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Date: Mon, 29 Jun 2020 20:11:49 +0200 From: Ard Biesheuvel <ardb@...nel.org> To: Fangrui Song <maskray@...gle.com> Cc: Kees Cook <keescook@...omium.org>, Arvind Sankar <nivedita@...m.mit.edu>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Dmitry Golovin <dima@...ovin.in>, clang-built-linux <clang-built-linux@...glegroups.com>, Masahiro Yamada <masahiroy@...nel.org>, Daniel Kiper <daniel.kiper@...cle.com>, Sedat Dilek <sedat.dilek@...il.com>, Nathan Chancellor <natechancellor@...il.com>, Arnd Bergmann <arnd@...db.de>, "H . J . Lu" <hjl@...rceware.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v3 7/7] x86/boot: Check that there are no runtime relocations On Mon, 29 Jun 2020 at 19:37, Fangrui Song <maskray@...gle.com> wrote: > > On 2020-06-29, Arvind Sankar wrote: > >On Mon, Jun 29, 2020 at 09:20:31AM -0700, Kees Cook wrote: > >> On Mon, Jun 29, 2020 at 06:11:59PM +0200, Ard Biesheuvel wrote: > >> > On Mon, 29 Jun 2020 at 18:09, Kees Cook <keescook@...omium.org> wrote: > >> > > > >> > > On Mon, Jun 29, 2020 at 10:09:28AM -0400, Arvind Sankar wrote: > >> > > > Add a linker script check that there are no runtime relocations, and > >> > > > remove the old one that tries to check via looking for specially-named > >> > > > sections in the object files. > >> > > > > >> > > > Drop the tests for -fPIE compiler option and -pie linker option, as they > >> > > > are available in all supported gcc and binutils versions (as well as > >> > > > clang and lld). > >> > > > > >> > > > Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu> > >> > > > Reviewed-by: Ard Biesheuvel <ardb@...nel.org> > >> > > > Reviewed-by: Fangrui Song <maskray@...gle.com> > >> > > > --- > >> > > > arch/x86/boot/compressed/Makefile | 28 +++----------------------- > >> > > > arch/x86/boot/compressed/vmlinux.lds.S | 8 ++++++++ > >> > > > 2 files changed, 11 insertions(+), 25 deletions(-) > >> > > > >> > > Reviewed-by: Kees Cook <keescook@...omium.org> > >> > > > >> > > question below ... > >> > > > >> > > > diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S > >> > > > index a4a4a59a2628..a78510046eec 100644 > >> > > > --- a/arch/x86/boot/compressed/vmlinux.lds.S > >> > > > +++ b/arch/x86/boot/compressed/vmlinux.lds.S > >> > > > @@ -42,6 +42,12 @@ SECTIONS > >> > > > *(.rodata.*) > >> > > > _erodata = . ; > >> > > > } > >> > > > + .rel.dyn : { > >> > > > + *(.rel.*) > >> > > > + } > >> > > > + .rela.dyn : { > >> > > > + *(.rela.*) > >> > > > + } > >> > > > .got : { > >> > > > *(.got) > >> > > > } > >> > > > >> > > Should these be marked (INFO) as well? > >> > > > >> > > >> > Given that sections marked as (INFO) will still be emitted into the > >> > ELF image, it does not really make a difference to do this for zero > >> > sized sections. > >> > >> Oh, I misunderstood -- I though they were _not_ emitted; I see now what > >> you said was not allocated. So, disk space used for the .got.plt case, > >> but not memory space used. Sorry for the confusion! > >> > >> -Kees > > About output section type (INFO): > https://sourceware.org/binutils/docs/ld/Output-Section-Type.html#Output-Section-Type > says "These type names are supported for backward compatibility, and are > rarely used." > > If all input section don't have the SHF_ALLOC flag, the output section > will not have this flag as well. This type is not useful... > > If .got and .got.plt were used, they should be considered dynamic > relocations which should be part of the loadable image. So they should > have the SHF_ALLOC flag. (INFO) will not be applicable anyway. > I don't care deeply either way, but Kees indicated that he would like to get rid of the 24 bytes of .got.plt magic entries that we have no need for. In fact, a lot of this mangling is caused by the fact that the linker is creating a relocatable binary, and assumes that it is a hosted binary that is loaded by a dynamic loader. It would actually be much better if the compiler and linker would take -ffreestanding into account, and suppress GOT entries, PLTs, dynamic program headers for shared libraries altogether.
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