linux-kernel - Re: [PATCH v5 06/36] x86/boot: Remove run-time relocations from head

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Message-ID: <20200807202056.GA1454138@rani.riverdale.lan>
Date:   Fri, 7 Aug 2020 16:20:56 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Fangrui Song <maskray@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 06/36] x86/boot: Remove run-time relocations from
 head_{32,64}.S

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@...omium.org> wrote:
> >
> > From: Arvind Sankar <nivedita@...m.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@...gle.com>
> > Reviewed-by: Kees Cook <keescook@...omium.org>
> > Reviewed-by: Ard Biesheuvel <ardb@...nel.org>
> > Reviewed-by: Fangrui Song <maskray@...gle.com>
> > Tested-by: Sedat Dilek <sedat.dilek@...il.com>
> > Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> > Signed-off-by: Kees Cook <keescook@...omium.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.