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Re: [Qemu-devel] [PATCH 3/5] migration/vmstate: fix array of ptr with nu
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH 3/5] migration/vmstate: fix array of ptr with nullptrs |
Date: |
Fri, 17 Feb 2017 19:42:37 +0000 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
* Halil Pasic (address@hidden) 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 <address@hidden>
> Reviewed-by: Guenther Hutzl <address@hidden>
>
> ---
> 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 | 33 +++++++++++++++++++++++++++++++--
> 2 files changed, 35 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..cb81cef 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);
That can return an error instead of asserting.
> + vmstate_info_nullptr.get(f, curr_elem, size, NULL);
You've ignored the return value of the get; that should be ret =
> + } 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 +755,27 @@ const VMStateInfo vmstate_info_uint64 = {
> .put = put_uint64,
> };
>
> +static int get_nullptr(QEMUFile *f, void *pv, size_t size, VMStateField
> *field)
> +
> +{
> + return qemu_get_byte(f) == VMS_NULLPTR_MARKER ? 0 : -EINVAL;
> +}
> +
> +static int put_nullptr(QEMUFile *f, void *pv, size_t size,
> + VMStateField *field, QJSON *vmdesc)
> +
> +{
> + assert(pv == NULL);
> + qemu_put_byte(f, VMS_NULLPTR_MARKER);
> + return 0;
> +}
Again that assert could just turn into a return -EINVAL
Dave
> +
> +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 / address@hidden / Manchester, UK
[Qemu-devel] [PATCH 2/5] migration/vmstate: split up vmstate_base_addr, Halil Pasic, 2017/02/16
[Qemu-devel] [PATCH 4/5] tests/test-vmstate.c: test array of ptr with null, Halil Pasic, 2017/02/16