From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 12838 invoked by alias); 27 Nov 2017 19:13:20 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Received: (qmail 12827 invoked by uid 89); 27 Nov 2017 19:13:19 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.7 required=5.0 tests=BAYES_00,KB_WAM_FROM_NAME_SINGLEWORD,SPF_HELO_PASS,T_RP_MATCHES_RCVD autolearn=no version=3.3.2 spammy=corrupted, az, sk:minimal X-HELO: mx1.redhat.com Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Mon, 27 Nov 2017 19:13:18 +0000 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id CA25DC0587D6; Mon, 27 Nov 2017 19:13:16 +0000 (UTC) Received: from [127.0.0.1] (ovpn04.gateway.prod.ext.ams2.redhat.com [10.39.146.4]) by smtp.corp.redhat.com (Postfix) with ESMTP id 62CD35D965; Mon, 27 Nov 2017 19:13:12 +0000 (UTC) Subject: Re: [PATCH v2] Add support for the --readnever command-line option (DWARF only) To: Sergio Durigan Junior References: <1467838463-15786-1-git-send-email-brobecker@adacore.com> <87o9ntddb6.fsf_-_@redhat.com> <2cb6d01b-b40b-0a73-2df4-65f4e2094731@redhat.com> <87efoodi79.fsf@redhat.com> <87a7zc9syx.fsf@redhat.com> <874lpj8lta.fsf@redhat.com> Cc: Yao Qi , Joel Brobecker , "gdb-patches@sourceware.org" From: Pedro Alves Message-ID: <8bd72067-dbb3-4c7f-1934-87df87c6f358@redhat.com> Date: Mon, 27 Nov 2017 19:13:00 -0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <874lpj8lta.fsf@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-SW-Source: 2017-11/txt/msg00696.txt.bz2 On 11/24/2017 08:26 PM, Sergio Durigan Junior wrote: > On Friday, November 24 2017, Pedro Alves wrote: > >> On 11/24/2017 04:54 AM, Sergio Durigan Junior wrote: >>> On Thursday, November 23 2017, Pedro Alves wrote: >>> >>>> On 11/23/2017 05:21 PM, Sergio Durigan Junior wrote: >>>> >>>>>>> +This option is currently limited to debug information in DWARF format. >>>>>>> +For all other format, this option has no effect. >>>>>> >>>>>> How hard would it be to just make it work? There's only stabs and mdebug >>>>>> left, I think? There should be a single a function somewhere that we can >>>>>> add an early return. And then we don't need to document this limitation... >>>>>> >>>>>> For example, in elf_symfile_read, we could just skip the elf_locate_sections >>>>>> call. In coffread.c we could skip reading stabs right after >>>>>> bfd_map_over_sections (abfd, coff_locate_sections....); >>>>>> >>>>>> Looking for: >>>>>> >>>>>> $ grep -h "^[a-z]*_build_psymtabs" gdb/ >>>>>> coffstab_build_psymtabs (struct objfile *objfile, >>>>>> elfstab_build_psymtabs (struct objfile *objfile, asection *stabsect, >>>>>> stabsect_build_psymtabs (struct objfile *objfile, char *stab_name, >>>>>> mdebug_build_psymtabs (minimal_symbol_reader &reader, >>>>>> elfmdebug_build_psymtabs (struct objfile *objfile, >>>>>> >>>>>> finds all the relevant places. >>>>>> >>>>>> Maybe it wouldn't be that hard to make this be an objfile flag >>>>>> afterall (like OBJF_READNOW is). That'd make it possible >>>>>> to add the location "-readnever" counterpart switch to add-symbol-file >>>>>> too, BTW: >>>> >>>> I meant "logical" instead of "location". I was staring at >>>> gdb/location.c at that time. :-P >>>> >>>>>> >>>>>> symfile.c: if (strcmp (arg, "-readnow") == 0) >>>>>> symfile.c: else if (strcmp (arg, "-readnow") == 0) >>>>> >>>>> Hm, I'll look into this. Just to make it clear: the idea is to have >>>>> both a --readnever global option and also a OBJF_READNEVER specific to >>>>> each objfile? >>>> >>>> Sure, the idea is to do something similar to what's done for --readnow. >>> >>> Sorry, but I guess I need a few more details on this. >>> >>> The way I understand the code at elf_symfile_read, the very first thing >>> to do would be to check if OBJF_READNEVER is set and return early if it >>> is. But it seems that you're proposing something a bit different when >>> you say that we should "... just skip the elf_locate_sections call." It >>> doesn't seem to me that is worth continuing on that function if >>> OBJF_READNEVER is present. >> >> No, you can't return early the very first thing, because >> --readnever is supposed to skip _debug_ info, not ELF/minimal symbols... >> So the "return early" would have to be _after_ the >> elf_read_minimal_symbols call: > > Hm, OK, it makes sense and I confess I thought "why is Pedro mentioning > elf_symfile_read if the feature is about skipping DWARF, not ELF?". > >> static void >> elf_symfile_read (struct objfile *objfile, symfile_add_flags symfile_flags) >> { >> bfd *abfd = objfile->obfd; >> struct elfinfo ei; >> >> memset ((char *) &ei, 0, sizeof (ei)); >> bfd_map_over_sections (abfd, elf_locate_sections, (void *) & ei); >> >> elf_read_minimal_symbols (objfile, symfile_flags, &ei); >> >> I don't know whether we can reorder that. Maybe we can. >> >> When I looked at this quickly yesterday, I saw that elf_location_sections >> is what finds the stabs and mdebug sections: >> >> static void >> elf_locate_sections (bfd *ignore_abfd, asection *sectp, void *eip) >> { >> struct elfinfo *ei; >> >> ei = (struct elfinfo *) eip; >> if (strcmp (sectp->name, ".stab") == 0) >> { >> ei->stabsect = sectp; >> } >> else if (strcmp (sectp->name, ".mdebug") == 0) >> { >> ei->mdebugsect = sectp; >> } >> } >> >> and it seemed to be that skipping the section location would make >> the parts of elf_symfile_read that actually read the symbols >> be no-ops, because the stabsect/mdebusect pointers would be NULL. >> >> But if returning early or something else works, that's fine. > > OK, thanks for clarifying. > >>> As for the *_build_psymtabs functions, I am doing exactly that: if >>> objfile->flags contains OBJF_READNEVER, then just return and do nothing. >> >> Sure, that should work too. It's just the difference between >> skipping checking whether debug info is available (skipping before >> calling into those), vs letting gdb do the work to figure out whether >> debug info is available, but then ignore it. >> The grep for "*_build_psymtabs" was intended as a pointer to find >> what the relevant code is, including to look at the code that >> is calling those functions, see if there's something to be done there. > > I think it makes more sense, logically speaking, to not mess with > elf_symfile_read and instead modify the *_build_psymtabs functions. Yet, somehow that logic didn't carry to the DWARF reader changes? ;-) dwarf2_has_info is the place that checks whether the objfile has DWARF sections, see dwarf2_per_objfile::dwarf2_per_objfile. So for DWARF, the patch is checking for readnever _before_ dwarf2_build_psymtabs is reached, by essentially skipping the "locate_sections" call. A small advantage of checking before is that you can skip a little bit more work. See the comment in elf_read_minimal_symbols about skipping work unless stabs, or this bit here: if (info->stabsects) { if (!info->stabstrsect) { error (_("The debugging information in `%s' is corrupted.\nThe " "file has a `.stabs' section, but no `.stabstr' section."), name); } ... It's really not a bit deal, but to me it'd be more consistent have all readers do the same logically. On another note: there's a symbol_file_add_separate call at the tail end of elf_symfile_read, where we seem to read separate info. It seems like that'd end up reading debug info, with "add-symbol-file -readnever"? Same in read_symbols, I guess. Or is OBJF_READNEVER etc. somehow propagated to the separate objfile? Thanks, Pedro Alves