Re: [PATCH v7 1/5] hw/nvram/fw_cfg: Add the FW_CFG_DATA_GENERATOR interface

[Laszlo Ersek]

On 05/28/20 19:31, Philippe Mathieu-Daudé wrote:
> The FW_CFG_DATA_GENERATOR allows any object to produce
> blob of data consumable by the fw_cfg device.
> 
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
> v7: addressed Laszlo's comments
> - fixed typos in description
> - return size_t instead of ssize_t; 0 for error
> - do not use 1-letter variable names
> - do not open-code 'fw_cfg-data-generator'
> - cast g_memdup() size argument as 'guint'
> - improved documentation
> ---
>  docs/specs/fw_cfg.txt     |  9 ++++++-
>  include/hw/nvram/fw_cfg.h | 52 +++++++++++++++++++++++++++++++++++++++
>  hw/nvram/fw_cfg.c         | 31 +++++++++++++++++++++++
>  3 files changed, 91 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/specs/fw_cfg.txt b/docs/specs/fw_cfg.txt
> index 8f1ebc66fa..bc16daa38a 100644
> --- a/docs/specs/fw_cfg.txt
> +++ b/docs/specs/fw_cfg.txt
> @@ -219,7 +219,7 @@ To check the result, read the "control" field:
>  
>  = Externally Provided Items =
>  
> -As of v2.4, "file" fw_cfg items (i.e., items with selector keys above
> +Since v2.4, "file" fw_cfg items (i.e., items with selector keys above
>  FW_CFG_FILE_FIRST, and with a corresponding entry in the fw_cfg file
>  directory structure) may be inserted via the QEMU command line, using
>  the following syntax:
> @@ -230,6 +230,13 @@ Or
>  
>      -fw_cfg [name=]<item_name>,string=<string>
>  
> +Since v5.1, QEMU allows some objects to generate fw_cfg-specific content,
> +the content is then associated with a "file" item using the 'gen_id' option
> +in the command line, using the following syntax:
> +
> +    -object <generator-type>,id=<generated_id>,[generator-specific-options] \
> +    -fw_cfg [name=]<item_name>,gen_id=<generated_id>
> +
>  See QEMU man page for more documentation.
>  
>  Using item_name with plain ASCII characters only is recommended.

I've looked at this hunk with a larger context, and I think it's really
good.

> diff --git a/include/hw/nvram/fw_cfg.h b/include/hw/nvram/fw_cfg.h
> index 25d9307018..8fbf2446c1 100644
> --- a/include/hw/nvram/fw_cfg.h
> +++ b/include/hw/nvram/fw_cfg.h
> @@ -9,11 +9,43 @@
>  #define TYPE_FW_CFG     "fw_cfg"
>  #define TYPE_FW_CFG_IO  "fw_cfg_io"
>  #define TYPE_FW_CFG_MEM "fw_cfg_mem"
> +#define TYPE_FW_CFG_DATA_GENERATOR_INTERFACE "fw_cfg-data-generator"
>  
>  #define FW_CFG(obj)     OBJECT_CHECK(FWCfgState,    (obj), TYPE_FW_CFG)
>  #define FW_CFG_IO(obj)  OBJECT_CHECK(FWCfgIoState,  (obj), TYPE_FW_CFG_IO)
>  #define FW_CFG_MEM(obj) OBJECT_CHECK(FWCfgMemState, (obj), TYPE_FW_CFG_MEM)
>  
> +#define FW_CFG_DATA_GENERATOR_CLASS(class) \
> +    OBJECT_CLASS_CHECK(FWCfgDataGeneratorClass, (class), \
> +                       TYPE_FW_CFG_DATA_GENERATOR_INTERFACE)
> +#define FW_CFG_DATA_GENERATOR_GET_CLASS(obj) \
> +    OBJECT_GET_CLASS(FWCfgDataGeneratorClass, (obj), \
> +                     TYPE_FW_CFG_DATA_GENERATOR_INTERFACE)
> +
> +typedef struct FWCfgDataGeneratorClass {
> +    /*< private >*/
> +    InterfaceClass parent_class;
> +    /*< public >*/
> +
> +    /**
> +     * get_data:
> +     * @obj: the object implementing this interface
> +     *
> +     * Returns: pointer to start of the generated item data
> +     *
> +     * The returned pointer is a QObject weak reference, @obj owns
> +     * the reference and may free it at any time in the future.
> +     */
> +    const void *(*get_data)(Object *obj);
> +    /**
> +     * get_length:
> +     * @obj: the object implementing this interface
> +     *
> +     * Returns: the size of the generated item data in bytes
> +     */
> +    size_t (*get_length)(Object *obj);
> +} FWCfgDataGeneratorClass;
> +
>  typedef struct fw_cfg_file FWCfgFile;
>  
>  #define FW_CFG_ORDER_OVERRIDE_VGA    70
> @@ -263,6 +295,26 @@ void fw_cfg_add_file_callback(FWCfgState *s, const char 
> *filename,
>  void *fw_cfg_modify_file(FWCfgState *s, const char *filename, void *data,
>                           size_t len);
>  
> +/**
> + * fw_cfg_add_from_generator:
> + * @s: fw_cfg device being modified
> + * @filename: name of new fw_cfg file item
> + * @gen_id: name of object implementing FW_CFG_DATA_GENERATOR interface
> + * @errp: pointer to a NULL initialized error object
> + *
> + * Add a new NAMED fw_cfg item with the content generated from the
> + * @gen_id object. The data generated by the @gen_id object/ is copied

(1) typo: "object/" (possibly a copy-paste error from my v6 review)

> + * into the data structure of the fw_cfg device.
> + * The next available (unused) selector key starting at FW_CFG_FILE_FIRST
> + * will be used; also, a new entry will be added to the file directory
> + * structure residing at key value FW_CFG_FILE_DIR, containing the item name,
> + * data size, and assigned selector key value.
> + *
> + * Returns: the size of the device tree image on success, or 0 on errors.

(2) typo (probably another copy-paste error):

s/device tree image/generated item data/

> + */
> +size_t fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
> +                                 const char *gen_id, Error **errp);
> +
>  FWCfgState *fw_cfg_init_io_dma(uint32_t iobase, uint32_t dma_iobase,
>                                  AddressSpace *dma_as);
>  FWCfgState *fw_cfg_init_io(uint32_t iobase);
> diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
> index 8dd50c2c72..6d2fa13042 100644
> --- a/hw/nvram/fw_cfg.c
> +++ b/hw/nvram/fw_cfg.c
> @@ -1032,6 +1032,31 @@ void *fw_cfg_modify_file(FWCfgState *s, const char 
> *filename,
>      return NULL;
>  }
>  
> +size_t fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
> +                                 const char *gen_id, Error **errp)
> +{
> +    FWCfgDataGeneratorClass *klass;
> +    Object *obj;
> +    size_t size;
> +
> +    obj = object_resolve_path_component(object_get_objects_root(), gen_id);
> +    if (!obj) {
> +        error_setg(errp, "Cannot find object ID %s", gen_id);
> +        return 0;
> +    }
> +    if (!object_dynamic_cast(obj, TYPE_FW_CFG_DATA_GENERATOR_INTERFACE)) {
> +        error_setg(errp, "Object '%s' is not a '%s' subclass",
> +                   TYPE_FW_CFG_DATA_GENERATOR_INTERFACE, gen_id);

(3) the order of the last two arguments is wrong; please reverse it.

> +        return 0;
> +    }
> +    klass = FW_CFG_DATA_GENERATOR_GET_CLASS(obj);
> +    size = klass->get_length(obj);

(4) I think we should not call fw_cfg_add_file() if get_length() returns
zero.

AFAICS, fw_cfg_add_file() will happily create a useless zero-size fw_cfg
entry, but then the same zero size that we return from
fw_cfg_add_from_generator() will indicate failure to the caller (and we
will also not have set "errp").

I think we need something like:

    if (size == 0) {
        error_setg(errp, "Object '%s' failed to generate fw_cfg data",
                   gen_id);
        return 0;
    }

That will also prevent an awkward call to g_memdup() with size==0.


Now, looking at patches #4 and #5, I think if no suitable cipher suite
is found, then qcrypto_tls_cipher_suites_get_data() will return NULL,
and qcrypto_tls_cipher_suites_get_length() will return zero. In turn, my
suggestion (4) here will prevent QEMU from starting up.

And I think that is just fine. The corresponding firmware function is
SetCipherSuites() in
"OvmfPkg/Library/TlsAuthConfigLib/TlsAuthConfigLib.c", and if it sees a
zero-sized "etc/edk2/https/ciphers" fw_cfg file, it will hang the
firmware, *on purpose*. (See the reason explained in comments there.)

Thus, it's much better if we prevent QEMU from starting up in the first
place.

> +    fw_cfg_add_file(s, filename, g_memdup(klass->get_data(obj), (guint)size),
> +                    size);
> +
> +    return size;
> +}
> +
>  static void fw_cfg_machine_reset(void *opaque)
>  {
>      MachineClass *mc = MACHINE_GET_CLASS(qdev_get_machine());
> @@ -1333,12 +1358,18 @@ static const TypeInfo fw_cfg_mem_info = {
>      .class_init    = fw_cfg_mem_class_init,
>  };
>  
> +static const TypeInfo fw_cfg_data_generator_interface_info = {
> +    .name = TYPE_FW_CFG_DATA_GENERATOR_INTERFACE,
> +    .parent = TYPE_INTERFACE,
> +    .class_size = sizeof(FWCfgDataGeneratorClass),
> +};
>  
>  static void fw_cfg_register_types(void)
>  {
>      type_register_static(&fw_cfg_info);
>      type_register_static(&fw_cfg_io_info);
>      type_register_static(&fw_cfg_mem_info);
> +    type_register_static(&fw_cfg_data_generator_interface_info);
>  }
>  
>  type_init(fw_cfg_register_types)
> 

With (1) through (4) fixed:

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks
Laszlo