From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 6563 invoked by alias); 8 Dec 2011 07:45:46 -0000 Received: (qmail 6551 invoked by uid 22791); 8 Dec 2011 07:45:43 -0000 X-SWARE-Spam-Status: No, hits=-1.8 required=5.0 tests=AWL,BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,RCVD_IN_DNSWL_NONE X-Spam-Check-By: sourceware.org Received: from caibbdcaaaaf.dreamhost.com (HELO homiemail-a46.g.dreamhost.com) (208.113.200.5) by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Thu, 08 Dec 2011 07:45:21 +0000 Received: from homiemail-a46.g.dreamhost.com (localhost [127.0.0.1]) by homiemail-a46.g.dreamhost.com (Postfix) with ESMTP id 91F113E405B; Wed, 7 Dec 2011 23:45:20 -0800 (PST) Received: from redwood.eagercon.com (c-76-102-3-160.hsd1.ca.comcast.net [76.102.3.160]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) (Authenticated sender: eager@eagerm.com) by homiemail-a46.g.dreamhost.com (Postfix) with ESMTPSA id 2A8693E4056; Wed, 7 Dec 2011 23:45:20 -0800 (PST) Message-ID: <4EE06B0F.3050202@eagerm.com> Date: Thu, 08 Dec 2011 07:45:00 -0000 From: Michael Eager User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:8.0) Gecko/20111115 Thunderbird/8.0 MIME-Version: 1.0 To: Alan Modra CC: gdb@sourceware.org, "Ryan S.Arnold" Subject: Re: Next over function with Secure PLT References: <4EE0088C.4070208@eagerm.com> <20111208040118.GB10960@bubble.grove.modra.org> In-Reply-To: <20111208040118.GB10960@bubble.grove.modra.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-IsSubscribed: yes Mailing-List: contact gdb-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-owner@sourceware.org X-SW-Source: 2011-12/txt/msg00009.txt.bz2 On 12/07/2011 08:01 PM, Alan Modra wrote: > On Wed, Dec 07, 2011 at 04:58:22PM -0800, Michael Eager wrote: >> When using PowerPC Secure PLT, trying to "next" over a >> library function in a shared library does not work correctly. >> Instead of skipping over the function, gdb steps through >> the PLT entry which shows up as code in >> call___do_global_ctors_aux. > > As an aside, if you use a newer linker symbols will be emitted on > each plt stub by default for -shared. Or I might have to backport the binutils patch. Another work- around is to force --emit-stub-sims. >> This doesn't happen when "nexting" over a library function >> in the executable. Next works the same as with a local function. >> >> When reading the executable, ppc_elf_get_synthetic_symtab() >> calls is_nonpic_glink_stub() to recognizes the PLT stub and >> then generates internal symbols for each PLT stub like foo@plt. >> It also creates an entry for __glink and __glink_PLTresolve. >> >> If I modify is_nonpic_glink_stub() to recognize the shared >> library PLT stub format, similar internal symbols are created >> and gdb seems to work correctly. > > Don't do that. You can't easily know which plt entry is loaded by a > PIC stub. > >> There's a comment before the call: >> /* If the stubs are those for -shared/-pie then we might have >> multiple stubs for each plt entry. If that is the case then >> there is no way to associate stubs with their plt entries short >> of figuring out the GOT pointer value used in the stub. */ >> if (!is_nonpic_glink_stub (abfd, glink, >> glink_vma - GLINK_ENTRY_SIZE - glink->vma)) >> >> What is this trying to tell me? What are the circumstances where >> there would be multiple stubs for each PLT entry? If there are >> multiple stubs, then this might create multiple foo@plt symbols >> with different values. Would this cause any problems? > > There can be multiple stubs using the same PLT entry when linking > -fPIC code. -fPIC effectively gives you a 64k GOT per file, with > the result that r30 may differ from one function to another in the > executable. Since r30 is used in a PIC stub to calculate the PLT > entry address, you need a different stub for different values of r30. OK. That was the detail I was missing. > gdb ought to just pattern match the stub code to recognize when the pc > is in a stub. I wanted to avoid pattern matching code because of the dependency it creates between code generation and debugging. But that shouldn't be much of an issue or hard to implement. -- Michael Eager eager@eagercon.com 1960 Park Blvd., Palo Alto, CA 94306 650-325-8077