From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-il1-x141.google.com (mail-il1-x141.google.com [IPv6:2607:f8b0:4864:20::141]) by sourceware.org (Postfix) with ESMTPS id 28A6C384607B; Thu, 1 Oct 2020 19:53:46 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 28A6C384607B Received: by mail-il1-x141.google.com with SMTP id u9so7205618ilj.7; Thu, 01 Oct 2020 12:53:46 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=+Q4klVUk84Ps6TdAhndncfSLD1CDzG0OvxxsqZh8IZ8=; b=t2nR/ombrG2n4SlnhDxzi6UunJ87stH4zMez1b/9FlWCW31QLUbwL5XLFKnghKV03a tf4qUbk8Ll+8v9+MAFn8CkfqkUfhbW/S6QWPgP0vNtCNZ/1B3TO0o6+fQ9SKCmjrr03P Pu2SdKfGtn8WzXhrqjI51PgfF0vyCN5TLqsMpp0IQC3uMgqdGqEecdIXYqlChUGr2i3Y kbqqL9i12E1sT6mIJvB8BEuPbpvz9kiC/FT7hiptVYZfKWTT+3iOKfMSoMtO6fr+H9lu K0gIbe54GwEFwGEaIu1pd1StD7yypSEuKZzFa5qHIIKz37pWpSnq1rKMN27TuYidYmyE GrwQ== X-Gm-Message-State: AOAM532Yc/Lh4wxg8/Cwd34TQOBm2SiJTUW9X9iTt1JagmBp+MSRSJQV 4KNQVPuxCZ/Dfd9iSgA6/YEy7n87xdPjVl3kh+ATah6x X-Google-Smtp-Source: ABdhPJzNXFf3fJijFM0HohsqzEBN2ue76sThXDxYds5xm79MGd440pkP7JTNoqIgxP43EPEMdSZm8JD3z7Lf7KsTg8U= X-Received: by 2002:a05:6e02:d06:: with SMTP id g6mr3742616ilj.151.1601582025607; Thu, 01 Oct 2020 12:53:45 -0700 (PDT) MIME-Version: 1.0 References: <20200928132613.btkqaoomv4fdnupn@jozef-acer-manjaro> <20200929044353.vx26ypc2oioqbmfb@gmail.com> <20200929100442.o3e66jbmqpv5cajh@jozef-acer-manjaro> <20200929193806.m6u6ra6oijqkstfo@gmail.com> <20200929213741.jqegh62d7jne5uyo@jozef-acer-manjaro> <20200930101831.vi7pxhstftemanfc@jozef-acer-manjaro> <20201001192211.samopetmxsdfwjnb@gmail.com> In-Reply-To: <20201001192211.samopetmxsdfwjnb@gmail.com> From: "H.J. Lu" Date: Thu, 1 Oct 2020 12:53:09 -0700 Message-ID: Subject: Re: [PATCH v2] Support for SHF_GNU_RETAIN ELF Section Flag To: Fangrui Song Cc: Roland McGrath , Binutils Development , GNU gABI gnu-gabi , Peter Smith , George Rimar , James Henderson , James Y Knight Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-3031.6 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: gnu-gabi@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gnu-gabi mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 01 Oct 2020 19:53:56 -0000 On Thu, Oct 1, 2020 at 12:22 PM Fangrui Song wrote: > > On 2020-09-30, Jozef Lawrynowicz wrote: > >On Tue, Sep 29, 2020 at 05:10:09PM -0700, Roland McGrath wrote: > >> Since group semantics mean that if any section in a group is retained the > >> whole group must be retained, it seems to me that making the feature a > >> group flag rather than a section flag makes sense. I don't think it really > >> has any downside. It's a little bit more structure to generate a group but > >> everything already knows how to do it so that should be no problem. While > >> it is true that multiple sections with the same name and different > >> attributes should already be fine, the place where multiple different > >> sections of the same name are usually seen today is when each different > >> section reusing a name is in a different group from the other sections with > >> the same name. > >> > > > >Let's assume: > > GRP_RETAIN > > Sections in a group with this flag set should always be included in > > the linked object, even if they appear unused. > > > >One issue I have with the group method is conceptual; groups are supposed > >to be used to group related sections. They are related because of their > >meaning from the application perspective, not because of their metadata > >(i.e. sections to be retained are related by their metadata because they > >all should be retained). > > > >Just because groups have a side effect that can be leveraged to > >get a section to be retained in the final link, why does that mean we > >need to leverage that property when it it's not actually the simplest or > >most obvious way to implement the new behavior? > > > >SHF_GNU_RETAIN describes in the simplest possible way that the section > >should be retained in the final link, without relying on other > >constructs. > > > >Just because groups are discarded or retained as a group, doesn't mean > >we need to leverage groups to try and implement retention or discarding > >functionality. > > > >Why wasn't SHF_EXCLUDE implemented as a group flag? After all, groups > >are included or excluded from the link together. > > GRP_EXCLUDE > > Sections in a group with GRP_EXCLUDE set should be discarded from > > the link. > > > >I mean, you could kind of use groups for anything when you decide > >grouping sections by metadata is OK. > >Why define SHT_NOBITS when you can create: > > GRP_NOBITS > > Sections in this group occupy no space in the file. They must have > > type SHT_PROGBITS. > > > >Retention of a section is a property of the section. We are misusing ELF > >constructs by using groups to indicate an arbitrary section needs to be > >retained. > > > >Another issue (if more is needed) is about how to name the groups. > >* If it is mandated that GRP_RETAIN groups have the same name e.g. > > "grp_retain", that means you can't put a section you want to > > retain in a different logical group that makes sense from the > > application perspective. So the other sections you would want to put > > in a group with the retained section need to all be put in a bundle > > with all the other GRP_RETAIN sections. > >* If GRP_RETAIN groups can have any name, so you can have multiple > > GRP_RETAIN groups, how does the compiler decide how to name the > > groups? It seems like it would be a mess. "grp_retain0", "grp_retain2" > > ... "grp_retain10" from one object file, "grp_retain0"... > > "grp_retain5" from another. Extra processing in the > > linker to clean up these group names and keep them unique would be > > required when performing a relocatable link. > > > >As a general point, what if I decide that there's enough pressure from > >the anti-SHF_GNU_RETAIN side that I change the implementation to use > >groups. But then those who already backed the flag prefer that method > >over using groups think the implementation should not have been changed. > > > >I feel like we already got enough backing for SHF_GNU_RETAIN between the > >GNU gABI and Binutils discussions. I understand, and welcome, more > >feedback, but I haven't been convinced any other method is obviously > >better than SHF_GNU_RETAIN, so why change it? > > > >I'm not saying it's possible to quantify which mechanism for "retain" is > >best, but SHF_GNU_RETAIN is the simplest, most obvious, and easiest to > >understand. Surely that gives it top marks? > > > >I'm also yet to hear one convincing reason why SHF_GNU_RETAIN is bad. > >As far as I can tell, the main argument against it stems from the fact > >that you can have two input sections with the same name but different > >SHF_GNU_RETAIN flag states. Not only is this a non-issue, groups have > >exactly the same problem! > > > >It would be great if a global maintainer to chime in on whether the > >attached patch is acceptable, because otherwise we are going to go back > >and forth forever. > > > >Thanks, > >Jozef > > Hi Jozef, > > I have checked with a few folks on the LLVM side. James Henderson and James Y > Knight seem to prefer a section flag to other mechanism. I prefer a > section flag, too (I was on the fence and wanted the alternatives to be considered). > > About the section flag 'R' in assembly: > > I'd still prefer an error to not add another exception to https://sourceware.org/pipermail/binutils/2020-February/109945.html > (a consensus GNU as and LLVM's integrated assembler have reached) > > .section retain,"a",@progbits > .section retain,"aR",@progbits # error for section flags change > > If a separate section is desired, I'd prefer an explicit ,unique,0 or ,unique,1 or ... > > .section retain,"a",@progbits > .section retain,"aR",@progbits,unique,0 # 'unique' is a binutils 2.35 feature (available in LLVM for a while) I don't think it is desirable since it requires tracking the unique number which isn't an issue with the compiler. But it is a hassle for handwritten assembly codes with #include. > > About the use case in GCC (and probably Clang in the future): > > // a.h > __attribute__((section("sec"))) inline void bar() { ... } > > // a.c > #include "a.h" > __attribute__((section("sec"), retain)) // retain may be the pre-existing used. > void foo() { ... } > > Perhaps there needs a function attribute to lower to ,unique,1 in assembly. -- H.J.