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

[Kees Cook]

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 <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, Sep 28, 2020 at 3:49 PM Nathan Chancellor
> > <natechancellor@xxxxxxxxx> 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@xxxxxxxxxx>
> > Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> > > Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> > > ---
> > >  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.*)

?

> 
> > 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

-- 
Kees Cook