rust: kasan/kbuild: fix missing flags on first build

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 <[email protected]>

Cc Sami Tolvanen <[email protected]>

Cc [email protected]

Fixes e3117404b411 ("kbuild: rust: Enable KASAN support")

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

---
By the way, I noticed that we are not getting asan-instrument-allocas enabled in neither C nor Rust -- upstream LLVM renamed it in commit 8176ee9b5dda ("[asan] Rename asan-instrument-allocas -> asan-instrument-dynamic-allocas")). But it happened a very long time ago (9 years ago), and the addition in the kernel is fairly old too, in 342061ee4ef3 ("kasan: support alloca() poisoning"). I assume it should either be renamed or removed? Happy to send a patch if so.

 scripts/Makefile.compiler | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

scripts/Makefile.compiler +0 -0

index 8956587b8547..7ed7f92a7daa 100644

scripts/Makefile.compiler +0 -0

 # rustc-option
 

base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8 --
2.49.0

The problem with this change is that some rustc flags will only be valid on some platforms. For example, if we check if a

Thus filter out the target.

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 <[email protected]>

Cc Sami Tolvanen <[email protected]>

Cc [email protected]

Fixes e3117404b411 ("kbuild: rust: Enable KASAN support")

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

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 <[email protected]>

Cc Sami Tolvanen <[email protected]>

Cc [email protected]

Fixes e3117404b411 ("kbuild: rust: Enable KASAN support")

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

---
By the way, I noticed that we are not getting asan-instrument-allocas enabled in neither C nor Rust -- upstream LLVM renamed it in commit 8176ee9b5dda ("[asan] Rename asan-instrument-allocas -> asan-instrument-dynamic-allocas")). But it happened a very long time ago (9 years ago), and the addition in the kernel is fairly old too, in 342061ee4ef3 ("kasan: support alloca() poisoning"). I assume it should either be renamed or removed? Happy to send a patch if so.

 scripts/Makefile.compiler | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

scripts/Makefile.compiler +1 -1

index 8956587b8547..7ed7f92a7daa 100644
@@ -80,7 +80,7 @@ ld-option = $(call try-run, $(LD) $(KBUILD_LDFLAGS) $(1) -v,$(1),$(2),$(3))
 # 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

base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8 --
2.49.0

Yo,

Quoted 40 lines · Miguel Ojeda

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.

Quoted 8 lines

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.

Quoted 93 lines · Conor Dooley

Yo,

Quoted 40 lines · Miguel Ojeda

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.

Quoted 8 lines

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.

Quoted 3 lines

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_ARM

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)

Quoted 94 lines · Alice Ryhl

Quoted 93 lines · Conor Dooley

Yo,

Quoted 40 lines · Miguel Ojeda

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.

Quoted 8 lines

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.

Quoted 7 lines

Quoted 3 lines

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_ARM

It's valid on x86 too, right?

Quoted 5 lines

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.

Quoted 11 lines

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)

Quoted 2 lines

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

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.