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From a91da7741464dadeb306a741b4fb562e49ffea57 Mon Sep 17 00:00:00 2001
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From: Peter Xu <peterx@redhat.com>
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Date: Tue, 7 Feb 2023 15:57:11 -0500
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Subject: [PATCH 5/8] util/userfaultfd: Support /dev/userfaultfd
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RH-Author: Peter Xu <peterx@redhat.com>
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RH-MergeRequest: 149: Support /dev/userfaultfd
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RH-Bugzilla: 2158704
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RH-Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
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RH-Acked-by: quintela1 <quintela@redhat.com>
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RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
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RH-Commit: [3/3] 5f427d8c18c210ff8f66724c9e358a7120619e69 (peterx/qemu-kvm)
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Teach QEMU to use /dev/userfaultfd when it existed and fallback to the
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system call if either it's not there or doesn't have enough permission.
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Firstly, as long as the app has permission to access /dev/userfaultfd, it
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always have the ability to trap kernel faults which QEMU mostly wants.
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Meanwhile, in some context (e.g. containers) the userfaultfd syscall can be
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forbidden, so it can be the major way to use postcopy in a restricted
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environment with strict seccomp setup.
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Signed-off-by: Peter Xu <peterx@redhat.com>
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Reviewed-by: Juan Quintela <quintela@redhat.com>
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Signed-off-by: Juan Quintela <quintela@redhat.com>
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(cherry picked from commit c40c0463413b941c13fe5f99a90c02d7d6584828)
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Signed-off-by: Peter Xu <peterx@redhat.com>
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---
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util/trace-events | 1 +
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util/userfaultfd.c | 32 ++++++++++++++++++++++++++++++++
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2 files changed, 33 insertions(+)
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diff --git a/util/trace-events b/util/trace-events
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index c8f53d7d9f..16f78d8fe5 100644
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--- a/util/trace-events
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+++ b/util/trace-events
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@@ -93,6 +93,7 @@ qemu_vfio_region_info(const char *desc, uint64_t region_ofs, uint64_t region_siz
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qemu_vfio_pci_map_bar(int index, uint64_t region_ofs, uint64_t region_size, int ofs, void *host) "map region bar#%d addr 0x%"PRIx64" size 0x%"PRIx64" ofs 0x%x host %p"
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#userfaultfd.c
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+uffd_detect_open_mode(int mode) "%d"
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uffd_query_features_nosys(int err) "errno: %i"
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uffd_query_features_api_failed(int err) "errno: %i"
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uffd_create_fd_nosys(int err) "errno: %i"
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diff --git a/util/userfaultfd.c b/util/userfaultfd.c
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index 4953b3137d..fdff4867e8 100644
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--- a/util/userfaultfd.c
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+++ b/util/userfaultfd.c
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@@ -18,10 +18,42 @@
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#include <poll.h>
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#include <sys/syscall.h>
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#include <sys/ioctl.h>
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+#include <fcntl.h>
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+
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+typedef enum {
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+ UFFD_UNINITIALIZED = 0,
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+ UFFD_USE_DEV_PATH,
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+ UFFD_USE_SYSCALL,
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+} uffd_open_mode;
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int uffd_open(int flags)
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{
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#if defined(__NR_userfaultfd)
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+ static uffd_open_mode open_mode;
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+ static int uffd_dev;
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+
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+ /* Detect how to generate uffd desc when run the 1st time */
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+ if (open_mode == UFFD_UNINITIALIZED) {
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+ /*
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+ * Make /dev/userfaultfd the default approach because it has better
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+ * permission controls, meanwhile allows kernel faults without any
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+ * privilege requirement (e.g. SYS_CAP_PTRACE).
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+ */
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+ uffd_dev = open("/dev/userfaultfd", O_RDWR | O_CLOEXEC);
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+ if (uffd_dev >= 0) {
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+ open_mode = UFFD_USE_DEV_PATH;
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+ } else {
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+ /* Fallback to the system call */
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+ open_mode = UFFD_USE_SYSCALL;
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+ }
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+ trace_uffd_detect_open_mode(open_mode);
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+ }
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+
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+ if (open_mode == UFFD_USE_DEV_PATH) {
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+ assert(uffd_dev >= 0);
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+ return ioctl(uffd_dev, USERFAULTFD_IOC_NEW, flags);
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+ }
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+
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return syscall(__NR_userfaultfd, flags);
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#else
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return -EINVAL;
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--
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2.31.1
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