From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lj1-x242.google.com (mail-lj1-x242.google.com [IPv6:2a00:1450:4864:20::242]) by sourceware.org (Postfix) with ESMTPS id 2CE58385702E for ; Wed, 23 Sep 2020 20:18:03 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 2CE58385702E Received: by mail-lj1-x242.google.com with SMTP id k25so770544ljk.0 for ; Wed, 23 Sep 2020 13:18:03 -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; bh=Gfwz7Pb68Yp5i2R64buqzIt055mbWYPHNdy/OYHu3pY=; b=bfV3Jyyja+a6IcYmuqghX6QnCrutL7y+U2cCaFzi5zuY23Oax4YamBpFczQHRXajCY WGOWpXRJJdDS0Rq5tsekQ2RNW80NFh/a2cIPkMif7JPK5mOFsUSpkqNEXq4m6TG2hfuk ndsVZadr+paIq1nFaLVOVsDYBs+MZbdSp700FmylJUr0qc/kvypB7kRgV8jTGocJEVsx rgICIymVtLDUhPUfaMbFpunir9cSSvh5YqYWbjB5EcehoCrs/+AAYCMB3wT55HMIkG5y 3cs3ZrUVrNDJ2AcnGUxGpr659k7mdB4DcXz8fREfbLqPWGd7MOdLhAgHJjtGwRWZlxD6 Z5rg== X-Gm-Message-State: AOAM533nN+0nicAffUmh1dn+ZtqaZBrmj/lQrju/TyKPhvZKkt9SfPql 5iLeXKvuf/4lWsJoLZxmYYxo61j5yBEcDaZMk+s= X-Google-Smtp-Source: ABdhPJyBRNWpFm2APCh0Q76tHZ1qzNQhnpeiBd5tTbTzhPAzGsD3Xmv4dPeiKczSQ4jEiODw9DLjoP317MLlDvED8rY= X-Received: by 2002:a2e:8153:: with SMTP id t19mr512401ljg.334.1600892281958; Wed, 23 Sep 2020 13:18:01 -0700 (PDT) MIME-Version: 1.0 References: <20200923010930.xtc4mgmxsoesohkn@gmail.com> <20200923095818.npbwybrm63vb4ejm@jozef-acer-manjaro> <20200923165211.fr4rqzp5uqqmrufq@jozef-acer-manjaro> <20200923184735.4k2tji4yro452bep@jozef-acer-manjaro> <20200923200437.mnegrmwebjuzmfeu@jozef-acer-manjaro> In-Reply-To: <20200923200437.mnegrmwebjuzmfeu@jozef-acer-manjaro> From: "H.J. Lu" Date: Wed, 23 Sep 2020 13:17:25 -0700 Message-ID: Subject: Re: [PATCH] Support SHF_GNU_RETAIN ELF section flag To: Michael Matz , Binutils Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-2.2 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: binutils@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Binutils mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 23 Sep 2020 20:18:04 -0000 On Wed, Sep 23, 2020 at 1:04 PM Jozef Lawrynowicz wrote: > > On Wed, Sep 23, 2020 at 12:03:28PM -0700, H.J. Lu via Binutils wrote: > > On Wed, Sep 23, 2020 at 11:47 AM Jozef Lawrynowicz > > wrote: > > > > > > On Wed, Sep 23, 2020 at 10:13:37AM -0700, H.J. Lu via Binutils wrote: > > > > On Wed, Sep 23, 2020 at 9:52 AM Jozef Lawrynowicz > > > > wrote: > > > > > > > > > > On Wed, Sep 23, 2020 at 01:51:56PM +0000, Michael Matz wrote: > > > > > > Hello, > > > > > > > > > > > > On Wed, 23 Sep 2020, H.J. Lu via Binutils wrote: > > > > > > > > > > > > > > I think that: > > > > > > > > > > > > > > > > > .section .text,"ax" > > > > > > > > > ... > > > > > > > > > foo: > > > > > > > > > ... > > > > > > > > > .retain > > > > > > > > > retained_fn: > > > > > > > > > ... > > > > > > > > > > > > > > > > is some nice syntactic sugar compared to: > > > > > > > > > > > > > > > > > .section .text,"ax" > > > > > > > > > ... > > > > > > > > > foo: > > > > > > > > > ... > > > > > > > > > .section .text,"axR" > > > > > > > > > retained_fn: > > > > > > > > > ... > > > > > > > > > > > > > > > > It's also partly for convenience; we have other directives which are > > > > > > > > synonyms or short-hand for each other. > > > > > > > > > > > > > > > > > > > > > > You don't need to keep the whole section when only one symbol should > > > > > > > be kept. Please drop the .retain directive. GCC, as and ld should do the > > > > > > > right thing with > > > > > > > > > > > > > > .section .text,"ax" > > > > > > > ... > > > > > > > foo: > > > > > > > ... > > > > > > > .section .text,"axR" > > > > > > > > > > > > > > retained_fn: > > > > > > > > > > > > > > where foo can be dropped and retained_fn will be kept. > > > > > > > > > > > > This is not what we discussed at the ABI list, the flag is per section, so > > > > > > either the whole section is retained or not. What you describe is > > > > > > something else that would work on a per symbol basis, which would have to > > > > > > be specified in a different way and might or might not be a good idea. > > > > > > But let's not conflate these two. > > > > > > > > > > Also, the linker cannot currently dissect a section and remove a > > > > > particular unused symbol anyway. Since garbage collection only operates > > > > > on the section level, marking the section itself as "retained" seems > > > > > most appropriate. > > > > > > > > It can be done. If you put your branch on > > > > > > > > https://gitlab.com/x86-binutils/binutils-gdb > > > > > > > > I can help you implement it. > > > > > > It's not something I have time to look into at the moment, for now the > > > aim is just to prevent garbage collection of sections. > > > > Linker and assembler already support it. You just need to add SHF_GNU_RETAIN > > to the framework. Check how SHF_GNU_MBIND works. > > Sorry, I don't understand. > > Are you saying that LD already supports the garbage collection of > individual unused symbol definitions from input sections? Whilst > retaining other symbol definitions which are required by the program? > I cannot find any reference to this. > > How does that relate to SHF_GNU_MBIND? I looked at all the references > to "mbind" in Binutils and nothing seemed related garbage collection of > sections, since SHF_GNU_MBIND is just used to indicate a particular > section should be placed in a special memory area. For section .text,"ax" ... foo: ... .section .text,"axR" retained_fn: you need to create a new .text section with SHF_GNU_RETAIN for retained_fn. See get_section in obj-elf.c. If you want to avoid merging .text section with SHF_GNU_RETAIN with other .text sections by ld -r, linker needs to distinguish sections of the same name with and without SHF_GNU_RETAIN. -- H.J.