Re: [PATCH v5 06/36] x86/boot: Remove run-time relocations from head_{32,64}.S

From: Arvind Sankar <nivedita@alum.mit.edu>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	linux-efi <linux-efi@vger.kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Will Deacon <will@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-arch <linux-arch@vger.kernel.org>,
	Fangrui Song <maskray@google.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	"maintainer:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)"
	<x86@kernel.org>, Russell King <linux@armlinux.org.uk>,
	Ard Biesheuvel <ardb@kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@suse.de>,
	Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Peter Collingbourne <pcc@google.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	James Morse <james.morse@arm.com>
Subject: Re: [PATCH v5 06/36] x86/boot: Remove run-time relocations from head_{32,64}.S
Date: Fri, 7 Aug 2020 16:20:56 -0400	[thread overview]
Message-ID: <20200807202056.GA1454138@rani.riverdale.lan> (raw)
In-Reply-To: <CAKwvOd=mY5=SWjGKA_KpvKnOPmJky_qMcyBYeFhskx6J=aJmNA@mail.gmail.com>

On Fri, Aug 07, 2020 at 11:12:29AM -0700, Nick Desaulniers wrote:
> On Fri, Jul 31, 2020 at 4:08 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > From: Arvind Sankar <nivedita@alum.mit.edu>
> >
> > The BFD linker generates run-time relocations for z_input_len and
> > z_output_len, even though they are absolute symbols.
> >
> > This is fixed for binutils-2.35 [1]. Work around this for earlier
> > versions by defining two variables input_len and output_len in addition
> > to the symbols, and use them via position-independent references.
> >
> > This eliminates the last two run-time relocations in the head code and
> > allows us to drop the -z noreloc-overflow flag to the linker.
> >
> > Move the -pie and --no-dynamic-linker LDFLAGS to LDFLAGS_vmlinux instead
> > of KBUILD_LDFLAGS. There shouldn't be anything else getting linked, but
> > this is the more logical location for these flags, and modversions might
> > call the linker if an EXPORT_SYMBOL is left over accidentally in one of
> > the decompressors.
> >
> > [1] https://sourceware.org/bugzilla/show_bug.cgi?id=25754
> >
> > Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> > Reviewed-by: Kees Cook <keescook@chromium.org>
> > Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
> > Reviewed-by: Fangrui Song <maskray@google.com>
> > Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> > Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> >  arch/x86/boot/compressed/Makefile  | 12 ++----------
> >  arch/x86/boot/compressed/head_32.S | 17 ++++++++---------
> >  arch/x86/boot/compressed/head_64.S |  4 ++--
> >  arch/x86/boot/compressed/mkpiggy.c |  6 ++++++
> >  4 files changed, 18 insertions(+), 21 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> > index 489fea16bcfb..7db0102a573d 100644
> > --- a/arch/x86/boot/compressed/Makefile
> > +++ b/arch/x86/boot/compressed/Makefile
> > @@ -51,16 +51,8 @@ UBSAN_SANITIZE :=n
> >  KBUILD_LDFLAGS := -m elf_$(UTS_MACHINE)
> >  # Compressed kernel should be built as PIE since it may be loaded at any
> >  # address by the bootloader.
> > -ifeq ($(CONFIG_X86_32),y)
> > -KBUILD_LDFLAGS += $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)
> > -else
> > -# To build 64-bit compressed kernel as PIE, we disable relocation
> > -# overflow check to avoid relocation overflow error with a new linker
> > -# command-line option, -z noreloc-overflow.
> > -KBUILD_LDFLAGS += $(shell $(LD) --help 2>&1 | grep -q "\-z noreloc-overflow" \
> > -       && echo "-z noreloc-overflow -pie --no-dynamic-linker")
> > -endif
> > -LDFLAGS_vmlinux := -T
> > +LDFLAGS_vmlinux := $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)
> 
> Oh, do these still need ld-option?  bfd and lld both support these
> flags. (Though in their --help, they mention single hyphen and double
> hyphen respectively.  Also, if we don't build this as PIE because the
> linker doesn't support the option, we probably want to fail the build?
> 

The check for pie doesn't, it's dropped in the next patch and pie is
used unconditionally.

no-dynamic-linker still needs the check as it was only supported from
binutils-2.26.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel