`HAVE_KMSAN_COMPILER [=n]` with Debian’s *clang-23*

[paulmenzel]

On Debian sid/unstable with clang-23 1:23~++20260204101751+dd02b404b020-1~exp1 from suite experimental, I am unable to select KMSAN:

$ git log --oneline --no-graph --no-decorate -1
f5459048c38a0 Merge tag 'i2c-for-7.0-final' of git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux
$ clang-23 --version
clang-23: warning: future releases of the clang compiler will prefer GCC installations containing libstdc++ include directories; '/usr/lib/gcc/x86_64-linux-gnu/15' would be chosen over '/usr/lib/gcc/x86_64-linux-gnu/16' [-Wgcc-install-dir-libstdcxx]
Debian clang version 23.0.0 (++20260204101751+dd02b404b020-1~exp1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-23/bin
$ make menuconfig CC=clang-23
[Search by pressing `/` for `KMSAN`]
Symbol: KMSAN [=n]
Type  : bool
Defined at lib/Kconfig.kmsan:8
  Prompt: KMSAN: detector of uninitialized values use
  Depends on: HAVE_ARCH_KMSAN [=y] && HAVE_KMSAN_COMPILER [=n] && DEBUG_KERNEL [=y] && !KASAN [=n] && !KCSAN [=n] && !PREEMPT_RT [=n]

I do not get, why HAVE_KMSAN_COMPILER=n, because the option should be there:

$ clang-23 -fsanitize=kernel-memory -c -x c /dev/null
clang-23: warning: future releases of the clang compiler will prefer GCC installations containing libstdc++ include directories; '/usr/lib/gcc/x86_64-linux-gnu/15' would be chosen over '/usr/lib/gcc/x86_64-linux-gnu/16' [-Wgcc-install-dir-libstdcxx]
$ echo $?
0

[nathanchance]

That instance of -Wgcc-install-dir-libstdcxx will cause all cc-option instances to fail because both the Kconfig (scripts/Kconfig.include) and Kbuild (scripts/Makefile.compiler) versions of cc-option add -Werror to catch truly unsupported flags in clang because the frontend may emit a warning for an unrecognized flag rather than error out. This is likely the root cause of #2158 as well).

# $(cc-option,<flag>)
# Return y if the compiler supports <flag>, n otherwise
cc-option = $(success,trap "rm -rf .tmp_$$" EXIT; mkdir .tmp_$$; $(CC) -Werror $(CLANG_FLAGS) $(1) -c -x c /dev/null -o .tmp_$$/tmp.o)

# __cc-option
# Usage: MY_CFLAGS += $(call __cc-option,$(CC),$(MY_CFLAGS),-march=winchip-c6,-march=i586)
__cc-option = $(call try-run,\
        $(1) -Werror $(2) $(3:-Wno-%=-W%) -c -x c /dev/null -o "$$TMP",$(3),$(4))

# cc-option
# Usage: cflags-y += $(call cc-option,-march=winchip-c6,-march=i586)

cc-option = $(call __cc-option, $(CC),\
        $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS),$(1),$(2))

This warning was added in clang-22:

https://releases.llvm.org/22.1.0/tools/clang/docs/ReleaseNotes.html#potentially-breaking-changes

Clang will now emit a warning if the auto-detected GCC installation directory (i.e. the one with the largest version number) does not contain libstdc++ include directories although a "complete" GCC installation directory containing the include directories is available. It is planned to change the auto-detection to prefer the "complete" directory in the future. The warning will disappear if the libstdc++ include directories are either installed or removed for all GCC installation directories considered by the auto-detection; see the output of clang -v for a list of those directories. If the GCC installations cannot be modified and maintaining the current choice of the auto-detection is desired, the GCC installation directory can be selected explicitly using the --gcc-install-dir command line argument. This will silence the warning. It can also be disabled using the -Wno-gcc-install-dir-libstdcxx command line flag.

It is possible that we should just never warn on this, especially since the kernel is a C project (not sure how we would fix this for Kconfig though, as it does not use KBUILD_CFLAGS):

diff --git a/scripts/Makefile.warn b/scripts/Makefile.warn
index 5567da6c7dfe..149cfb9d36e0 100644
--- a/scripts/Makefile.warn
+++ b/scripts/Makefile.warn
@@ -49,6 +49,10 @@ KBUILD_CFLAGS += $(call cc-option, -Wno-format-truncation-non-kprintf)
 # because -Wuninitialized will still flag when an uninitialized const variable
 # is used.
 KBUILD_CFLAGS += $(call cc-option, -Wno-default-const-init-unsafe)
+
+# Some users may have multiple GCC installations but not their development
+# headers installed, which does not matter for the kernel
+KBUILD_CFLAGS += $(call cc-option, -Wno-gcc-install-dir-libstdcxx)
 else

 # gcc inanely warns about local variables called 'main'

But I suspect this would be resolvable on your side by installing libstdc++-16-dev to go along with libgcc-16-dev, as I only see this warning when libgcc-16-dev is installed without libstdcc++-16-dev? I don't use Debian so I have not seen this warning before.

[paulmenzel]

Great analysis. Thank you. sudo apt install libstdc++-16-dev made the issue go away.