6.12-stable review patch. If anyone has any objections, please let me know.
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From: Miguel Ojeda ojeda@kernel.org
commit 46e24a545cdb4556f8128c90ecc34eeae52477a0 upstream.
If KASAN is enabled, and one runs in a clean repository e.g.:
make LLVM=1 prepare make LLVM=1 prepare
Then the Rust code gets rebuilt, which should not happen.
The reason is some of the LLVM KASAN `rustc` flags are added in the second run:
-Cllvm-args=-asan-instrumentation-with-call-threshold=10000 -Cllvm-args=-asan-stack=0 -Cllvm-args=-asan-globals=1 -Cllvm-args=-asan-kernel-mem-intrinsic-prefix=1
Further runs do not rebuild Rust because the flags do not change anymore.
Rebuilding like that in the second run is bad, even if this just happens with KASAN enabled, but missing flags in the first one is even worse.
The root issue is that we pass, for some architectures and for the moment, a generated `target.json` file. That file is not ready by the time `rustc` gets called for the flag test, and thus the flag test fails just because the file is not available, e.g.:
$ ... --target=./scripts/target.json ... -Cllvm-args=... error: target file "./scripts/target.json" does not exist
There are a few approaches we could take here to solve this. For instance, we could ensure that every time that the config is rebuilt, we regenerate the file and recompute the flags. Or we could use the LLVM version to check for these flags, instead of testing the flag (which may have other advantages, such as allowing us to detect renames on the LLVM side).
However, it may be easier than that: `rustc` is aware of the `-Cllvm-args` regardless of the `--target` (e.g. I checked that the list printed is the same, plus that I can check for these flags even if I pass a completely unrelated target), and thus we can just eliminate the dependency completely.
Thus filter out the target.
This does mean that `rustc-option` cannot be used to test a flag that requires the right target, but we don't have other users yet, it is a minimal change and we want to get rid of custom targets in the future.
We could only filter in the case `target.json` is used, to make it work in more cases, but then it would be harder to notice that it may not work in a couple architectures.
Cc: Matthew Maurer mmaurer@google.com Cc: Sami Tolvanen samitolvanen@google.com Cc: stable@vger.kernel.org Fixes: e3117404b411 ("kbuild: rust: Enable KASAN support") Tested-by: Alice Ryhl aliceryhl@google.com Link: https://lore.kernel.org/r/20250408220311.1033475-1-ojeda@kernel.org Signed-off-by: Miguel Ojeda ojeda@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- scripts/Makefile.compiler | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/scripts/Makefile.compiler +++ b/scripts/Makefile.compiler @@ -76,7 +76,7 @@ ld-option = $(call try-run, $(LD) $(KBUI # TODO: remove RUSTC_BOOTSTRAP=1 when we raise the minimum GNU Make version to 4.4 __rustc-option = $(call try-run,\ echo '#![allow(missing_docs)]#![feature(no_core)]#![no_core]' | RUSTC_BOOTSTRAP=1\ - $(1) --sysroot=/dev/null $(filter-out --sysroot=/dev/null,$(2)) $(3)\ + $(1) --sysroot=/dev/null $(filter-out --sysroot=/dev/null --target=%,$(2)) $(3)\ --crate-type=rlib --out-dir=$(TMPOUT) --emit=obj=- - >/dev/null,$(3),$(4))
# rustc-option