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 X-Spam-Level: X-Spam-Status: No, score=-1.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EAA6AC433E0 for ; Wed, 24 Jun 2020 17:57:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CB10C207E8 for ; Wed, 24 Jun 2020 17:57:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1593021477; bh=QhcLJu5XeabSx4iawRJSemJSRZyIebuJjQB6tYl8B5c=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=WrUBCvUTogaFnl32qA366jsPR57x+73Tx1ACkPQ9TAlU9QQn9IbBv0MRVpzfe3+lt pYsFYYwUVMbxffxt4tHAmP/JcVCBFrHn0ZjxuLcD8WhqtJG1zVnpmYIDkV68DLKyhU C5UZGLfqiXycvuQ3SAKywhuBmCw9/rdnmS47mjYM= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2405869AbgFXR55 (ORCPT ); Wed, 24 Jun 2020 13:57:57 -0400 Received: from mail.kernel.org ([198.145.29.99]:58472 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2405743AbgFXR55 (ORCPT ); Wed, 24 Jun 2020 13:57:57 -0400 Received: from quaco.ghostprotocols.net (unknown [179.97.37.151]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 480082078E; Wed, 24 Jun 2020 17:57:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1593021476; bh=QhcLJu5XeabSx4iawRJSemJSRZyIebuJjQB6tYl8B5c=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=vn4DGikepn6Cwcr5GONOveuq+gSy3somBP4NvHgYUR6vL9BltCdSfAAEBie5zEmll Zqugc9efXjjclkICYIfHA5HODRmEA8UxK+DwhRUlyMR4iM6YCYqBD+PNvmXjQkblwt 52zkA+I+fFH5FvD8aHvhOcD757MEZtLcDswxAo8M= Received: by quaco.ghostprotocols.net (Postfix, from userid 1000) id 5E4AD405FF; Wed, 24 Jun 2020 14:57:54 -0300 (-03) Date: Wed, 24 Jun 2020 14:57:54 -0300 From: Arnaldo Carvalho de Melo To: Andrii Nakryiko Cc: Lorenz Bauer , bpf , dwarves@vger.kernel.org, kernel-team Subject: Re: pahole generates invalid BTF for code compiled with recent clang Message-ID: <20200624175754.GD20203@kernel.org> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Url: http://acmel.wordpress.com Sender: bpf-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org Em Wed, Jun 24, 2020 at 10:41:10AM -0700, Andrii Nakryiko escreveu: > On Wed, Jun 24, 2020 at 4:07 AM Lorenz Bauer wrote: > > > > Hi, > > > > If pahole -J is used on an ELF that has BTF info from clang, it > > produces an invalid > > output. This is because pahole rewrites the .BTF section (which > > includes a new string > > table) but it doesn't touch .BTF.ext at all. > > Why do you run `pahole -J` on BPF .o file? Clang already generates > .BTF (and .BTF.ext, of course) for you. > > pahole -J is supposed to be used for vmlinux, not for clang-compiled > -target BPF object files. yeah, I was thinking this was for a vmlinux generated by clang, which, from the commands below (the suffix _prog.o) should have told me this is a target BPF object file. But then, if one insists for some reason in generating BTF from the DWARF in a BPF target object file, stripping .BTF.ext, if present, is the right thing to do at this point. - Arnaldo > > > > To demonstrate, on a recent check out of bpf-next: > > $ cp connect4_prog.o connect4_pahole.o > > $ pahole -J connect4_pahole.o > > $ llvm-objcopy-10 --dump-section .BTF=pahole-btf.bin > > --dump-section .BTF.ext=pahole-btf-ext.bin connect4_pahole.o > > $ llvm-objcopy-10 --dump-section .BTF=btf.bin --dump-section > > .BTF.ext=btf-ext.bin connect4_prog.o > > $ sha1sum *.bin > > 1b5c7407dd9fd13f969931d32f6b864849e66a68 btf.bin > > 4c43efcc86d3cd908ddc77c15fc4a35af38d842b btf-ext.bin > > 2a60767a3a037de66a8d963110601769fa0f198e pahole-btf.bin > > 4c43efcc86d3cd908ddc77c15fc4a35af38d842b pahole-btf-ext.bin > > > > This problem crops up when compiling old kernels like 4.19 which have > > an extra pahole > > build step with clang-10. > > I was under impression that clang generates .BTF and .BTF.ext only for > -target BPF. In this case, kernel is compiled for "real" target arch, > so there shouldn't be .BTF.ext in the first place? If that's not the > case, then I guess it's a bug in Clang. > > > > > I think a possible fix is to strip .BTF.ext if .BTF is rewritten. > > > > Best > > Lorenz > > > > -- > > Lorenz Bauer | Systems Engineer > > 6th Floor, County Hall/The Riverside Building, SE1 7PB, UK > > > > www.cloudflare.com -- - Arnaldo From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnaldo Carvalho de Melo Subject: Re: pahole generates invalid BTF for code compiled with recent clang Date: Wed, 24 Jun 2020 14:57:54 -0300 Message-ID: <20200624175754.GD20203@kernel.org> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: dwarves-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Andrii Nakryiko Cc: Lorenz Bauer , bpf , dwarves-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, kernel-team List-Id: dwarves@vger.kernel.org Em Wed, Jun 24, 2020 at 10:41:10AM -0700, Andrii Nakryiko escreveu: > On Wed, Jun 24, 2020 at 4:07 AM Lorenz Bauer wrote: > > > > Hi, > > > > If pahole -J is used on an ELF that has BTF info from clang, it > > produces an invalid > > output. This is because pahole rewrites the .BTF section (which > > includes a new string > > table) but it doesn't touch .BTF.ext at all. > > Why do you run `pahole -J` on BPF .o file? Clang already generates > .BTF (and .BTF.ext, of course) for you. > > pahole -J is supposed to be used for vmlinux, not for clang-compiled > -target BPF object files. yeah, I was thinking this was for a vmlinux generated by clang, which, from the commands below (the suffix _prog.o) should have told me this is a target BPF object file. But then, if one insists for some reason in generating BTF from the DWARF in a BPF target object file, stripping .BTF.ext, if present, is the right thing to do at this point. - Arnaldo > > > > To demonstrate, on a recent check out of bpf-next: > > $ cp connect4_prog.o connect4_pahole.o > > $ pahole -J connect4_pahole.o > > $ llvm-objcopy-10 --dump-section .BTF=pahole-btf.bin > > --dump-section .BTF.ext=pahole-btf-ext.bin connect4_pahole.o > > $ llvm-objcopy-10 --dump-section .BTF=btf.bin --dump-section > > .BTF.ext=btf-ext.bin connect4_prog.o > > $ sha1sum *.bin > > 1b5c7407dd9fd13f969931d32f6b864849e66a68 btf.bin > > 4c43efcc86d3cd908ddc77c15fc4a35af38d842b btf-ext.bin > > 2a60767a3a037de66a8d963110601769fa0f198e pahole-btf.bin > > 4c43efcc86d3cd908ddc77c15fc4a35af38d842b pahole-btf-ext.bin > > > > This problem crops up when compiling old kernels like 4.19 which have > > an extra pahole > > build step with clang-10. > > I was under impression that clang generates .BTF and .BTF.ext only for > -target BPF. In this case, kernel is compiled for "real" target arch, > so there shouldn't be .BTF.ext in the first place? If that's not the > case, then I guess it's a bug in Clang. > > > > > I think a possible fix is to strip .BTF.ext if .BTF is rewritten. > > > > Best > > Lorenz > > > > -- > > Lorenz Bauer | Systems Engineer > > 6th Floor, County Hall/The Riverside Building, SE1 7PB, UK > > > > www.cloudflare.com -- - Arnaldo