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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id i21sm12412582pfa.18.2020.07.31.22.35.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 31 Jul 2020 22:35:15 -0700 (PDT) Date: Fri, 31 Jul 2020 22:35:14 -0700 From: Kees Cook To: Arvind Sankar Cc: Thomas Gleixner , Will Deacon , Catalin Marinas , Mark Rutland , Ard Biesheuvel , Peter Collingbourne , James Morse , Borislav Petkov , Ingo Molnar , Russell King , Masahiro Yamada , 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: <202007312233.1BA0E2EFC@keescook> References: <20200731230820.1742553-1-keescook@chromium.org> <20200731230820.1742553-33-keescook@chromium.org> <20200801014755.GA2700342@rani.riverdale.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200801014755.GA2700342@rani.riverdale.lan> 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 09:47:55PM -0400, Arvind Sankar wrote: > 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. I did not find that universally true. I found some sections be written out with a 0 size. Some I could remove from disk with NOLOAD, others did not like that so much. > 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. What do you mean about "only if it is first moved to the end"? Would it be zero-sized if it was closer to .text? -- Kees Cook