Forbid rustc from using -f{function,data}-sections (fixes #20 panic)

Some kernel configs were panicking at the Rust example driver
initialization. The `__MOD` static value had a bogus value,
which meant that trying to initialize it was dropping the object
that was, supposedly, there.

The memory corruption happened during rootfs unpacking,
which explains why it only happened in some configs (like in CI)
and why it also didn't happen if there was an early error
during unpacking.

That memory corruption, in turn, was caused because the
`__MOD` symbol was being placed after the end of the
kernel reserve. That happened due to the kernel's linker
script not supporting unique sections per symbol for dead
code data elimination -- yet. Some arches do, but until
we can rely on that, we need to disable their generation
in rustc's side for the moment.

Since we discussed to have the target spec on our side,
and since `-Z function-sections=false` was added just
a month ago, I went with the spec route.

Other symbols were being placed in unexpected places,
which should be fixed now too.

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

Makefile

@@ -510,11 +510,9 @@ KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
 KBUILD_CPPFLAGS := -D__KERNEL__
 KBUILD_RUSTCFLAGS :=
 # TODO: a simple way to update `Cargo.lock` when we add a new driver
-# TODO: another option is using explicit target specs, e.g.
-# `--target=$(srctree)/arch/$(SRCARCH)/rust-target-spec.json`
 KBUILD_CARGOFLAGS := $(CARGO_VERBOSE) --locked \
 		    -Z build-std=core,alloc -Z unstable-options \
-		    --out-dir=out --target=x86_64-linux-kernel
+		    --out-dir=out --target=$(PWD)/$(srctree)/arch/$(SRCARCH)/rust/target.json
 KBUILD_AFLAGS_KERNEL :=
 KBUILD_CFLAGS_KERNEL :=
 KBUILD_RUSTCFLAGS_KERNEL :=

arch/x86/rust/target.json

@@ -0,0 +1,34 @@
+{
+  "arch": "x86_64",
+  "code-model": "kernel",
+  "cpu": "x86-64",
+  "data-layout": "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128",
+  "disable-redzone": true,
+  "eliminate-frame-pointer": false,
+  "env": "gnu",
+  "features": "-mmx,-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-3dnow,-3dnowa,-avx,-avx2,+soft-float",
+  "function-sections": false,
+  "is-builtin": true,
+  "linker-flavor": "gcc",
+  "linker-is-gnu": true,
+  "llvm-target": "x86_64-elf",
+  "max-atomic-width": 64,
+  "needs-plt": true,
+  "os": "none",
+  "panic-strategy": "abort",
+  "position-independent-executables": true,
+  "pre-link-args": {
+    "gcc": [
+      "-Wl,--as-needed",
+      "-Wl,-z,noexecstack",
+      "-m64"
+    ]
+  },
+  "relocation-model": "static",
+  "relro-level": "full",
+  "stack-probes": true,
+  "target-c-int-width": "32",
+  "target-endian": "little",
+  "target-pointer-width": "64",
+  "vendor": "unknown"
+}

rust/kernel/build.rs

@@ -96,7 +96,8 @@ fn main() {
 
     let kernel_args = prepare_cflags(&cflags, &kernel_dir);
 
-    let target = env::var("TARGET").unwrap();
+    // TODO: pass the proper triple to bindgen
+    let target = "x86_64-linux-kernel";
 
     let mut builder = bindgen::Builder::default()
         .use_core()