On Wed, Jan 24, 2024 at 8:00 AM Nathan Chancellor nathan@kernel.org wrote:

The kernel builds with -fno-PIE, so commit 883354afbc10 ("um: link vmlinux with -no-pie") added the compiler linker flag '-no-pie' via cc-option because '-no-pie' was only supported in GCC 6.1.0 and newer.

While this works for GCC, this does not work for clang because cc-option uses '-c', which stops the pipeline right before linking, so '-no-pie' is unconsumed and clang warns, causing cc-option to fail just as it would if the option was entirely unsupported:

$ clang -Werror -no-pie -c -o /dev/null-x c /dev/null

A nit. A missing space in-between.

I fixed "/dev/null-x" to "/dev/null -x" when I applied the patch.

clang-16: error: argument unused during compilation: '-no-pie' [-Werror,-Wunused-command-line-argument]

A recent version of clang exposes this because it generates a relocation under '-mcmodel=large' that is not supported in PIE mode:

/usr/sbin/ld: init/main.o: relocation R_X86_64_32 against symbol `saved_command_line' can not be used when making a PIE object; recompile with -fPIE /usr/sbin/ld: failed to set dynamic section sizes: bad value clang: error: linker command failed with exit code 1 (use -v to see invocation)

Remove the cc-option check altogether. It is wasteful to invoke the compiler to check for '-no-pie' because only one supported compiler version does not support it, GCC 5.x (as it is supported with the minimum version of clang and GCC 6.1.0+). Use a combination of the gcc-min-version macro and CONFIG_CC_IS_CLANG to unconditionally add '-no-pie' with CONFIG_LD_SCRIPT_DYN=y, so that it is enabled with all compilers that support this. Furthermore, using gcc-min-version can help turn this back into

LINK-$(CONFIG_LD_SCRIPT_DYN) += -no-pie

when the minimum version of GCC is bumped past 6.1.0.

Cc: stable@vger.kernel.org Closes: https://github.com/ClangBuiltLinux/linux/issues/1982 Signed-off-by: Nathan Chancellor nathan@kernel.org

---

arch/um/Makefile | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/um/Makefile b/arch/um/Makefile index 82f05f250634..34957dcb88b9 100644 --- a/arch/um/Makefile +++ b/arch/um/Makefile @@ -115,7 +115,9 @@ archprepare: $(Q)$(MAKE) $(build)=$(HOST_DIR)/um include/generated/user_constants.h

LINK-$(CONFIG_LD_SCRIPT_STATIC) += -static -LINK-$(CONFIG_LD_SCRIPT_DYN) += $(call cc-option, -no-pie) +ifdef CONFIG_LD_SCRIPT_DYN +LINK-$(call gcc-min-version, 60100)$(CONFIG_CC_IS_CLANG) += -no-pie +endif LINK-$(CONFIG_LD_SCRIPT_DYN_RPATH) += -Wl,-rpath,/lib

CFLAGS_NO_HARDENING := $(call cc-option, -fno-PIC,) $(call cc-option, -fno-pic,) \

-- 2.43.0

-- Best Regards Masahiro Yamada

On Wed, Jan 24, 2024 at 8:00 AM Nathan Chancellor nathan@kernel.org wrote:

After the linked LLVM change, building ARCH=um defconfig results in a segmentation fault in modpost.

Yeah, this is a mistake in my commits. The NULL pointer access should be fixed, but that is a separate issue.

Anyway, I applied this patch.

Prior to commit a23e7584ecf3 ("modpost: unify 'sym' and 'to' in default_mismatch_handler()"), there was a warning:

WARNING: modpost: vmlinux.o(__ex_table+0x88): Section mismatch in reference to the .ltext:(unknown) WARNING: modpost: The relocation at __ex_table+0x88 references section ".ltext" which is not in the list of authorized sections. If you're adding a new section and/or if this reference is valid, add ".ltext" to the list of authorized sections to jump to on fault. This can be achieved by adding ".ltext" to OTHER_TEXT_SECTIONS in scripts/mod/modpost.c.

The linked LLVM change moves global objects to the '.ltext' (and '.ltext.*' with '-ffunction-sections') sections with '-mcmodel=large', which ARCH=um uses. These sections should be handled just as '.text' and '.text.*' are, so add them to TEXT_SECTIONS.

Cc: stable@vger.kernel.org Closes: https://github.com/ClangBuiltLinux/linux/issues/1981 Link: https://github.com/llvm/llvm-project/commit/4bf8a688956a759b7b6b8d94f42d25c1... Signed-off-by: Nathan Chancellor nathan@kernel.org

---

scripts/mod/modpost.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c index cb6406f485a9..f7c4d3fe4381 100644 --- a/scripts/mod/modpost.c +++ b/scripts/mod/modpost.c @@ -807,7 +807,8 @@ static void check_section(const char *modname, struct elf_info *elf,

#define DATA_SECTIONS ".data", ".data.rel" #define TEXT_SECTIONS ".text", ".text.*", ".sched.text", \

•           ".kprobes.text", ".cpuidle.text", ".noinstr.text"
  

•           ".kprobes.text", ".cpuidle.text", ".noinstr.text", \
  

•           ".ltext", ".ltext.*"
  

#define OTHER_TEXT_SECTIONS ".ref.text", ".head.text", ".spinlock.text", \ ".fixup", ".entry.text", ".exception.text", \ ".coldtext", ".softirqentry.text"

-- 2.43.0