From: Sedat Dilek <sedat.dilek@gmail.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Thomas Gleixner <tglx@linutronix.de>,
Arvind Sankar <nivedita@alum.mit.edu>,
Fangrui Song <maskray@google.com>,
clang-built-linux <clang-built-linux@googlegroups.com>,
e5ten.arch@gmail.com,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
Masahiro Yamada <masahiroy@kernel.org>,
Ard Biesheuvel <ardb@kernel.org>,
Kees Cook <keescook@chromium.org>,
LKML <linux-kernel@vger.kernel.org>,
"# 3.4.x" <stable@vger.kernel.org>
Subject: Re: [PATCH v2] x86/boot/compressed: Disable relocation relaxation
Date: Sat, 15 Aug 2020 23:09:54 +0200 [thread overview]
Message-ID: <CA+icZUXhPakmkVOEHOq7FON+3ETRNN8DHEiRws-BJaAUad4hOg@mail.gmail.com> (raw)
In-Reply-To: <CAKwvOdmHxsLzou=6WN698LOGq9ahWUmztAHfUYYAUcgpH1FGRA@mail.gmail.com>
On Sat, Aug 15, 2020 at 10:57 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> Hi Ingo,
> I saw you picked up Arvind's other series into x86/boot. Would you
> mind please including this, as well? Our CI is quite red for x86...
>
> EOM
>
+1
- Sedat -
> On Sat, Aug 15, 2020 at 8:49 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> >
> > On Wed, Aug 12, 2020 at 2:43 AM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> > >
> > > The x86-64 psABI [0] specifies special relocation types
> > > (R_X86_64_[REX_]GOTPCRELX) for indirection through the Global Offset
> > > Table, semantically equivalent to R_X86_64_GOTPCREL, which the linker
> > > can take advantage of for optimization (relaxation) at link time. This
> > > is supported by LLD and binutils versions 2.26 onwards.
> > >
> > > The compressed kernel is position-independent code, however, when using
> > > LLD or binutils versions before 2.27, it must be linked without the -pie
> > > option. In this case, the linker may optimize certain instructions into
> > > a non-position-independent form, by converting foo@GOTPCREL(%rip) to $foo.
> > >
> > > This potential issue has been present with LLD and binutils-2.26 for a
> > > long time, but it has never manifested itself before now:
> > > - LLD and binutils-2.26 only relax
> > > movq foo@GOTPCREL(%rip), %reg
> > > to
> > > leaq foo(%rip), %reg
> > > which is still position-independent, rather than
> > > mov $foo, %reg
> > > which is permitted by the psABI when -pie is not enabled.
> > > - gcc happens to only generate GOTPCREL relocations on mov instructions.
> > > - clang does generate GOTPCREL relocations on non-mov instructions, but
> > > when building the compressed kernel, it uses its integrated assembler
> > > (due to the redefinition of KBUILD_CFLAGS dropping -no-integrated-as),
> > > which has so far defaulted to not generating the GOTPCRELX
> > > relocations.
> > >
> > > Nick Desaulniers reports [1,2]:
> > > A recent change [3] to a default value of configuration variable
> > > (ENABLE_X86_RELAX_RELOCATIONS OFF -> ON) in LLVM now causes Clang's
> > > integrated assembler to emit R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX
> > > relocations. LLD will relax instructions with these relocations based
> > > on whether the image is being linked as position independent or not.
> > > When not, then LLD will relax these instructions to use absolute
> > > addressing mode (R_RELAX_GOT_PC_NOPIC). This causes kernels built with
> > > Clang and linked with LLD to fail to boot.
> > >
> > > Patch series [4] is a solution to allow the compressed kernel to be
> > > linked with -pie unconditionally, but even if merged is unlikely to be
> > > backported. As a simple solution that can be applied to stable as well,
> > > prevent the assembler from generating the relaxed relocation types using
> > > the -mrelax-relocations=no option. For ease of backporting, do this
> > > unconditionally.
> > >
> > > [0] https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/linker-optimization.tex#L65
> > > [1] https://lore.kernel.org/lkml/20200807194100.3570838-1-ndesaulniers@google.com/
> > > [2] https://github.com/ClangBuiltLinux/linux/issues/1121
> > > [3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0
> > > [4] https://lore.kernel.org/lkml/20200731202738.2577854-1-nivedita@alum.mit.edu/
> > >
> > > Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> > > Reported-by: Nick Desaulniers <ndesaulniers@google.com>
> > > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> > > Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> >
> > Thanks for the patch.
> >
> > Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> >
> > - Sedat -
> >
> > [1] https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674409705
> >
> > > ---
> > > arch/x86/boot/compressed/Makefile | 2 ++
> > > 1 file changed, 2 insertions(+)
> > >
> > > diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> > > index 3962f592633d..ff7894f39e0e 100644
> > > --- a/arch/x86/boot/compressed/Makefile
> > > +++ b/arch/x86/boot/compressed/Makefile
> > > @@ -43,6 +43,8 @@ KBUILD_CFLAGS += -Wno-pointer-sign
> > > KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
> > > KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
> > > KBUILD_CFLAGS += -D__DISABLE_EXPORTS
> > > +# Disable relocation relaxation in case the link is not PIE.
> > > +KBUILD_CFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no)
> > >
> > > KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__
> > > GCOV_PROFILE := n
> > > --
> > > 2.26.2
> > >
> > > --
> > > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group.
> > > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@googlegroups.com.
> > > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20200812004308.1448603-1-nivedita%40alum.mit.edu.
>
>
>
> --
> Thanks,
> ~Nick Desaulniers
next prev parent reply other threads:[~2020-08-15 21:45 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-07 19:41 [PATCH] x86/boot: avoid relaxable symbols with Clang Nick Desaulniers
2020-08-07 21:29 ` Arvind Sankar
2020-08-07 21:54 ` Nick Desaulniers
2020-08-08 1:43 ` Arvind Sankar
2020-08-10 18:32 ` Nick Desaulniers
2020-08-11 17:36 ` [PATCH] x86/boot/compressed: Disable relocation relaxation for non-pie link Arvind Sankar
2020-08-11 17:58 ` Nick Desaulniers
2020-08-11 22:44 ` Arvind Sankar
2020-08-11 23:04 ` Nick Desaulniers
2020-08-11 23:43 ` Arvind Sankar
2020-08-11 23:51 ` Nick Desaulniers
2020-08-12 0:41 ` Arvind Sankar
2020-08-12 0:43 ` [PATCH v2] x86/boot/compressed: Disable relocation relaxation Arvind Sankar
2020-08-12 17:42 ` Nick Desaulniers
2020-08-15 15:49 ` Sedat Dilek
2020-08-15 20:56 ` Nick Desaulniers
2020-08-15 21:09 ` Sedat Dilek [this message]
2020-08-25 14:56 ` Arvind Sankar
2020-09-04 15:23 ` Arvind Sankar
2020-09-13 22:34 ` Arvind Sankar
2020-09-14 5:43 ` Ard Biesheuvel
2020-09-14 9:16 ` Ingo Molnar
2020-09-14 9:35 ` Sedat Dilek
2020-09-14 17:16 ` [tip: x86/urgent] " tip-bot2 for Arvind Sankar
2020-08-12 17:39 ` [PATCH] x86/boot/compressed: Disable relocation relaxation for non-pie link Nick Desaulniers
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