Re: [PATCH 3/3] util/userfaultfd: Support /dev/userfaultfd

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 25/1/23 23:40, 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 <peterx@redhat.com>
> ---
>   util/trace-events  |  1 +
>   util/userfaultfd.c | 36 ++++++++++++++++++++++++++++++++++++
>   2 files changed, 37 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..360ecf8084 100644
> --- a/util/userfaultfd.c
> +++ b/util/userfaultfd.c
> @@ -18,10 +18,46 @@
>   #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 uffd_open_mode open_mode;

'open_mode' could be reduced to uffd_detect_open_mode()'s
scope.

> +static int uffd_dev;
> +
> +static uffd_open_mode uffd_detect_open_mode(void)
> +{
> +    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;

If 'open_mode' isn't relevant, this function could return uffd_dev/-1 
instead. Not really an improvement :)

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

> +}
>   
>   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;