Re: [Qemu-devel] [PATCH 1/5] block: Auto-generate node_names for each BDS entry

[Benoît Canet]

The Thursday 15 May 2014 à 08:06:14 (-0400), Jeff Cody wrote :
> On Thu, May 15, 2014 at 01:58:59PM +0200, Benoît Canet wrote:
> > The Wednesday 14 May 2014 à 23:20:15 (-0400), Jeff Cody wrote :
> > > Currently, node_name is only filled in when done so explicitly by the
> > > user.  If no node_name is specified, then the node name field is not
> > > populated.
> > > 
> > > If node_names are automatically generated when not specified, that means
> > > that all block job operations can be done by reference to the unique
> > > node_name field.  This eliminates ambiguity in filename pathing
> > > (relative filenames, or file descriptors, symlinks, mounts, etc..) that
> > > qemu currently needs to deal with.
> > > 
> > > If a node name is specified, then it will not be automatically
> > > generated for that BDS entry.
> > > 
> > > If it is automatically generated, it will be prefaced with "__qemu##",
> > > followed by 8 characters of a unique number, followed by 8 random
> > > ASCII characters in the range of 'A-Z'.  Some sample generated node-name
> > > strings:
> > >     __qemu##00000000IAIYNXXR
> > >     __qemu##00000002METXTRBQ
> > >     __qemu##00000001FMBORDWG
> > > 
> > > The prefix is to aid in identifying it as a qemu-generated name, the
> > > numeric portion is to guarantee uniqueness in a given qemu session, and
> > > the random characters are to further avoid any accidental collisions
> > > with user-specified node-names.
> > > 
> > > Signed-off-by: Jeff Cody <address@hidden>
> > > ---
> > >  block.c | 16 +++++++++++++++-
> > >  1 file changed, 15 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/block.c b/block.c
> > > index c90c71a..81945d3 100644
> > > --- a/block.c
> > > +++ b/block.c
> > > @@ -838,12 +838,26 @@ static int bdrv_open_flags(BlockDriverState *bs, 
> > > int flags)
> > >      return open_flags;
> > >  }
> > >  
> > > +#define GEN_NODE_NAME_PREFIX    "__qemu##"
> > > +#define GEN_NODE_NAME_MAX_LEN   (sizeof(GEN_NODE_NAME_PREFIX) + 8 + 8)
> > >  static void bdrv_assign_node_name(BlockDriverState *bs,
> > >                                    const char *node_name,
> > >                                    Error **errp)
> > >  {
> > > +    char gen_node_name[GEN_NODE_NAME_MAX_LEN];
> > 
> > The room for the '\0' string termination seems to be missing:
> > 
> >     char gen_node_name[GEN_NODE_NAME_MAX_LEN + 1];
> >
> 
> The array includes room for it, note the use of 'sizeof()':
>     #define GEN_NODE_NAME_MAX_LEN   (sizeof(GEN_NODE_NAME_PREFIX) + 8 + 8)
> 
> sizeof() includes the '\0' in the length, while strlen() does not;
> e.g.:
>     sizeof("four") = 5
>     strlen("four") = 4
> 
> > > +    static uint32_t counter; /* simple counter to guarantee uniqueness */
> > > +
> > > +    /* if node_name is NULL, auto-generate a node name */
> > >      if (!node_name) {
> > > -        return;
> > > +        int len;
> > > +        snprintf(gen_node_name, GEN_NODE_NAME_MAX_LEN,
> > > +                 "%s%08x", GEN_NODE_NAME_PREFIX, counter++);
> > > +        len = strlen(gen_node_name);
> > > +        while (len < GEN_NODE_NAME_MAX_LEN - 1) {
> > > +            gen_node_name[len++] = g_random_int_range('A', 'Z');
> > > +        }
> > 
> > Is this code generating only 7 random chars instead of 8 ?
> > 
> 
> It generates 8 random characters (the sample node-name strings in the
> commit message were pulled straight from the QMP command
> 'query-named-block-nodes')
> 
> > > +        gen_node_name[GEN_NODE_NAME_MAX_LEN - 1] = '\0';
> > 
> > Could be:
> >         gen_node_name[GEN_NODE_NAME_MAX_LEN] = '\0';
> > if the array is properly declared.
> >
> 
> That would go over the array bounds by 1.

Yes I missed the use of sizeof().
I am happy to have learnt that.
Sorry for the noise.

> 
> > > +        node_name = gen_node_name;
> > >      }
> > >  
> > >      /* empty string node name is invalid */
> > > -- 
> > > 1.8.3.1
> > >