Re: [Qemu-devel] [PATCH v2 3/5] migration/vmstate: fix array of ptr with nullptrs

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Halil Pasic (pasic@linux.vnet.ibm.com) wrote:
> Make VMS_ARRAY_OF_POINTER cope with null pointers. Previously the
> reward for trying to migrate an array with some null pointers in it was
> an illegal memory access, that is a swift and painless death of the
> process.  Let's make vmstate cope with this scenario.
> 
> The general approach is, when we encounter a null pointer (element),
> instead of following the pointer to save/load the data behind it, we
> save/load a placeholder. This way we can detect if we expected a null
> pointer at the load side but not null data was saved instead.
> 
> Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
> Reviewed-by: Guenther Hutzl <hutzl@linux.vnet.ibm.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
> We will need this to load/save some on demand created state in the
> (s390x) channel subsystem (see ChannelSubSys.css in hw/s390x/css.c for
> an example).
> ---
>  include/migration/vmstate.h |  4 ++++
>  migration/vmstate.c         | 40 ++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 42 insertions(+), 2 deletions(-)
> 
> diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
> index 63e7b02..f2dbf84 100644
> --- a/include/migration/vmstate.h
> +++ b/include/migration/vmstate.h
> @@ -253,6 +253,10 @@ extern const VMStateInfo vmstate_info_uint16;
>  extern const VMStateInfo vmstate_info_uint32;
>  extern const VMStateInfo vmstate_info_uint64;
>  
> +/** Put this in the stream when migrating a null pointer.*/
> +#define VMS_NULLPTR_MARKER (0x30U) /* '0' */
> +extern const VMStateInfo vmstate_info_nullptr;
> +
>  extern const VMStateInfo vmstate_info_float64;
>  extern const VMStateInfo vmstate_info_cpudouble;
>  
> diff --git a/migration/vmstate.c b/migration/vmstate.c
> index 836a7a4..78b3cd4 100644
> --- a/migration/vmstate.c
> +++ b/migration/vmstate.c
> @@ -117,7 +117,11 @@ int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
>                  if (field->flags & VMS_ARRAY_OF_POINTER) {
>                      curr_elem = *(void **)curr_elem;
>                  }
> -                if (field->flags & VMS_STRUCT) {
> +                if (!curr_elem) {
> +                    /* if null pointer check placeholder and do not follow */
> +                    assert(field->flags & VMS_ARRAY_OF_POINTER);
> +                    ret = vmstate_info_nullptr.get(f, curr_elem, size, NULL);
> +                } else if (field->flags & VMS_STRUCT) {
>                      ret = vmstate_load_state(f, field->vmsd, curr_elem,
>                                               field->vmsd->version_id);
>                  } else {
> @@ -332,7 +336,11 @@ void vmstate_save_state(QEMUFile *f, const VMStateDescription *vmsd,
>                      assert(curr_elem);
>                      curr_elem = *(void **)curr_elem;
>                  }
> -                if (field->flags & VMS_STRUCT) {
> +                if (!curr_elem) {
> +                    /* if null pointer write placeholder and do not follow */
> +                    assert(field->flags & VMS_ARRAY_OF_POINTER);
> +                    vmstate_info_nullptr.put(f, curr_elem, size, NULL, NULL);
> +                } else if (field->flags & VMS_STRUCT) {
>                      vmstate_save_state(f, field->vmsd, curr_elem, vmdesc_loop);
>                  } else {
>                      field->info->put(f, curr_elem, size, field, vmdesc_loop);
> @@ -747,6 +755,34 @@ const VMStateInfo vmstate_info_uint64 = {
>      .put  = put_uint64,
>  };
>  
> +static int get_nullptr(QEMUFile *f, void *pv, size_t size, VMStateField *field)
> +
> +{
> +    if (qemu_get_byte(f) == VMS_NULLPTR_MARKER) {
> +        return  0;
> +    }
> +    error_report("vmstate: get_nullptr expected VMS_NULLPTR_MARKER");
> +    return -EINVAL;
> +}
> +
> +static int put_nullptr(QEMUFile *f, void *pv, size_t size,
> +                        VMStateField *field, QJSON *vmdesc)
> +
> +{
> +    if (pv == NULL) {
> +        qemu_put_byte(f, VMS_NULLPTR_MARKER);
> +        return 0;
> +    }
> +    error_report("vmstate: put_nullptr must be called with pv == NULL");
> +    return -EINVAL;
> +}
> +
> +const VMStateInfo vmstate_info_nullptr = {
> +    .name = "uint64",
> +    .get  = get_nullptr,
> +    .put  = put_nullptr,
> +};
> +
>  /* 64 bit unsigned int. See that the received value is the same than the one
>     in the field */
>  
> -- 
> 2.8.4
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK