Date:   Mon, 12 Oct 2020 14:22:03 -0700
From:   Fāng-ruì Sòng <maskray@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] arm/build: Always handle .ARM.exidx and .ARM.extab sections

On Mon, Oct 12, 2020 at 2:11 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@...glegroups.com> wrote:
>
> On Mon, Sep 28, 2020 at 3:49 PM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > 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 <ndesaulniers@...gle.com>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
> Please submit to:
> https://www.arm.linux.org.uk/developer/patches/add.php
>
> >
> > Fixes: 5a17850e251a ("arm/build: Warn on orphan section placement")
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1152
> > Reported-by: kernel test robot <lkp@...el.com>
> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > ---
> >  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.

> 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?
>
>
> > +               *(.ARM.extab*)
> >  #endif
> >         }
> >
> >
> > base-commit: 6e0bf0e0e55000742a53c5f3b58f8669e0091a11
> > --
>
>
> --
> Thanks,
> ~Nick Desaulniers
>
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-- 
宋方睿

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