[PATCH] arm64: Restrict ARM64_BTI_KERNEL to clang 12.0.0 and newer

[PATCH] arm64: Restrict ARM64_BTI_KERNEL to clang 12.0.0 and newer

[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
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.

Rather than play whack-a-mole with the config options, just update this
condition to require clang 12.0.0, which should 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
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 arch/arm64/Kconfig | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index e07e7de9ac49..c2127595980f 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -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://reviews.llvm.org/D85649
+	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: f55966571d5eb2876a11e48e798b4592fa1ffbb7
-- 
2.32.0.93.g670b81a890

Re: [PATCH] arm64: Restrict ARM64_BTI_KERNEL to clang 12.0.0 and newer

[Will Deacon]

Hi Nathan,

On Thu, Jul 08, 2021 at 05:06:27PM -0700, Nathan Chancellor wrote:
> 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"

> 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

Re: [PATCH] arm64: Restrict ARM64_BTI_KERNEL to clang 12.0.0 and newer

[Nick Desaulniers]

On Mon, Jul 12, 2021 at 5:24 AM Will Deacon <will@kernel.org> wrote:
>
> Hi Nathan,
>
> On Thu, Jul 08, 2021 at 05:06:27PM -0700, Nathan Chancellor wrote:
> > 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"
>
> > 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?

Nathan, consider if any of the below can be reused to reword the commit message?
```
Prior to 51c2ee6d121c, allmodconfig with clang-11 would not select
ARM64_BTI_KERNEL because GCOV_KERNEL already had been.

After 51c2ee6d121c, allmodconfig with clang-11 does select
ARM64_BTI_KERNEL because GCOV_KERNEL cannot be selected.

clang-11 had basic support for BTI, but failed to instrument compiler
generated functions properly.  This produces the following stream of
warnings for clang-11 allmodconfig builds of ARCH=arm64:

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

This was fixed in clang-12 via https://reviews.llvm.org/D85649.
```
Perhaps its of interest to find which config in (allmodconfig minus
GCOV_KERNEL) but not in (defconfig) produces these, since I don't
think we see them in defconfig builds of clang-11?
https://github.com/ClangBuiltLinux/continuous-integration2/runs/3009146591?check_suite_focus=true

>
> 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



-- 
Thanks,
~Nick Desaulniers

[PATCH v2] arm64: Restrict ARM64_BTI_KERNEL to clang 12.0.0 and newer

[Nathan Chancellor]

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 <nathan@kernel.org>
---

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(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index e07e7de9ac49..b5b13a932561 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -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

Re: [PATCH v2] arm64: Restrict ARM64_BTI_KERNEL to clang 12.0.0 and newer

[Nick Desaulniers]

On Mon, Jul 12, 2021 at 2:51 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> 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 <nathan@kernel.org>
> ---
>
> 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.

Perfect, sorry for being a pedant on the commit message, but this
provides much more context should we need to revisit this again in the
future.  Never say never, (again)!  Thanks very much for this patch.

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

>
>  arch/arm64/Kconfig | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index e07e7de9ac49..b5b13a932561 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -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
>


-- 
Thanks,
~Nick Desaulniers

Re: [PATCH v2] arm64: Restrict ARM64_BTI_KERNEL to clang 12.0.0 and newer

[Will Deacon]

On Mon, 12 Jul 2021 14:46:37 -0700, Nathan Chancellor wrote:
> 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
> 
> [...]

Thanks Nathan for turning this around so quickly. The new commit message
is much better.

Applied to arm64 (for-next/fixes), thanks!

[1/1] arm64: Restrict ARM64_BTI_KERNEL to clang 12.0.0 and newer
      https://git.kernel.org/arm64/c/8cdd23c23c3d

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev