From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kees Cook Subject: Re: [PATCH] vmlinux.lds.h: Coalesce transient LLVM dead code elimination sections Date: Thu, 2 Jul 2020 08:54:53 -0700 Message-ID: <202007020853.5F15B5DDD@keescook> References: <20200702085400.2643527-1-danny@kdrag0n.dev> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60890 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725878AbgGBPyz (ORCPT ); Thu, 2 Jul 2020 11:54:55 -0400 Received: from mail-pl1-x642.google.com (mail-pl1-x642.google.com [IPv6:2607:f8b0:4864:20::642]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A751CC08C5C1 for ; Thu, 2 Jul 2020 08:54:55 -0700 (PDT) Received: by mail-pl1-x642.google.com with SMTP id d10so11473859pls.5 for ; Thu, 02 Jul 2020 08:54:55 -0700 (PDT) Content-Disposition: inline In-Reply-To: <20200702085400.2643527-1-danny@kdrag0n.dev> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Danny Lin Cc: Arnd Bergmann , Nathan Chancellor , Nick Desaulniers , Sami Tolvanen , Fangrui Song , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com, stable@vger.kernel.org On Thu, Jul 02, 2020 at 01:54:00AM -0700, Danny Lin wrote: > A recent LLVM 11 commit [1] made LLD stop implicitly coalescing some > temporary LLVM sections, namely .{data,bss}..compoundliteral.XXX: > > [30] .data..compoundli PROGBITS ffffffff9ac9a000 19e9a000 > 000000000000cea0 0000000000000000 WA 0 0 32 > [31] .rela.data..compo RELA 0000000000000000 40965440 > 0000000000001d88 0000000000000018 I 2238 30 8 > [32] .data..compoundli PROGBITS ffffffff9aca6ea0 19ea6ea0 > 00000000000033c0 0000000000000000 WA 0 0 32 > [33] .rela.data..compo RELA 0000000000000000 409671c8 > 0000000000000948 0000000000000018 I 2238 32 8 > [...] > [2213] .bss..compoundlit NOBITS ffffffffa3000000 1d85c000 > 00000000000000a0 0000000000000000 WA 0 0 32 > [2214] .bss..compoundlit NOBITS ffffffffa30000a0 1d85c000 > 0000000000000040 0000000000000000 WA 0 0 32 > [...] > > While these extra sections don't typically cause any breakage, they do > inflate the vmlinux size due to the overhead of storing metadata for > thousands of extra sections. > > It's also worth noting that for some reason, some downstream Android > kernels can't boot at all if these sections aren't coalesced. > > This issue isn't limited to any specific architecture; it affects arm64 > and x86 if CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is forced on. > > Example on x86 allyesconfig: > Before: 2241 sections, 1170972 KiB > After: 56 sections, 1171169 KiB > > [1] https://github.com/llvm/llvm-project/commit/9e33c096476ab5e02ab1c8442cc3cb4e32e29f17 > > Link: https://github.com/ClangBuiltLinux/linux/issues/958 > Cc: stable@vger.kernel.org # v4.4+ > Suggested-by: Fangrui Song > Signed-off-by: Danny Lin > --- > include/asm-generic/vmlinux.lds.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h > index db600ef218d7..18968cba87c7 100644 > --- a/include/asm-generic/vmlinux.lds.h > +++ b/include/asm-generic/vmlinux.lds.h > @@ -94,10 +94,10 @@ > */ > #ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION > #define TEXT_MAIN .text .text.[0-9a-zA-Z_]* > -#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX* > +#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX* .data..compoundliteral* > #define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]* > #define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* > -#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* > +#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..compoundliteral* Are there .data.. and .bss.. sections we do NOT want to collect? i.e. why not include .data..* and .bss..* ? -- Kees Cook