linux-kernel - [PATCH] x86/Kconfig: Fix CONFIG_CC_HAS_SANE_STACKPROTECTOR when cross compiling with clang

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Date:   Fri, 17 Jun 2022 11:08:46 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Daniel Kolesa <daniel@...aforge.org>,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        llvm@...ts.linux.dev, Nathan Chancellor <nathan@...nel.org>,
        stable@...r.kernel.org
Subject: [PATCH] x86/Kconfig: Fix CONFIG_CC_HAS_SANE_STACKPROTECTOR when cross compiling with clang

Chimera Linux notes that CONFIG_CC_HAS_SANE_STACKPROTECTOR cannot be
enabled when cross compiling an x86_64 kernel with clang, even though it
does work when natively compiling.

When building on aarch64:

  $ make -sj"$(nproc)" ARCH=x86_64 LLVM=1 defconfig

  $ grep STACKPROTECTOR .config

When building on x86_64:

  $ make -sj"$(nproc)" ARCH=x86_64 LLVM=1 defconfig

  $ grep STACKPROTECTOR .config
  CONFIG_CC_HAS_SANE_STACKPROTECTOR=y
  CONFIG_HAVE_STACKPROTECTOR=y
  CONFIG_STACKPROTECTOR=y
  CONFIG_STACKPROTECTOR_STRONG=y

When clang is invoked without a '--target' flag, code is generated for
the default target, which is usually the host (it is configurable via
cmake). As a result, the has-stack-protector scripts will generate code
for the default target but check for x86 specific segment registers,
which cannot succeed if the default target is not x86.

$(CLANG_FLAGS) contains an explicit '--target' flag so pass that
variable along to the has-stack-protector scripts so that the stack
protector can be enabled when cross compiling with clang. The 32-bit
stack protector cannot currently be enabled with clang, as it does not
support '-mstack-protector-guard-symbol', so this results in no
functional change for ARCH=i386 when cross compiling.

Link: https://github.com/chimera-linux/cports/commit/0fb7e506d5f83fdf2104feb22cdac34934561226
Link: https://github.com/llvm/llvm-project/issues/48553
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---

Fixes: 2a61f4747eea ("stack-protector: test compiler capability in Kconfig and drop AUTO mode")

might be appropriate; I am conflicted on fixes tags for problems that
that arise due to use cases that were not considered at the time of a
change, as it feels wrong to blame the commit for not looking far enough
into the future where it might be common for people to have workstations
running another architecture other than x86_64.

Chimera appears to use a 5.15 kernel so a

Cc: stable@...r.kernel.org

might be nice but some maintainers are picky about that so I leave it up
to you all.

 arch/x86/Kconfig | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index be0b95e51df6..076adde7ead9 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -391,8 +391,8 @@ config PGTABLE_LEVELS

 config CC_HAS_SANE_STACKPROTECTOR
 	bool
-	default $(success,$(srctree)/scripts/gcc-x86_64-has-stack-protector.sh $(CC)) if 64BIT
-	default $(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC))
+	default $(success,$(srctree)/scripts/gcc-x86_64-has-stack-protector.sh $(CC) $(CLANG_FLAGS)) if 64BIT
+	default $(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC) $(CLANG_FLAGS))
 	help
 	  We have to make sure stack protector is unconditionally disabled if
 	  the compiler produces broken code or if it does not let us control

base-commit: b13baccc3850ca8b8cccbf8ed9912dbaa0fdf7f3
-- 
2.36.1