Peter Xu <pet...@redhat.com> 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.
>
> Reviewed-by: Philippe Mathieu-Daudé <phi...@linaro.org>
> Signed-off-by: Peter Xu <pet...@redhat.com>
Hi
Can we change this code to not use the global variable.
> ---
> util/trace-events | 1 +
> util/userfaultfd.c | 37 +++++++++++++++++++++++++++++++++++++
> 2 files changed, 38 insertions(+)
>
> diff --git a/util/trace-events b/util/trace-events
> index c8f53d7d9f..16f78d8fe5 100644
> --- a/util/trace-events
> +++ b/util/trace-events
> @@ -93,6 +93,7 @@ qemu_vfio_region_info(const char *desc, uint64_t
> region_ofs, uint64_t region_siz
> 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"
>
> #userfaultfd.c
> +uffd_detect_open_mode(int mode) "%d"
> uffd_query_features_nosys(int err) "errno: %i"
> uffd_query_features_api_failed(int err) "errno: %i"
> uffd_create_fd_nosys(int err) "errno: %i"
> diff --git a/util/userfaultfd.c b/util/userfaultfd.c
> index 9845a2ec81..7dceab51d6 100644
> --- a/util/userfaultfd.c
> +++ b/util/userfaultfd.c
> @@ -18,10 +18,47 @@
> #include <poll.h>
> #include <sys/syscall.h>
> #include <sys/ioctl.h>
> +#include <fcntl.h>
> +
> +typedef enum {
> + UFFD_UNINITIALIZED = 0,
> + UFFD_USE_DEV_PATH,
> + UFFD_USE_SYSCALL,
> +} uffd_open_mode;
> +
> +static int uffd_dev;
> +
> +static uffd_open_mode uffd_detect_open_mode(void)
> +{
> + static uffd_open_mode open_mode;
> +
> + 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);
> + }
> +
> + return open_mode;
> +}
>
> int uffd_open(int flags)
> {
> #if defined(__linux__) && defined(__NR_userfaultfd)
> + if (uffd_detect_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;
static int open_userfaultd(void)
{
/*
* 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).
*/
int uffd = open("/dev/userfaultfd", O_RDWR | O_CLOEXEC);
if (uffd >= 0) {
return uffd;
}
return -1;
}
int uffd_open(int flags)
{
#if defined(__linux__) && defined(__NR_userfaultfd)
static int uffd = -2;
if (uffd == -2) {
uffd = open_userfaultd();
}
if (uffd >= 0) {
return ioctl(uffd, USERFAULTFD_IOC_NEW, flags);
}
return syscall(__NR_userfaultfd, flags);
#else
return -EINVAL;
27 lines vs 42
No need for enum type
No need for global variable
What do you think?
Later, Juan.
