From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 33EBFC43219 for ; Wed, 2 Nov 2022 09:22:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229975AbiKBJWd (ORCPT ); Wed, 2 Nov 2022 05:22:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57420 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231341AbiKBJVU (ORCPT ); Wed, 2 Nov 2022 05:21:20 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2FE6365A2; Wed, 2 Nov 2022 02:20:08 -0700 (PDT) Date: Wed, 02 Nov 2022 09:20:05 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1667380806; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=XCaIO1zUHpx41jxtR0vGc0NZJ7OQkBuM2m8u8UjxdHo=; b=PcYpkm+q3HRc7m19SblIX7HrOb2jKeWbrcVWUbi8vyzUGiGkrvqfDsXMf2bFRlYSsc2rDX kL8jjZmMpnBKpadO4on4DU88N42gvQROENwhO2ln1P0hhi+CuHOz6Si5XNLjnTlH6tVgZP aXJ+PHJL+Pqvgs5SUOvTa9uqv8aVVBPLHPMwGq/GEKi+SG+fncYtCkiIffZfvXZyUBwmvH KP6Y1yuTCqfOk0OeCjrdRNebmPmvMvlEDggpbRX8x3v9YDL8B4f5sqCk4k0rKC20ECQE6S +zAAq8ZToW0h5Basq1UkSdDUIzHVB+1w+1PYdet0ZsRavqGWrelf6m1rwImo8Q== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1667380806; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=XCaIO1zUHpx41jxtR0vGc0NZJ7OQkBuM2m8u8UjxdHo=; b=8CPITZNFi+Dv+G261vs28vH2Ad4gjNmaiToIOG5lDsSnIe7SLAhYUoq7dV3tV1FmrKUHpa AIk45FDF0fvMGCCg== From: "tip-bot2 for Peter Zijlstra" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: x86/core] objtool: Avoid O(bloody terrible) behaviour -- an ode to libelf Cc: "Peter Zijlstra (Intel)" , Yujie Liu , x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20221028194453.461658986@infradead.org> References: <20221028194453.461658986@infradead.org> MIME-Version: 1.0 Message-ID: <166738080540.7716.10737738024182403591.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following commit has been merged into the x86/core branch of tip: Commit-ID: 13f60e80e15dd0657c90bcca372ba045630ed9de Gitweb: https://git.kernel.org/tip/13f60e80e15dd0657c90bcca372ba045630ed9de Author: Peter Zijlstra AuthorDate: Fri, 28 Oct 2022 20:29:51 +02:00 Committer: Peter Zijlstra CommitterDate: Tue, 01 Nov 2022 13:44:08 +01:00 objtool: Avoid O(bloody terrible) behaviour -- an ode to libelf Due to how gelf_update_sym*() requires an Elf_Data pointer, and how libelf keeps Elf_Data in a linked list per section, elf_update_symbol() ends up having to iterate this list on each update to find the correct Elf_Data for the index'ed symbol. By allocating one Elf_Data per new symbol, the list grows per new symbol, giving an effective O(n^2) insertion time. This is obviously bloody terrible. Therefore over-allocate the Elf_Data when an extention is needed. Except it turns out libelf disregards Elf_Scn::sh_size in favour of the sum of Elf_Data::d_size. IOW it will happily write out all the unused space and fill it with: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND entries (aka zeros). Which obviously violates the STB_LOCAL placement rule, and is a general pain in the backside for not being the desired behaviour. Manually fix-up the Elf_Data size to avoid this problem before calling elf_update(). This significantly improves performance when adding a significant number of symbols. Signed-off-by: Peter Zijlstra (Intel) Tested-by: Yujie Liu Link: https://lkml.kernel.org/r/20221028194453.461658986@infradead.org --- tools/objtool/elf.c | 89 ++++++++++++++++++++++++++-- tools/objtool/include/objtool/elf.h | 2 +- 2 files changed, 84 insertions(+), 7 deletions(-) diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c index 3ad89d9..36dc787 100644 --- a/tools/objtool/elf.c +++ b/tools/objtool/elf.c @@ -634,6 +634,12 @@ static int elf_update_symbol(struct elf *elf, struct section *symtab, /* end-of-list */ if (!symtab_data) { + /* + * Over-allocate to avoid O(n^2) symbol creation + * behaviour. The down side is that libelf doesn't + * like this; see elf_truncate_section() for the fixup. + */ + int num = max(1U, sym->idx/3); void *buf; if (idx) { @@ -647,28 +653,34 @@ static int elf_update_symbol(struct elf *elf, struct section *symtab, if (t) shndx_data = elf_newdata(t); - buf = calloc(1, entsize); + buf = calloc(num, entsize); if (!buf) { WARN("malloc"); return -1; } symtab_data->d_buf = buf; - symtab_data->d_size = entsize; + symtab_data->d_size = num * entsize; symtab_data->d_align = 1; symtab_data->d_type = ELF_T_SYM; - symtab->sh.sh_size += entsize; symtab->changed = true; + symtab->truncate = true; if (t) { - shndx_data->d_buf = &sym->sec->idx; - shndx_data->d_size = sizeof(Elf32_Word); + buf = calloc(num, sizeof(Elf32_Word)); + if (!buf) { + WARN("malloc"); + return -1; + } + + shndx_data->d_buf = buf; + shndx_data->d_size = num * sizeof(Elf32_Word); shndx_data->d_align = sizeof(Elf32_Word); shndx_data->d_type = ELF_T_WORD; - symtab_shndx->sh.sh_size += sizeof(Elf32_Word); symtab_shndx->changed = true; + symtab_shndx->truncate = true; } break; @@ -770,6 +782,14 @@ non_local: return NULL; } + symtab->sh.sh_size += symtab->sh.sh_entsize; + symtab->changed = true; + + if (symtab_shndx) { + symtab_shndx->sh.sh_size += sizeof(Elf32_Word); + symtab_shndx->changed = true; + } + return sym; } @@ -1286,6 +1306,60 @@ int elf_write_reloc(struct elf *elf, struct reloc *reloc) return 0; } +/* + * When Elf_Scn::sh_size is smaller than the combined Elf_Data::d_size + * do you: + * + * A) adhere to the section header and truncate the data, or + * B) ignore the section header and write out all the data you've got? + * + * Yes, libelf sucks and we need to manually truncate if we over-allocate data. + */ +static int elf_truncate_section(struct elf *elf, struct section *sec) +{ + u64 size = sec->sh.sh_size; + bool truncated = false; + Elf_Data *data = NULL; + Elf_Scn *s; + + s = elf_getscn(elf->elf, sec->idx); + if (!s) { + WARN_ELF("elf_getscn"); + return -1; + } + + for (;;) { + /* get next data descriptor for the relevant section */ + data = elf_getdata(s, data); + + if (!data) { + if (size) { + WARN("end of section data but non-zero size left\n"); + return -1; + } + return 0; + } + + if (truncated) { + /* when we remove symbols */ + WARN("truncated; but more data\n"); + return -1; + } + + if (!data->d_size) { + WARN("zero size data"); + return -1; + } + + if (data->d_size > size) { + truncated = true; + data->d_size = size; + } + + size -= data->d_size; + } +} + int elf_write(struct elf *elf) { struct section *sec; @@ -1296,6 +1370,9 @@ int elf_write(struct elf *elf) /* Update changed relocation sections and section headers: */ list_for_each_entry(sec, &elf->sections, list) { + if (sec->truncate) + elf_truncate_section(elf, sec); + if (sec->changed) { s = elf_getscn(elf->elf, sec->idx); if (!s) { diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h index d285331..9e96a61 100644 --- a/tools/objtool/include/objtool/elf.h +++ b/tools/objtool/include/objtool/elf.h @@ -38,7 +38,7 @@ struct section { Elf_Data *data; char *name; int idx; - bool changed, text, rodata, noinstr, init; + bool changed, text, rodata, noinstr, init, truncate; }; struct symbol {