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Fri, 31 Jul 2020 18:47:57 -0700 (PDT) From: Arvind Sankar X-Google-Original-From: Arvind Sankar Date: Fri, 31 Jul 2020 21:47:55 -0400 To: Kees Cook Cc: Thomas Gleixner , Will Deacon , Catalin Marinas , Mark Rutland , Ard Biesheuvel , Peter Collingbourne , James Morse , Borislav Petkov , Ingo Molnar , Russell King , Masahiro Yamada , Arvind Sankar , Nick Desaulniers , Nathan Chancellor , Arnd Bergmann , x86@kernel.org, clang-built-linux@googlegroups.com, linux-arch@vger.kernel.org, linux-efi@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v5 32/36] x86/boot/compressed: Reorganize zero-size section asserts Message-ID: <20200801014755.GA2700342@rani.riverdale.lan> References: <20200731230820.1742553-1-keescook@chromium.org> <20200731230820.1742553-33-keescook@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200731230820.1742553-33-keescook@chromium.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 31, 2020 at 04:08:16PM -0700, Kees Cook wrote: > For readability, move the zero-sized sections to the end after DISCARDS > and mark them NOLOAD for good measure. > > Signed-off-by: Kees Cook > --- > arch/x86/boot/compressed/vmlinux.lds.S | 42 +++++++++++++++----------- > 1 file changed, 25 insertions(+), 17 deletions(-) > > diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S > index 3c2ee9a5bf43..42dea70a5091 100644 > --- a/arch/x86/boot/compressed/vmlinux.lds.S > +++ b/arch/x86/boot/compressed/vmlinux.lds.S > @@ -42,18 +42,16 @@ SECTIONS > *(.rodata.*) > _erodata = . ; > } > - .rel.dyn : { > - *(.rel.*) > - } > - .rela.dyn : { > - *(.rela.*) > - } > - .got : { > - *(.got) > - } > .got.plt : { > *(.got.plt) > } > + ASSERT(SIZEOF(.got.plt) == 0 || > +#ifdef CONFIG_X86_64 > + SIZEOF(.got.plt) == 0x18, > +#else > + SIZEOF(.got.plt) == 0xc, > +#endif > + "Unexpected GOT/PLT entries detected!") > > .data : { > _data = . ; > @@ -85,13 +83,23 @@ SECTIONS > ELF_DETAILS > > DISCARDS > -} > > -ASSERT(SIZEOF(.got) == 0, "Unexpected GOT entries detected!") > -#ifdef CONFIG_X86_64 > -ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT entries detected!") > -#else > -ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0xc, "Unexpected GOT/PLT entries detected!") > -#endif > + /* > + * Sections that should stay zero sized, which is safer to > + * explicitly check instead of blindly discarding. > + */ > + .got (NOLOAD) : { > + *(.got) > + } > + ASSERT(SIZEOF(.got) == 0, "Unexpected GOT entries detected!") > > -ASSERT(SIZEOF(.rel.dyn) == 0 && SIZEOF(.rela.dyn) == 0, "Unexpected run-time relocations detected!") > + /* ld.lld does not like .rel* sections being made "NOLOAD". */ > + .rel.dyn : { > + *(.rel.*) > + } > + ASSERT(SIZEOF(.rel.dyn) == 0, "Unexpected run-time relocations (.rel) detected!") > + .rela.dyn : { > + *(.rela.*) > + } > + ASSERT(SIZEOF(.rela.dyn) == 0, "Unexpected run-time relocations (.rela) detected!") > +} > -- > 2.25.1 > There's no point in marking zero-size sections NOLOAD -- if the ASSERT's passed, they won't be present in the file at all anyway. The only section for which there might be a point is .got.plt, which is non-empty on 32-bit, and only if it is first moved to the end. That saves a few bytes.