From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 12695 invoked by alias); 24 Aug 2017 14:42:25 -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 12596 invoked by uid 89); 24 Aug 2017 14:42:25 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-0.9 required=5.0 tests=BAYES_00,KAM_LAZY_DOMAIN_SECURITY,RP_MATCHES_RCVD,SPF_HELO_PASS autolearn=no version=3.3.2 spammy=Hx-languages-length:2088 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; Thu, 24 Aug 2017 14:42:23 +0000 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 3AE5A75714; Thu, 24 Aug 2017 14:42:22 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 3AE5A75714 Authentication-Results: ext-mx03.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx03.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=palves@redhat.com 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 58B2F7E23C; Thu, 24 Aug 2017 14:42:21 +0000 (UTC) Subject: Re: [PATCH v2] gdbserver: linux_low: elf_64_file_p cache results To: Jon Ringle References: <1503549910-24770-1-git-send-email-jon@ringle.org> <402cb282-9cd1-2725-f6de-ec5f9eb15e0d@redhat.com> Cc: gdb-patches@sourceware.org, Jon Ringle From: Pedro Alves Message-ID: Date: Thu, 24 Aug 2017 14:42:00 -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: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-SW-Source: 2017-08/txt/msg00470.txt.bz2 On 08/24/2017 03:14 PM, Jon Ringle wrote: > On Thu, Aug 24, 2017 at 5:26 AM, Pedro Alves wrote: >> Hi Jon, >> >> On 08/24/2017 05:45 AM, jon@ringle.org wrote: >> >>> The problem lied in the fact that the function elf_64_file_p() (call on >>> line 7184), was returning -1 because it could not open the file (with errno >>> set to EACCESS). This was because the inferior's code was dropping root >>> privileges and no longer was able to read the file. >> >> I feel like I'm missing something. If it's the inferior that >> is dropping privileges, how can that affect gdbserver? It's gdbserver >> that opens the file, not the inferior. >> >> It'd be nice to have a testcase for this. Would it be possible to come >> up with a small reproducer? >> > > I learned something while coming up with a small reproducer. The real > program I'm debugging is a systemd service and has a > CapabilityBoundingSet. The CapabilityBoundingSet turns out to be a > crucial point for reproducing. > > I setup a systemd service: > $ cat /lib/systemd/system/droproot-test.service > [Unit] > Description=gdbserver droproot test service > > [Service] > Type=simple > ExecStart=/home/jringle/build/gdbserver/gdbserver :5555 > /home/jringle/git/droproot-test/droproot-test jringle > Restart=on-success > CapabilityBoundingSet=CAP_SETGID CAP_SETUID OK, I don't know much about this, but it sounds like without that, your "droproot" test wouldn't be able to call setgid/setuid successfully to change to the "jringle" user. I'm still mystified about why can't gdbserver read the file after "droproot" has changed user. I assume gdbserver is running as root? Why wouldn't a gdbserver running as root be able to read "jringle"'s /proc file? Does CAP_PTRACE make a difference? I have to wonder whether there's a better way to do this.. gdbserver needs to read other /proc files, some not cacheable. I fear that you may have run into just one case so far, and that we may run into problems if we take this route. Thanks, Pedro Alves