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Peter Anvin" , x86@kernel.org, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com, Michael Matz Subject: Re: [PATCH 2/2] x86/boot/compressed: Remove unnecessary sections from bzImage Message-ID: <20200222074242.GA17358@ubuntu-m2-xlarge-x86> References: <20200109150218.16544-1-nivedita@alum.mit.edu> <20200109150218.16544-2-nivedita@alum.mit.edu> <20200222050845.GA19912@ubuntu-m2-xlarge-x86> <20200222065521.GA11284@zn.tnic> <20200222070218.GA27571@ubuntu-m2-xlarge-x86> <20200222072144.asqaxlv364s6ezbv@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200222072144.asqaxlv364s6ezbv@google.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 21, 2020 at 11:21:44PM -0800, Fangrui Song wrote: > On 2020-02-22, Nathan Chancellor wrote: > > On Sat, Feb 22, 2020 at 07:55:21AM +0100, Borislav Petkov wrote: > > > On Fri, Feb 21, 2020 at 10:08:45PM -0700, Nathan Chancellor wrote: > > > > On Thu, Jan 09, 2020 at 10:02:18AM -0500, Arvind Sankar wrote: > > > > > Discarding the sections that are unused in the compressed kernel saves > > > > > about 10 KiB on 32-bit and 6 KiB on 64-bit, mostly from .eh_frame. > > > > > > > > > > Signed-off-by: Arvind Sankar > > > > > --- > > > > > arch/x86/boot/compressed/vmlinux.lds.S | 5 +++++ > > > > > 1 file changed, 5 insertions(+) > > > > > > > > > > diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S > > > > > index 508cfa6828c5..12a20603d92e 100644 > > > > > --- a/arch/x86/boot/compressed/vmlinux.lds.S > > > > > +++ b/arch/x86/boot/compressed/vmlinux.lds.S > > > > > @@ -73,4 +73,9 @@ SECTIONS > > > > > #endif > > > > > . = ALIGN(PAGE_SIZE); /* keep ZO size page aligned */ > > > > > _end = .; > > > > > + > > > > > + /* Discard all remaining sections */ > > > > > + /DISCARD/ : { > > > > > + *(*) > > > > > + } > > > > > } > > > > > -- > > > > > 2.24.1 > > > > > > > > > > > > > This patch breaks linking with ld.lld: > > > > > > > > $ make -j$(nproc) -s CC=clang LD=ld.lld O=out.x86_64 distclean defconfig bzImage > > > > ld.lld: error: discarding .shstrtab section is not allowed > > > > > > Well, why is it not allowed? And why isn't the GNU linker complaining? > > > > No idea, unfortunately I am not a linker expert and the patch that > > changes this in lld does not really explain why it adds this > > restriction: > > > > https://github.com/llvm/llvm-project/commit/1e799942b37d04f30b73f6a9e792d551dadafeea > > > > CC'ing Fangrui as I don't know George's email and he is usually > > responsive to ld.lld issues/questions. > > > > Cheers, > > Nathan > > In GNU ld, it seems that .shstrtab .symtab and .strtab are special > cased. Neither the input section description *(.shstrtab) nor *(*) > discards .shstrtab . I feel that this is a weird case (probably even a bug) > that lld should not implement. > > In GNU ld, the following is not useful, while lld will discard the > synthesized .strtab as requested: > > /DISCARD/ : { *(.strtab) } > > I think it is better making the intention (retaining .shstrtab) > explicit, by adding a .shstrtab beside /DISCARD/ : > > SECTIONS { > ... > .shstrtab : { *(.shstrtab) } > /DISCARD/ : { *(*) } > } Thanks for the clarity. With your suggestion (diff below), I see the following error: arch/x86/boot/compressed/vmlinux: no symbols ld.lld: error: undefined symbol: ZO_input_data >>> referenced by arch/x86/boot/header.o:(.header+0x59) ld.lld: error: undefined symbol: ZO_z_input_len >>> referenced by arch/x86/boot/header.o:(.header+0x5D) make[3]: *** [../arch/x86/boot/Makefile:108: arch/x86/boot/setup.elf] It seems like the section still isn't being added? Cheers, Nathan diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S index 12a20603d92e..a6c3c7440c27 100644 --- a/arch/x86/boot/compressed/vmlinux.lds.S +++ b/arch/x86/boot/compressed/vmlinux.lds.S @@ -74,6 +74,10 @@ SECTIONS . = ALIGN(PAGE_SIZE); /* keep ZO size page aligned */ _end = .; + .shstrtab : { + *(.shstrtab) + } + /* Discard all remaining sections */ /DISCARD/ : { *(*)