Re: [PATCH] arm/build: Always handle .ARM.exidx and .ARM.extab sections

On Mon, Oct 12, 2020 at 02:22:03PM -0700, Fāng-ruì Sòng wrote:

On Mon, Oct 12, 2020 at 2:11 PM 'Nick Desaulniers' via Clang Built
Linux [off-list ref] wrote:

quoted

On Mon, Sep 28, 2020 at 3:49 PM Nathan Chancellor
[off-list ref] wrote:

quoted

After turning on warnings for orphan section placement, enabling
CONFIG_UNWINDER_FRAME_POINTER instead of CONFIG_UNWINDER_ARM causes
thousands of warnings when clang + ld.lld are used:

$ scripts/config --file arch/arm/configs/multi_v7_defconfig \
                 -d CONFIG_UNWINDER_ARM \
                 -e CONFIG_UNWINDER_FRAME_POINTER
$ make -skj"$(nproc)" ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- LLVM=1 defconfig zImage
ld.lld: warning: init/built-in.a(main.o):(.ARM.extab) is being placed in '.ARM.extab'
ld.lld: warning: init/built-in.a(main.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(main.o):(.ARM.extab.ref.text) is being placed in '.ARM.extab.ref.text'
ld.lld: warning: init/built-in.a(do_mounts.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(do_mounts.o):(.ARM.extab) is being placed in '.ARM.extab'
ld.lld: warning: init/built-in.a(do_mounts_rd.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(do_mounts_rd.o):(.ARM.extab) is being placed in '.ARM.extab'
ld.lld: warning: init/built-in.a(do_mounts_initrd.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(initramfs.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(initramfs.o):(.ARM.extab) is being placed in '.ARM.extab'
ld.lld: warning: init/built-in.a(calibrate.o):(.ARM.extab.init.text) is being placed in '.ARM.extab.init.text'
ld.lld: warning: init/built-in.a(calibrate.o):(.ARM.extab) is being placed in '.ARM.extab'

These sections are handled by the ARM_UNWIND_SECTIONS define, which is
only added to the list of sections when CONFIG_ARM_UNWIND is set.
CONFIG_ARM_UNWIND is a hidden symbol that is only selected when
CONFIG_UNWINDER_ARM is set so CONFIG_UNWINDER_FRAME_POINTER never
handles these sections. According to the help text of
CONFIG_UNWINDER_ARM, these sections should be discarded so that the
kernel image size is not affected.

My apologies for taking so long to review this.

I have a suspicion that these come from forcing on configs that
Kconfig/menuconfig would block, and aren't clang or lld specific, yet
are exposed by the new linker warnings for orphan section placement
(good).  That said, we definitely have OEMs in Android land that still
prefer the older unwinder.

From https://developer.arm.com/documentation/ihi0038/b/ (click
download in top left), section 4.4.1 "Sections" has a note:

Tables are not required for ABI compliance at the C/Assembler level
but are required for C++.

Review-by: Nick Desaulniers [off-list ref]
Tested-by: Nick Desaulniers <ndesaulniers@google.com>

Please submit to:
https://www.arm.linux.org.uk/developer/patches/add.php

quoted

Fixes: 5a17850e251a ("arm/build: Warn on orphan section placement")
Link: https://github.com/ClangBuiltLinux/linux/issues/1152
Reported-by: kernel test robot <redacted>
Signed-off-by: Nathan Chancellor <redacted>
---
 arch/arm/kernel/vmlinux.lds.S | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index 5f4922e858d0..a2c0d96b0580 100644
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S

@@ -40,6 +40,10 @@ SECTIONS
                ARM_DISCARD
 #ifndef CONFIG_SMP_ON_UP
                *(.alt.smp.init)
+#endif
+#ifndef CONFIG_ARM_UNWIND
+               *(.ARM.exidx*)

I don't think we need the wildcard, as without this line, I see:

ld.lld: warning: <internal>:(.ARM.exidx) is being placed in '.ARM.exidx'

We may need the wildcard if there are -ffunction-sections builds.
In clang, .ARM.exidx* cannot be removed even with -fno-unwind-tables
-fno-exceptions.

Does it need to be:

	*(.ARM.exidx) *(.ARM.exidx.*)
	*(.ARM.extab) *(.ARM.extab.*)

?

quoted

though I do see binutils linker scripts use precisely what you have.
So I guess that's fine.

I guess we can't reuse `ARM_UNWIND_SECTIONS` since the ALIGN and
linker-script-defined-symbols would be weird in a DISCARD clause?

quoted

+               *(.ARM.extab*)
 #endif
        }


base-commit: 6e0bf0e0e55000742a53c5f3b58f8669e0091a11
--

--
Thanks,
~Nick Desaulniers

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宋方睿

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Kees Cook

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