From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from simark.ca (simark.ca [158.69.221.121]) by sourceware.org (Postfix) with ESMTPS id 6AF803858D28; Fri, 17 Dec 2021 15:51:35 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 6AF803858D28 Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=simark.ca Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=simark.ca Received: from [172.16.0.95] (192-222-180-24.qc.cable.ebox.net [192.222.180.24]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by simark.ca (Postfix) with ESMTPSA id A91311EDEE; Fri, 17 Dec 2021 10:51:34 -0500 (EST) Subject: Re: rseq and gdb To: Florian Weimer , Mathieu Desnoyers Cc: gdb@sourceware.org, Simon Marchi , libc-alpha References: <1507387605.36435.1639688959308.JavaMail.zimbra@efficios.com> <87wnk4fe3t.fsf@oldenburg.str.redhat.com> From: Simon Marchi Message-ID: Date: Fri, 17 Dec 2021 10:51:34 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 MIME-Version: 1.0 In-Reply-To: <87wnk4fe3t.fsf@oldenburg.str.redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: tl Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-4.3 required=5.0 tests=BAYES_00, KAM_DMARC_STATUS, NICE_REPLY_A, SPF_HELO_PASS, SPF_PASS, TXREP, URIBL_BLACK autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: gdb@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gdb mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 17 Dec 2021 15:51:36 -0000 On 2021-12-16 4:12 p.m., Florian Weimer via Gdb wrote: > * Mathieu Desnoyers: > >> I suspect that gdb should ideally do something to allow it to >> single-step through rseq critical sections. In librseq [1], we emit >> the __rseq_cs_ptr_array and __rseq_exit_point_array sections to allow >> gdb to know about rseq critical sections and skip over those critical >> sections as needed. Otherwise single-stepping over each instruction of >> a rseq critical section will loop forever. > > That's probably something that should be expressed in the DWARF data. I thought about this a little bit, and I don't think it belongs in DWARF: - If it's in DWARF, it means that by default (without installing the corresponding -dbg/-debug package), the information is not available. The user might have debug info for their program / libraries, but not for libc.so, so GDB won't know about the critical sections in libc.so. GDB should ideally know about critical sections even if there's not debug info. Let's say that they do "continue" while having a software watchpoint, GDB will be in "single step all instructions" mode. If execution happens to cross a critical rseq section, execution will hang. So it seems better to have this information in the main binary (not the separate debug info). It should be very small, just a few addresses, so I don't think size will be an issue. - Currently, the ELF sections are emitted by macros that add some assembly directives and labels, that's pretty straightforward. If it's in DWARF, I suppose it needs to go through the compiler. The rseq library needs to communicate the information to the compiler, which will then put it somewhere in a DWARF section. Nothing impossible, I suppose, is there a precedent for this way of doing things? - A philosophical argument more than practical: the goal of DWARF is to express the mapping between the source language and the machine code, rseq critical sections are more low level platform details. A slightly corner use case I am wondering about is GDB attaching to a process that uses an rseq-using library (libc or another) whose .so doesn't exist on disk anymore (perhaps because the package has been upgraded in the mean time). With the rseq sections information in the binary (regardless if it is in a simple ELF section or in DWARF), GDB wouldn't be able to load it then. The current simple ELF section is marked as allocated (at least, that's what Mathieu told me :)), which means the information is somewhere in memory, but to find it GDB would need to get its symbol from an address, which is not possible without the binary. The only solution I could see for this is if the dynamic linker structures gave us that information (either the address of the allocated section or the tables themselves). >> Now that glibc plans to enable rseq by default starting with glibc 2.35, >> it appears to be a good timing to raise this topic with the gdb community. > > It's less of an issue than the CRIU problem that we discussed because it > will only affect attempts to debug actually rseq-using applications. > (The CRIU problem affects everything.) Not saying that debugging > support isn't important, just trying to put it into perspective. 8-) Indeed. Although now is time to think about it, so we don't make choices we regret later. Simon