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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: <20200222072144.asqaxlv364s6ezbv@google.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20200222070218.GA27571@ubuntu-m2-xlarge-x86> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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/ : { *(*) } }