Re: [Qemu-devel] [Qemu-block] [PATCH 3/3] blockdev: Add dynamic module loading for block drivers

[Fam Zheng <famz@redhat.com>]

On Fri, 06/24 11:37, Daniel P. Berrange wrote:
> On Fri, Jun 24, 2016 at 11:04:43AM +0100, Stefan Hajnoczi wrote:
> > On Wed, Jun 22, 2016 at 05:35:54PM -0400, Colin Lord wrote:
> > 
> > >  
> > > @@ -447,8 +466,15 @@ int get_tmp_filename(char *filename, int size)
> > >  static BlockDriver *find_hdev_driver(const char *filename)
> > >  {
> > >      int score_max = 0, score;
> > > +    size_t i;
> > >      BlockDriver *drv = NULL, *d;
> > >  
> > > +    for (i = 0; i < ARRAY_SIZE(block_driver_modules); ++i) {
> > > +        if (block_driver_modules[i].has_probe_device) {
> > > +            block_module_load_one(block_driver_modules[i].library_name);
> > > +        }
> > > +    }
> > 
> > This patch series needs to solve probing so that we don't end up loading
> > all block drivers.  Fam's suggestion for a built-in probe.c sounds good
> > to me.
> 
> Do we really care if probing loads all drivers ? Last time we discussed
> this I thought we decided that because probing almost always leads to
> security vulnerabilities, no one should use it by default and so we
> don't really need to worry about optimizing it.

Does this mean we can drop "has_probe" and "has_probe_device" from the
generated header and load all modules for probe?  If so, I'd like to again
stress my preference for a "simplified" code "parser":

All of current block_init() calls except quorum can be converted with this:

    #define BLOCK_DRIVER_EXPORT(fmt_name, prot_name, drv) \
        static void bdrv_ ## fmt_name ## _ ## prot_name(void) \
        { \
            if (strlen(#fmt_name)) { \
                drv->format_name = #fmt_name; \
            } \
            if (strlen(#prot_name)) { \
                drv->protocol_name = #prot_name; \
            } \
            bdrv_register(&(drv)); \
        } \
        block_init(bdrv_ ## fmt_name ## prot_name)

curl.c:

    BLOCK_DRIVER_EXPORT(http, http, bdrv_http);
    BLOCK_DRIVER_EXPORT(https, https, bdrv_https);
    BLOCK_DRIVER_EXPORT(ftp, ftp, bdrv_ftp);
    ...

iscsi.c (on top of patch 1):

    BLOCK_DRIVER_EXPORT(iscsi, iscsi, bdrv_iscsi);

vmdk.c:

    BLOCK_DRIVER_EXPORT(vmdk,, bdrv_vmdk);

Then the python generator greps for BLOCK_DRIVER_EXPORT, instead of 'static
BlockDriver', which is much more reliable (in the sense of whitespace subtlety,
field name changing, etc).

Fam