear();
-    add_env_if_present(&mut cmd, "KDIR");
-    add_env_if_present(&mut cmd, "CLANG");
-    add_env_if_present(&mut cmd, "PATH");
-    let output = cmd.output().unwrap();
-    if !output.status.success() {
-        eprintln!("kernel-cflags-finder did not succeed");
-        eprintln!("stdout: {}", std::str::from_utf8(&output.stdout).unwrap());
-        eprintln!("stderr: {}", std::str::from_utf8(&output.stderr).unwrap());
-        std::process::exit(1);
-    }
+    println!("cargo:rerun-if-env-changed=CC");
+    println!("cargo:rerun-if-env-changed=abs_srctree");
+    println!("cargo:rerun-if-env-changed=c_flags");
+
+    let kernel_dir = env::var("abs_srctree").expect("Must be invoked from kernel makefile");
+    let kernel_cflags = env::var("c_flags").expect("Add 'export c_flags' to Kbuild");
+
+    let kernel_args = prepare_cflags(&kernel_cflags, &kernel_dir);
 
     let target = env::var("TARGET").unwrap();
 
@@ -155,8 +155,8 @@ fn main() {
         .rustfmt_bindings(true);
 
     builder = builder.clang_arg(format!("--target={}", target));
-    for arg in shlex::split(std::str::from_utf8(&output.stdout).unwrap()).unwrap() {
-        builder = builder.clang_arg(arg.to_string());
+    for arg in kernel_args.iter() {
+        builder = builder.clang_arg(arg.clone());
     }
 
     println!("cargo:rerun-if-changed=src/bindings_helper.h");
@@ -182,14 +182,14 @@ 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"));
+    handle_kernel_symbols_cfg(&PathBuf::from(&kernel_dir).join("Module.symvers"));
 
     let mut builder = cc::Build::new();
-    builder.compiler(env::var("CLANG").unwrap_or_else(|_| "clang".to_string()));
+    builder.compiler(env::var("CC").unwrap_or_else(|_| "clang".to_string()));
     builder.target(&target);
     builder.warnings(false);
     builder.file("src/helpers.c");
-    for arg in shlex::split(std::str::from_utf8(&output.stdout).unwrap()).unwrap() {
+    for arg in kernel_args.iter() {
         builder.flag(&arg);
     }
     builder.compile("helpers");

hello-world/Kbuild

@@ -3,6 +3,9 @@ helloworld-objs := hello_world.rust.o
 
 CARGO ?= cargo
 
+export c_flags
+export abs_srctree ?= ${CURDIR}
+
 $(src)/target/x86_64-linux-kernel/debug/libhello_world.a: $(src)/Cargo.toml $(wildcard $(src)/src/*.rs)
 	cd $(src); $(CARGO) build -Z build-std=core,alloc --target=x86_64-linux-kernel
 

hello-world/Makefile

@@ -1,7 +1,12 @@
 KDIR ?= /lib/modules/$(shell uname -r)/build
 
+CLANG ?= clang
+ifeq ($(origin CC),default)
+CC := ${CLANG}
+endif
+
 all:
-	$(MAKE) -C $(KDIR) M=$(CURDIR)
+	$(MAKE) -C $(KDIR) M=$(CURDIR) CC=$(CC) CONFIG_CC_IS_CLANG=y
 
 clean:
-	$(MAKE) -C $(KDIR) M=$(CURDIR) clean
+	$(MAKE) -C $(KDIR) M=$(CURDIR) CC=$(CC) clean

kernel-cflags-finder/.gitignore

@@ -3,6 +3,9 @@ testmodule-objs := $(TEST_NAME).rust.o
 
 CARGO ?= cargo
 
+export c_flags
+export abs_srctree ?= ${CURDIR}
+
 $(src)/target/x86_64-linux-kernel/debug/lib%.a: $(src)/$(TEST_PATH)/Cargo.toml $(wildcard $(src)/$(TEST_PATH)/src/*.rs)
 	cd $(src)/$(TEST_PATH); CARGO_TARGET_DIR=../target $(CARGO) build -Z build-std=core,alloc --target=x86_64-linux-kernel
 	cd $(src)/$(TEST_PATH); CARGO_TARGET_DIR=../target $(CARGO) clippy -Z build-std=core,alloc --target=x86_64-linux-kernel -- -Dwarnings

kernel-cflags-finder/Makefile

@@ -1,7 +1,12 @@
 KDIR ?= /lib/modules/$(shell uname -r)/build
 
+CLANG ?= clang
+ifeq ($(origin CC),default)
+CC := ${CLANG}
+endif
+
 all:
-	$(MAKE) -C $(KDIR) M=$(CURDIR)
+	$(MAKE) -C $(KDIR) M=$(CURDIR) CC=$(CC) CONFIG_CC_IS_CLANG=y
 
 clean:
-	$(MAKE) -C $(KDIR) M=$(CURDIR) clean
+	$(MAKE) -C $(KDIR) M=$(CURDIR) CC=$(CC) clean