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

[Daniel P . Berrangé]

On Wed, Jan 25, 2023 at 05:40:16PM -0500, 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;
> +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);

qemu_open(), otherwise FD passing from the mgmt app won't work.

> +        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;
> +}

This leaves the /dev/userfaultfd FD open forever once it has been used
once. Is this really needed ? IIUC, the place where we call this is
not going to be impacted if we open + close it every time we need to
create a new FD, and it'll simplify this code right down.

>  
>  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;
> -- 
> 2.37.3
> 
> 

With regards,
Daniel
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