Re: [PATCH v3 2/2] util/userfaultfd: Support /dev/userfaultfd

[Juan Quintela]

Peter Xu <peterx@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.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>

> 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 <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;

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.