[PATCH 0/2] Fix UML build with clang-18 and newer

[PATCH 0/2] Fix UML build with clang-18 and newer

[Nathan Chancellor]

Hi all,

This series resolves two independent but related issues that were
recently exposed by two LLVM changes.

https://github.com/llvm/llvm-project/commit/ec92d74a0ef89b9dd46aee6ec8aca6bfd3c66a54
exposes that '-no-pie' is not getting added to the linker flags with
clang, resulting in building objects with '-fno-PIE' that are linked
with '-pie', to which the linker rightfully errors with:

  /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

https://github.com/llvm/llvm-project/commit/4bf8a688956a759b7b6b8d94f42d25c13c7af130
adds '.ltext' (and '.ltext.*' with '-ffunction-sections') when using
'-mcmodel=large' (which UML does), which causes a segmentation fault
with modpost.

I have tested these patches with all supported versions of clang,
noticing no regressions.

---
Nathan Chancellor (2):
      um: Fix adding '-no-pie' for clang
      modpost: Add '.ltext' and '.ltext.*' to TEXT_SECTIONS

 arch/um/Makefile      | 4 +++-
 scripts/mod/modpost.c | 3 ++-
 2 files changed, 5 insertions(+), 2 deletions(-)
---
base-commit: 0dd3ee31125508cd67f7e7172247f05b7fd1753a
change-id: 20240118-fix-uml-clang-18-e365b0503a29

Best regards,
-- 
Nathan Chancellor <nathan@kernel.org>

[PATCH 1/2] um: Fix adding '-no-pie' for clang

[Nathan Chancellor]

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
  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

[PATCH 2/2] modpost: Add '.ltext' and '.ltext.*' to TEXT_SECTIONS

[Nathan Chancellor]

After the linked LLVM change, building ARCH=um defconfig results in a
segmentation fault in modpost. 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/4bf8a688956a759b7b6b8d94f42d25c13c7af130
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

Re: [PATCH 0/2] Fix UML build with clang-18 and newer

[Masahiro Yamada]

On Wed, Jan 24, 2024 at 8:00 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> Hi all,
>
> This series resolves two independent but related issues that were
> recently exposed by two LLVM changes.
>
> https://github.com/llvm/llvm-project/commit/ec92d74a0ef89b9dd46aee6ec8aca6bfd3c66a54
> exposes that '-no-pie' is not getting added to the linker flags with
> clang, resulting in building objects with '-fno-PIE' that are linked
> with '-pie', to which the linker rightfully errors with:
>
>   /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
>
> https://github.com/llvm/llvm-project/commit/4bf8a688956a759b7b6b8d94f42d25c13c7af130
> adds '.ltext' (and '.ltext.*' with '-ffunction-sections') when using
> '-mcmodel=large' (which UML does), which causes a segmentation fault
> with modpost.
>
> I have tested these patches with all supported versions of clang,
> noticing no regressions.


Both applied to linux-kbuild/fixes.

Thanks.




> ---
> Nathan Chancellor (2):
>       um: Fix adding '-no-pie' for clang
>       modpost: Add '.ltext' and '.ltext.*' to TEXT_SECTIONS
>
>  arch/um/Makefile      | 4 +++-
>  scripts/mod/modpost.c | 3 ++-
>  2 files changed, 5 insertions(+), 2 deletions(-)
> ---
> base-commit: 0dd3ee31125508cd67f7e7172247f05b7fd1753a
> change-id: 20240118-fix-uml-clang-18-e365b0503a29
>
> Best regards,
> --
> Nathan Chancellor <nathan@kernel.org>
>


-- 
Best Regards
Masahiro Yamada

Re: [PATCH 1/2] um: Fix adding '-no-pie' for clang

[Masahiro Yamada]

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

Re: [PATCH 2/2] modpost: Add '.ltext' and '.ltext.*' to TEXT_SECTIONS

[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/4bf8a688956a759b7b6b8d94f42d25c13c7af130
> 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
>


-- 
Best Regards
Masahiro Yamada