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David Alan Gilbert" , Daniel P . =?utf-8?Q?Berrang=C3=A9?= Subject: Re: [PATCH v3 2/2] util/userfaultfd: Support /dev/userfaultfd In-Reply-To: <20230207205711.1187216-3-peterx@redhat.com> (Peter Xu's message of "Tue, 7 Feb 2023 15:57:11 -0500") References: <20230207205711.1187216-1-peterx@redhat.com> <20230207205711.1187216-3-peterx@redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) Date: Wed, 08 Feb 2023 20:06:35 +0100 Message-ID: <87wn4s9dtw.fsf@secure.mitica> MIME-Version: 1.0 Content-Type: text/plain Received-SPF: pass client-ip=170.10.129.124; envelope-from=quintela@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: quintela@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Peter Xu wrote: > Teach QEMU to use /dev/userfaultfd when it existed and fallback to the > system call if either it's not there or doesn't have enough permission. > > Firstly, as long as the app has permission to access /dev/userfaultfd, it > always have the ability to trap kernel faults which QEMU mostly wants. > Meanwhile, in some context (e.g. containers) the userfaultfd syscall can be > forbidden, so it can be the major way to use postcopy in a restricted > environment with strict seccomp setup. > > Signed-off-by: Peter Xu Reviewed-by: Juan Quintela > diff --git a/util/userfaultfd.c b/util/userfaultfd.c > index 4953b3137d..fdff4867e8 100644 > --- a/util/userfaultfd.c > +++ b/util/userfaultfd.c > @@ -18,10 +18,42 @@ > #include > #include > #include > +#include > + > +typedef enum { > + UFFD_UNINITIALIZED = 0, > + UFFD_USE_DEV_PATH, > + UFFD_USE_SYSCALL, > +} uffd_open_mode; Having an enum or a magic -2 value, better the enum O:-) > int uffd_open(int flags) > { > #if defined(__NR_userfaultfd) > + static uffd_open_mode open_mode; > + static int uffd_dev; > + > + /* Detect how to generate uffd desc when run the 1st time */ > + if (open_mode == UFFD_UNINITIALIZED) { > + /* > + * Make /dev/userfaultfd the default approach because it has better > + * permission controls, meanwhile allows kernel faults without any > + * privilege requirement (e.g. SYS_CAP_PTRACE). > + */ > + uffd_dev = open("/dev/userfaultfd", O_RDWR | O_CLOEXEC); > + if (uffd_dev >= 0) { > + open_mode = UFFD_USE_DEV_PATH; > + } else { > + /* Fallback to the system call */ > + open_mode = UFFD_USE_SYSCALL; > + } > + trace_uffd_detect_open_mode(open_mode); > + } > + > + if (open_mode == UFFD_USE_DEV_PATH) { > + assert(uffd_dev >= 0); > + return ioctl(uffd_dev, USERFAULTFD_IOC_NEW, flags); > + } > + > return syscall(__NR_userfaultfd, flags); > #else > return -EINVAL; The rest is ok, thanks. queued.