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Peter Anvin" , x86@kernel.org, Nick Desaulniers , Dmitry Golovin , clang-built-linux@googlegroups.com, Ard Biesheuvel , Masahiro Yamada , Daniel Kiper , linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/4] x86/boot: Remove runtime relocations from compressed kernel Message-ID: <20200526003703.qr2yq6fgxpl6wcua@google.com> References: <20200524212816.243139-1-nivedita@alum.mit.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20200524212816.243139-1-nivedita@alum.mit.edu> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-05-24, Arvind Sankar wrote: >The compressed kernel currently contains bogus runtime relocations in >the startup code in head_{32,64}.S, which are generated by the linker, >but must not actually be processed at runtime. > >This generates warnings when linking with the BFD linker, and errors >with LLD, which defaults to erroring on runtime relocations in read-only >sections. It also requires the -z noreloc-overflow hack for the 64-bit >kernel, which prevents us from linking it as -pie on an older BFD linker >(<= 2.26) or on LLD, because the locations that are to be apparently >relocated are only 32-bits in size and so cannot normally have >R_X86_64_RELATIVE relocations. > >This series aims to get rid of these relocations. It is based on >efi/next (efi-changes-for-v5.8), where the latest patches touch the >head code to eliminate the global offset table. > >The first patch is an independent fix for LLD, to avoid an orphan >section in arch/x86/boot/setup.elf [0]. > >The second patch gets rid of almost all the relocations. It uses >standard PIC addressing technique for 32-bit, i.e. loading a register >with the address of _GLOBAL_OFFSET_TABLE_ and then using GOTOFF >references to access variables. For 64-bit, there is 32-bit code that >cannot use RIP-relative addressing, and also cannot use the 32-bit >method, since GOTOFF references are 64-bit only. This is instead handled >using a macro to replace a reference like gdt with (gdt-startup_32) >instead. The assembler will generate a PC32 relocation entry, with >addend set to (.-startup_32), and these will be replaced with constants >at link time. This works as long as all the code using such references >lives in the same section as startup_32, i.e. in .head.text. > >The third patch addresses a remaining issue with the BFD linker, which >insists on generating runtime relocations for absolute symbols. We use >z_input_len and z_output_len, defined in the generated piggy.S file, as >symbols whose absolute "addresses" are actually the size of the >compressed payload and the size of the decompressed kernel image >respectively. LLD does not generate relocations for these two symbols, >but the BFD linker does. To get around this, piggy.S is extended to also >define two u32 variables (in .rodata) with the lengths, and the head >code is modified to use those instead of the symbol addresses. > >An alternative way to handle z_input_len/z_output_len would be to just >include piggy.S in head_{32,64}.S instead of as a separate object file, >since the GNU assembler doesn't generate relocations for symbols set to >constants. > >The last patch adds a check in the linker script to ensure that no >runtime relocations get reintroduced. Since the GOT has been eliminated >as well, the compressed kernel has no runtime relocations whatsoever any >more. > >[0] https://lore.kernel.org/lkml/20200521152459.558081-1-nivedita@alum.mit.edu/ > >Arvind Sankar (4): > x86/boot: Add .text.startup to setup.ld > x86/boot: Remove runtime relocations from .head.text code > x86/boot: Remove runtime relocations from head_{32,64}.S > x86/boot: Check that there are no runtime relocations > > arch/x86/boot/compressed/Makefile | 36 +--------- > arch/x86/boot/compressed/head_32.S | 59 +++++++-------- > arch/x86/boot/compressed/head_64.S | 99 +++++++++++++++----------- > arch/x86/boot/compressed/mkpiggy.c | 6 ++ > arch/x86/boot/compressed/vmlinux.lds.S | 11 +++ > arch/x86/boot/setup.ld | 2 +- > 6 files changed, 109 insertions(+), 104 deletions(-) > >-- >2.26.2 All 4 commits look good. Reviewed-by: Fangrui Song