From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from simark.ca by simark.ca with LMTP id mFG1OKC9sl/uZgAAWB0awg (envelope-from ) for ; Mon, 16 Nov 2020 12:57:52 -0500 Received: by simark.ca (Postfix, from userid 112) id E445D1F08B; Mon, 16 Nov 2020 12:57:52 -0500 (EST) X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on simark.ca X-Spam-Level: X-Spam-Status: No, score=0.2 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,MAILING_LIST_MULTI,RDNS_NONE,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.2 Received: from sourceware.org (unknown [8.43.85.97]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by simark.ca (Postfix) with ESMTPS id 8A5201E552 for ; Mon, 16 Nov 2020 12:57:52 -0500 (EST) Received: from server2.sourceware.org (localhost [IPv6:::1]) by sourceware.org (Postfix) with ESMTP id F34973857806; Mon, 16 Nov 2020 17:57:51 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org F34973857806 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sourceware.org; s=default; t=1605549472; bh=KS07YASg8HWokkAfTfcx8N0c9VfmsQmAN83UvPEUPWY=; h=To:Subject:References:Date:In-Reply-To:List-Id:List-Unsubscribe: List-Archive:List-Post:List-Help:List-Subscribe:From:Reply-To:Cc: From; b=IdhKxOkP8G2/3vB4ZlG04m1npOPLuMaqKXJu4JVPWBTdIO1XN1KxxJFewcDW5Y8gB wuaJ+0TSApyzWv25yufPUqaa734Ld4nyS1Nw7oQC1Uak/JjJd1RJUUEDOOzrkj6Phv xm12IC7RY/ft7dMcQVxGfE7BRD2T5BZKCnmWN2t8= Received: from mail.sergiodj.net (mail.sergiodj.net [167.114.15.217]) by sourceware.org (Postfix) with ESMTPS id 825663857806 for ; Mon, 16 Nov 2020 17:57:48 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 825663857806 Received: from localhost (bras-base-toroon1016w-grc-37-76-65-22-220.dsl.bell.ca [76.65.22.220]) by mail.sergiodj.net (Postfix) with ESMTPSA id D3C75A04CB; Mon, 16 Nov 2020 12:57:47 -0500 (EST) To: Simon Marchi Subject: Re: [PATCH] Search for DWZ files in debug-file-directories as well References: <20201114234842.2334396-1-sergiodj@sergiodj.net> <47d6fb03-c296-df0e-6613-0cd08b3c7b0f@simark.ca> X-URL: http://blog.sergiodj.net Date: Mon, 16 Nov 2020 12:57:47 -0500 In-Reply-To: <47d6fb03-c296-df0e-6613-0cd08b3c7b0f@simark.ca> (Simon Marchi's message of "Sun, 15 Nov 2020 20:25:03 -0500") Message-ID: <87k0ul2nhg.fsf@paluero> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-BeenThere: gdb-patches@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gdb-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: Sergio Durigan Junior via Gdb-patches Reply-To: Sergio Durigan Junior Cc: Mark Wielaard , gdb-patches@sourceware.org Errors-To: gdb-patches-bounces@sourceware.org Sender: "Gdb-patches" On Sunday, November 15 2020, Simon Marchi wrote: > On 2020-11-14 6:48 p.m., Sergio Durigan Junior via Gdb-patches wrote: >> When Debian (and Ubuntu) builds its binaries, it (still) doesn't use >> dwz's "--relative" option. This causes their debuginfo files to >> carry a .gnu_debugaltlink section containing a full pathname to the >> DWZ alt debug file, like this: >> >> $ readelf -wk /usr/bin/cat >> Contents of the .gnu_debugaltlink section: >> >> Separate debug info file: /usr/lib/debug/.dwz/x86_64-linux-gnu/coreutils.debug >> Build-ID (0x14 bytes): >> ee 76 5d 71 97 37 ce 46 99 44 32 bb e8 a9 1a ef 99 96 88 db >> >> Contents of the .gnu_debuglink section: >> >> Separate debug info file: 06d3bee37b8c7e67b31cb2689cb351102ae73b.debug >> CRC value: 0x53267655 >> >> This usually works OK, because most of the debuginfo files installed >> via apt will be present in /usr/lib/debug anyway. However, imagine >> the following scenario: >> >> - You are using /usr/bin/cat, it crashes on you and generates a >> corefile. >> >> - You don't want/need to "apt install" the debuginfo file for >> coreutils from the repositories. Instead, you already have the >> debuginfo files in a separate directory (e.g., $HOME/dbgsym). >> >> - You start GDB and "set debug-file-directory $HOME/dbgsym". >> You then get the following message: >> >> $ gdb -ex 'set debug-file-directory ./dbgsym/usr/lib/debug' -ex 'file /bin/cat' -ex 'core-file ./cat.core' >> GNU gdb (Ubuntu 10.1-0ubuntu1) 10.1 >> ... >> Reading symbols from /bin/cat... >> Reading symbols from /home/sergio/gdb/dbgsym/usr/lib/debug/.build-id/bc/06d3bee37b8c7e67b31cb2689cb351102ae73b.debug... >> could not find '.gnu_debugaltlink' file for /home/sergio/gdb/dbgsym/usr/lib/debug/.build-id/bc/06d3bee37b8c7e67b31cb2689cb351102ae73b.debug >> >> This error happens because GDB is trying to locate the build-id >> link (inside /home/sergio/gdb/dbgsym/usr/lib/debug/.build-id) for the >> DWZ alt debug file, which doesn't exist. Arguably, this is a problem >> with how dh_dwz works in Debian, and it's something I'm also planning >> to tackle. But, back at the problem at hand. >> >> Besides not being able to find the build-id link in the directory >> mentioned above, GDB also tried to open the DWZ alt file using its >> filename. The problem here is that, since we don't have the distro's >> debuginfo installed, it can't find anything under /usr/lib/debug that >> satisfies it. >> >> It occurred to me that a good way to workaround this problem is to >> actually try to locate the DWZ alt debug file inside the >> debug-file-directories (that were likely provided by the user). So >> this is what the proposed patch does. >> >> The idea here is simple: get the filename extracted from the >> .gnu_debugaltlink section, and manipulate it in order to replace the >> initial part of the path (everything before "/.dwz/") by whatever >> debug-file-directories the user might have provided. >> >> I talked with Mark Wielaard and he agrees this is a sensible approach. >> In fact, apparently this is something that eu-readelf also does. >> >> I regtested this code, and no regressions were found. > > Hi Sergio, Bonjour, Simon, Thanks for the review. > I don't really have an opinion on this at the moment because I don't > know much about how dwz files are used in practice. Is having a ".dwz" > somewhat standard? Yeah, both Fedora and Debian/Ubuntu generate and use DWZ files extensively. Fedora has the 'find-debuginfo.sh' script which is called during rpmbuild and takes care of the debuginfo/DWZ generation: https://github.com/rpm-software-management/rpm/blob/HEAD/scripts/find-debuginfo.sh#L510 Debian/Ubuntu have the dh_dwz step which is invoked via debhelper during the package build: https://salsa.debian.org/debian/debhelper/-/blob/master/dh_dwz >> @@ -2238,6 +2236,61 @@ dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd) >> if (dwz_bfd == NULL) >> dwz_bfd = build_id_to_debug_bfd (buildid_len, buildid); >> >> + if (dwz_bfd == nullptr) >> + { >> + /* If the user has provided us with different >> + debug-file-directories, we can try them in order. */ >> + size_t dwz_pos = filename.find ("/.dwz/"); >> + >> + if (dwz_pos != std::string::npos) >> + { >> + std::string tmpfilename = filename.erase (0, dwz_pos); > > "filename.erase", I think that modifies filename in place. If so, is > this what you intended? Oh, you're right, both the .erase and the .insert methods modify their arguments in-place. This is not what I intended, but it's an easy fix. Thanks for pointing it out. >> + std::vector> debugdir_vec >> + = dirnames_to_char_ptr_vec (debug_file_directory); >> + >> + for (const gdb::unique_xmalloc_ptr &debugdir : debugdir_vec) >> + { >> + /* The idea is to iterate over the >> + debug-file-directories provided by the user and >> + replace the hard-coded path in the "filename" by each >> + debug-file-directory. >> + >> + For example, suppose that filename is: >> + >> + /usr/lib/debug/.dwz/foo.debug >> + >> + And suppose that we have "$HOME/bar" as the >> + debug-file-directory. We would then adjust filename >> + to look like: >> + >> + $HOME/bar/.dwz/foo.debug >> + >> + which would hopefully allow us to find the alt debug >> + file. */ >> + std::string ddir = debugdir.get (); >> + >> + if (filename.size () > ddir.size () >> + && filename.compare (0, ddir.size (), ddir) == 0) >> + continue; > > What's the intent of this condition? Can you add a comment to make it > explicit? It's just a check to verify whether "filename" already refers to a file under the "ddir" debug directory. If it does, then there's nothing to do here (because the function already tried to open the file before). I'll add a comment on top of this check. >> + >> + std::string new_filename = tmpfilename.insert (0, ddir); > > Same question as above for "tmpfilename.insert". Same answer as above :-). Thanks, -- Sergio GPG key ID: 237A 54B1 0287 28BF 00EF 31F4 D0EB 7628 65FC 5E36 Please send encrypted e-mail if possible https://sergiodj.net/