On Tue, Sep 2, 2025 at 12:12 PM Conor Dooley conor@kernel.org wrote:
On Tue, Sep 02, 2025 at 08:29:29AM +0000, Alice Ryhl wrote:
On Mon, Sep 01, 2025 at 06:45:54PM +0100, Conor Dooley wrote:
Yo,
On Wed, Apr 09, 2025 at 12:03:11AM +0200, Miguel Ojeda wrote:
If KASAN is enabled, and one runs in a clean repository e.g.:
make LLVM=1 prepare make LLVM=1 prepareThen 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=1Further 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 existThere 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.
Hmm, while this might be true, I think it should not actually have been true. Commit ca627e636551e ("rust: cfi: add support for CFI_CLANG with Rust") added a cc-option check to the rust kconfig symbol, checking if the c compiler supports the integer normalisations stuff: depends on !CFI_CLANG || RUSTC_VERSION >= 107900 && $(cc-option,-fsanitize=kcfi -fsanitize-cfi-icall-experimental-normalize-integers) and also sets the relevant options in the makefile: ifdef CONFIG_RUST # Always pass -Zsanitizer-cfi-normalize-integers as CONFIG_RUST selects # CONFIG_CFI_ICALL_NORMALIZE_INTEGERS. RUSTC_FLAGS_CFI := -Zsanitizer=kcfi -Zsanitizer-cfi-normalize-integers KBUILD_RUSTFLAGS += $(RUSTC_FLAGS_CFI) export RUSTC_FLAGS_CFI endif but it should also have added a rustc-option check as, unfortunately, support for kcfi in rustc is target specific. This results in build breakages where the arch supports CFI_CLANG and RUST, but the target in use does not have the kcfi flag set. I attempted to fix this by adding: diff --git a/arch/Kconfig b/arch/Kconfig index d1b4ffd6e0856..235709fb75152 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -916,6 +916,7 @@ config HAVE_CFI_ICALL_NORMALIZE_INTEGERS_CLANG config HAVE_CFI_ICALL_NORMALIZE_INTEGERS_RUSTC def_bool y depends on HAVE_CFI_ICALL_NORMALIZE_INTEGERS_CLANG + depends on $(rustc-option,-C panic=abort -Zsanitizer=kcfi -Zsanitizer-cfi-normalize-integers) depends on RUSTC_VERSION >= 107900 # With GCOV/KASAN we need this fix: https://github.com/rust-lang/rust/pull/129373 depends on (RUSTC_LLVM_VERSION >= 190103 && RUSTC_VERSION >= 108200) || \ but of course this does not work for cross compilation, as you're stripping the target information out and so the check passes on my host even though my intended RUSTC_BOOTSTRAP=1 rustc -C panic=abort -Zsanitizer=kcfi -Zsanitizer-cfi-normalize-integers -Ctarget-cpu=generic-rv64 --target=riscv64imac-unknown-none-elf is a failure.
I dunno too much about rustc itself, but I suspect that adding kcfi to the target there is a "free" win, but that'll take time to trickle down and the minimum version rustc version for the kernel isn't going to have that.
I'm not really sure what your target.json suggestion below is, so just reporting so that someone that understands the alternative solutions can fix this.
Probably right now we have to do this cfg by
depends on CONFIG_ARMIt's valid on x86 too, right?
to prevent riscv if rustc has the missing setting set on riscv. Once we add it to riscv, we change it to
depends on CONFIG_ARM || (RUSTC_VERSION >= ??? || CONFIG_RISCV)I kinda shied away from something like this since there was already a cc-option on the other half and checking different versions per arch becomes a mess - but yeah it kinda is a no-brainer to do it here when rustc-option is kinda broken.
I guess the temporary fix is then:
config HAVE_CFI_ICALL_NORMALIZE_INTEGERS_RUSTC def_bool y depends on HAVE_CFI_ICALL_NORMALIZE_INTEGERS_CLANG depends on ARM64 || x86_64 depends on RUSTC_VERSION >= 107900 # With GCOV/KASAN we need this fix: https://github.com/rust-lang/rust/pull/129373 depends on (RUSTC_LLVM_VERSION >= 190103 && RUSTC_VERSION >= 108200) || \ (!GCOV_KERNEL && !KASAN_GENERIC && !KASAN_SW_TAGS)
because there's no 32-bit target with SanitizerSet::KCFI in rustc either AFAICT. Then later on it'd become more like:
config HAVE_CFI_ICALL_NORMALIZE_INTEGERS_RUSTC def_bool y depends on HAVE_CFI_ICALL_NORMALIZE_INTEGERS_CLANG depends on RISCV || ((ARM64 || x86_64) && RUSTC_VERSION >= 107900) depends on (ARM64 || x86_64) || (RISCV && RUSTC_VERSION >= 999999) # With GCOV/KASAN we need this fix: https://github.com/rust-lang/rust/pull/129373 depends on (RUSTC_LLVM_VERSION >= 190103 && RUSTC_VERSION >= 108200) || \ (!GCOV_KERNEL && !KASAN_GENERIC && !KASAN_SW_TAGS)
but that exact sort of mess is what becomes unwieldy fast since that doesn't even cover 32-bit arm.
I think a better way of writing it is like this:
depends on ARCH1 || ARCH2 || ARCH3 depends on !ARCH1 || RUSTC_VERSION >= 000000 depends on !ARCH2 || RUSTC_VERSION >= 000000 depends on !ARCH3 || RUSTC_VERSION >= 000000
Alice