arm64: Restrict ARM64_BTI_KERNEL to clang 12.0.0 and newer

Commit 97fed779f2a6 ("arm64: bti: Provide Kconfig for kernel mode BTI") disabled CONFIG_ARM64_BTI_KERNEL when CONFIG_GCOV_KERNEL was enabled and compilign with clang because of warnings that occur because LLVM was not

emitting PAC/BTI instructions for compiler generated functions.

This was an adequate fix to avoid the warnings with allmodconfig until commit 51c2ee6d121c ("Kconfig: Introduce ARCH_WANTS_NO_INSTR and CC_HAS_NO_PROFILE_FN_ATTR"), which prevented CONFIG_GCOV_KERNEL from being selected with clang 12.0.0 and older because it does not support the no_profile_instrument_function attribute. As a result, CONFIG_ARM64_BTI_KERNEL gets enabled and there are more warnings of this nature.

Hi Nathan,

Quoted 3 lines · Nathan Chancellor

Commit 97fed779f2a6 ("arm64: bti: Provide Kconfig for kernel mode BTI") disabled CONFIG_ARM64_BTI_KERNEL when CONFIG_GCOV_KERNEL was enabled and compilign with clang because of warnings that occur because LLVM was not

Typo: "compilign"

Quoted 9 lines

emitting PAC/BTI instructions for compiler generated functions.

This was an adequate fix to avoid the warnings with allmodconfig until commit 51c2ee6d121c ("Kconfig: Introduce ARCH_WANTS_NO_INSTR and CC_HAS_NO_PROFILE_FN_ATTR"), which prevented CONFIG_GCOV_KERNEL from being selected with clang 12.0.0 and older because it does not support the no_profile_instrument_function attribute. As a result, CONFIG_ARM64_BTI_KERNEL gets enabled and there are more warnings of this nature.

I'm half asleep today, but I'm struggling to follow what GCOV_KERNEL has to do with the warnings here. Prior to 51c2ee6d121c, you could still go and turn that option off and so wouldn't the same warnings be triggered in that configuration?

In other words, I think there are two aspects to this patch:

1. Removing the (now useless) !GCOV_KERNEL dependency
2. Requiring Clang >= version 12

but the commit message doesn't really say where these warnings are coming from or why Clang 12 gets rid of them.

Thanks,

Will

Commit 97fed779f2a6 ("arm64: bti: Provide Kconfig for kernel mode BTI") disabled CONFIG_ARM64_BTI_KERNEL when CONFIG_GCOV_KERNEL was enabled and compiling with clang because of warnings that were seen with allmodconfig because LLVM was not emitting PAC/BTI instructions for compiler generated functions:

warning: some functions compiled with BTI and some compiled without BTI
warning: not setting BTI in feature flags

This depedency was fine for avoiding the warnings with allmodconfig until commit 51c2ee6d121c ("Kconfig: Introduce ARCH_WANTS_NO_INSTR and CC_HAS_NO_PROFILE_FN_ATTR"), which prevents CONFIG_GCOV_KERNEL from being enabled with clang 12.0.0 or older because those versions do not support the no_profile_instrument_function attribute.

As a result, CONFIG_ARM64_BTI_KERNEL gets enabled with allmodconfig and there are more warnings like the ones above due to CONFIG_KASAN, which suffers from the same problem as CONFIG_GCOV_KERNEL. This was most likely not noticed at the time because allmodconfig + CONFIG_GCOV_KERNEL=n was not tested. defconfig + CONFIG_KASAN=y is enough to reproduce the same warnings as above.

The root cause of the warnings was resolved in LLVM during the 12.0.0 release so rather than play whack-a-mole with the dependencies, just update CONFIG_ARM64_BTI_KERNEL to require clang 12.0.0, which will have all of the issues ironed out.

Link https://github.com/ClangBuiltLinux/linux/issues/1428

Link https://github.com/ClangBuiltLinux/continuous-integration2/runs/3010034706?check_suite_focus=true

Link https://github.com/ClangBuiltLinux/continuous-integration2/runs/3010035725?check_suite_focus=true

Link https://github.com/llvm/llvm-project/commit/a88c722e687e6780dcd6a58718350dc76fcc4cc9

Signed-off-by Nathan Chancellor <[email protected]>

---

v1 -> v2:

* Fix typo ("compilign" to "compiling") per Will.

* Clarify that these warnings are reproducible back at the time of 97fed779f2a6 with allmodconfig + CONFIG_GCOV_KERNEL=n but that configuration was most likely not tested.

* Clarify that the warnings come from CONFIG_KASAN, which suffers from the same issue at CONFIG_GCOV_KERNEL because there are compiler generated functions.

* Clarify that the root cause of the warnings is resolved in 12.0.0 and link to the GitHub commit rather than the Phabricator instance, in case LLVM moves to GitHub reviews in the future.

Hopefully this is adequate explanation for what is going on, thanks to Will and Nick for providing input on the commit message.

 arch/arm64/Kconfig | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

arch/arm64/Kconfig +2 -1

index e07e7de9ac49..b5b13a932561 100644
@@ -1605,7 +1605,8 @@ config ARM64_BTI_KERNEL
        depends on CC_HAS_BRANCH_PROT_PAC_RET_BTI
        # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94697
        depends on !CC_IS_GCC || GCC_VERSION >= 100100
-       depends on !(CC_IS_CLANG && GCOV_KERNEL)
+       # https://github.com/llvm/llvm-project/commit/a88c722e687e6780dcd6a58718350dc76fcc4cc9
+       depends on !CC_IS_CLANG || CLANG_VERSION >= 120000
        depends on (!FUNCTION_GRAPH_TRACER || DYNAMIC_FTRACE_WITH_REGS)
        help
          Build the kernel with Branch Target Identification annotations

base-commit: e73f0f0ee7541171d89f2e2491130c7771ba58d3 --
2.32.0.93.g670b81a890

Commit 97fed779f2a6 ("arm64: bti: Provide Kconfig for kernel mode BTI") disabled CONFIG_ARM64_BTI_KERNEL when CONFIG_GCOV_KERNEL was enabled and compiling with clang because of warnings that were seen with allmodconfig because LLVM was not emitting PAC/BTI instructions for compiler generated functions:

warning: some functions compiled with BTI and some compiled without BTI
warning: not setting BTI in feature flags

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