Re: [PATCH v3 4/9] x86/build: Warn on orphan section placement

[Kees Cook <keescook@chromium.org>]

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On Mon, Jun 29, 2020 at 04:54:13PM +0200, Marco Elver wrote:
> On Sat, 27 Jun 2020 at 17:44, Kees Cook <keescook@chromium.org> wrote:
> >
> > On Thu, Jun 25, 2020 at 02:36:27AM +0800, kernel test robot wrote:
> > > I love your patch! Perhaps something to improve:
> > > [...]
> > > config: x86_64-randconfig-a012-20200624 (attached as .config)
> >
> > CONFIG_KCSAN=y
> >
> > > compiler: clang version 11.0.0 (https://github.com/llvm/llvm-project 1d4c87335d5236ea1f35937e1014980ba961ae34)
> > > [...]
> > > All warnings (new ones prefixed by >>):
> > >
> > >    ld.lld: warning: drivers/built-in.a(mfd/mt6397-irq.o):(.init_array.0) is being placed in '.init_array.0'
> >
> > As far as I can tell, this is a Clang bug. But I don't know the
> > internals here, so I've opened:
> > https://bugs.llvm.org/show_bug.cgi?id=46478
> >
> > and created a work-around patch for the kernel:
> 
> Thanks, minor comments below.
> 
> With KCSAN this is:
> 
> Tested-by: Marco Elver <elver@google.com>

Thanks!

> 
> 
> > commit 915f2c343e59a14f00c68f4d7afcfdc621de0674
> > Author: Kees Cook <keescook@chromium.org>
> > Date:   Sat Jun 27 08:07:54 2020 -0700
> >
> >     vmlinux.lds.h: Avoid KCSAN's unwanted sections
> 
> Since you found that it's also KASAN, this probably wants updating.

Yeah, I found that while testing the v4 series and updated the patch
there.

> >     KCSAN (-fsanitize=thread) produces unwanted[1] .eh_frame and .init_array.*
> >     sections. Add them to DISCARDS, except with CONFIG_CONSTRUCTORS, which
> >     wants to keep .init_array.* sections.
> >
> >     [1] https://bugs.llvm.org/show_bug.cgi?id=46478
> >
> >     Signed-off-by: Kees Cook <keescook@chromium.org>
> >
> > diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> > index f8a5b2333729..41c8c73de6c4 100644
> > --- a/arch/x86/Makefile
> > +++ b/arch/x86/Makefile
> > @@ -195,7 +195,9 @@ endif
> >  # Workaround for a gcc prelease that unfortunately was shipped in a suse release
> >  KBUILD_CFLAGS += -Wno-sign-compare
> >  #
> > -KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
> > +KBUILD_AFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables
> > +KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables
> > +KBUILD_LDFLAGS += $(call ld-option,--no-ld-generated-unwind-info)
> 
> Why are they needed? They are not mentioned in the commit message.

This was a mis-applied chunk (I also noticed this in the v4).

> > +/*
> > + * Clang's -fsanitize=thread produces unwanted sections (.eh_frame
> > + * and .init_array.*), but CONFIG_CONSTRUCTORS wants to keep any
> > + * .init_array.* sections.
> > + * https://bugs.llvm.org/show_bug.cgi?id=46478
> > + */
> > +#if defined(CONFIG_KCSAN) && !defined(CONFIG_CONSTRUCTORS)
> 
> CONFIG_KASAN as well?
> 
> > +#define KCSAN_DISCARDS                                                 \
> > +       *(.init_array) *(.init_array.*)                                 \
> > +       *(.eh_frame)
> > +#elif defined(CONFIG_KCSAN) && defined(CONFIG_CONSTRUCTORS)
> > +#define KCSAN_DISCARDS                                                 \
> > +       *(.eh_frame)
> > +#else
> > +#define KCSAN_DISCARDS
> > +#endif
> > +
> >  #define DISCARDS                                                       \
> >         /DISCARD/ : {                                                   \
> >         EXIT_DISCARDS                                                   \
> >         EXIT_CALL                                                       \
> > +       KCSAN_DISCARDS                                                  \
> 
> Maybe just 'SANITIZER_DISCARDS'?

Sure! I will rename it.

> 
> >         *(.discard)                                                     \
> >         *(.discard.*)                                                   \
> >         *(.modinfo)                                                     \
> >
> > --
> > Kees Cook

-- 
Kees Cook