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d="scan'208";a="16675484" Subject: Re: [PATCH] mini-os: Avoid segfaults in tc{g,s}etattr To: Wei Liu , Jason Andryuk References: <20200427034019.6251-1-jandryuk@gmail.com> <20200427075429.mshevnm2ype7tq32@function> <20200428111645.pa6xfs6t6rifu6fu@liuwe-devbox-debian-v2.j3c5onc20sse1dnehy4noqpfcg.zx.internal.cloudapp.net> From: Andrew Cooper Message-ID: <3ed7eb87-070c-28ea-4f8a-aa4421cea93a@citrix.com> Date: Tue, 28 Apr 2020 12:24:29 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <20200428111645.pa6xfs6t6rifu6fu@liuwe-devbox-debian-v2.j3c5onc20sse1dnehy4noqpfcg.zx.internal.cloudapp.net> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Content-Language: en-GB X-ClientProxiedBy: AMSPEX02CAS01.citrite.net (10.69.22.112) To AMSPEX02CL02.citrite.net (10.69.22.126) X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: minios-devel@lists.xenproject.org, Samuel Thibault , Jan Beulich , xen-devel Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 28/04/2020 12:16, Wei Liu wrote: >>>> --- >>>> I can't get ioemu-stubdom to start without this. With this, the guest >>>> just reboots immediately, but it does that with a non-stubdom >>>> device_model_version="qemu-xen-traditional" . The same guest disk image >>>> (cirros 0.5.1) boots with a linux stubdom or non-stubdom Ubuntu >>>> qemu-system-x86_64. >> Ubuntu gcc-9 adds -fcf-protection by default. Somehow that flag >> caused rombios (I think) to restart. Setting -fcf-protection=none >> like below (probably just the EMBEDDED_EXTRA_CFLAGS part) lets rombios >> start properly. All it does is insert ENDBR{32,64} instructions, which are nops on older processors. I suspect that it is not the -fcf-protection= directly, but some change in alignment of a critical function. >> The hypervisor needs it as well via >> EXTRA_CFLAGS_XEN_CORE=-fcf-protection=none and maybe also added to >> xen/arch/x86/boot/build32.mk . > Are you able to turn this into a proper patch? I suspect you will need > to test the availability of this new (?) flag. > > Also Cc Jan and Andrew because it affects hypervisor build too. I need to chase this up.  It is a GCC bug breaking the hypervisor build, and I'm moderately disinclined to hack around it, seeing as -fcf-protection is something we want in due course. The bug is that GCC falsely declares that -fcf-protection is incompatible with -mindirect-thunk=extern, despite me spending a week during the Spectre embargo period specifically arranging for the two to be compatible, because we knew we'd want to build retpoline-safe binaries which could also use CET on newer hardware. ~Andrew