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

[ojeda]

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 ojeda@kernel.org

[kloenk]

Applied it on my patch, because I want/have to compile with O=build. This breaks with the following output, will try to fix it tomorrow:

error: target path "/persist/data/kloenk/proj/linux/../arch/x86/rust/target.json" is not a valid file

my kernel source lives in /persist/data/kloenk/proj/linux/ so he goes one layer to far up.

[ojeda]

Argh... Yeah, I did the $(PWD) trick there since otherwise Cargo does not find the target file because we cd into the folder:

cd $(src) && $(CARGO) build $(cargo_flags)

Hmm... Perhaps we can use Cargo's manifest path option instead of using cd and then the target file is easily found? Not sure if then the target path is also relative to the manifest or not. Otherwise, we can compute the absolute path from the srctree variable.

[kloenk]

manifest path seems like it could work. will try to implement it in my patch.

[kloenk]

I already impemented it, forgot that xD. But I used -p

build -p $(shell basename $(basename $(src))) --out-dir $(src) $(cargo_flags)

[ojeda]

Let me merge this to simplify.