On Fri, Mar 10, 2023 at 10:16 PM Eric Blake <eblake@xxxxxxxxxx> wrote: > > The uapi <linux/nbd.h> header declares a 'char handle[8]' per request; > which is overloaded in English (are you referring to "handle" the > verb, such as handling a signal or writing a callback handler, or > "handle" the noun, the value used in a lookup table to correlate a > response back to the request). Many client-side NBD implementations > (both servers and clients) have instead used 'u64 cookie' or similar, > as it is easier to directly assign an integer than to futz around with > memcpy. In fact, upstream documentation is now encouraging this shift > in terminology: https://lists.debian.org/nbd/2023/03/msg00031.html > > Accomplish this by use of an anonymous union to provide the alias for > anyone getting the definition from the uapi; this does not break > existing clients, while exposing the nicer name for those who prefer > it. Note that block/nbd.c still uses the term handle (in fact, it > actually combines a 32-bit cookie and a 32-bit tag into the 64-bit > handle), but that internal usage is not changed the public uapi, since > no compliant NBD server has any reason to inspect or alter the 64 > bits sent over the socket. > > Signed-off-by: Eric Blake <eblake@xxxxxxxxxx> > --- > include/uapi/linux/nbd.h | 10 ++++++++-- > 1 file changed, 8 insertions(+), 2 deletions(-) > > diff --git a/include/uapi/linux/nbd.h b/include/uapi/linux/nbd.h > index 8797387caaf7..f58f2043f62e 100644 > --- a/include/uapi/linux/nbd.h > +++ b/include/uapi/linux/nbd.h > @@ -81,7 +81,10 @@ enum { > struct nbd_request { > __be32 magic; /* NBD_REQUEST_MAGIC */ > __be32 type; /* See NBD_CMD_* */ > - char handle[8]; > + union { > + char handle[8]; > + __be64 cookie; > + }; > __be64 from; > __be32 len; > } __attribute__((packed)); > @@ -93,6 +96,9 @@ struct nbd_request { > struct nbd_reply { > __be32 magic; /* NBD_REPLY_MAGIC */ > __be32 error; /* 0 = ok, else error */ > - char handle[8]; /* handle you got from request */ > + union { > + char handle[8]; /* handle you got from request */ > + __be64 cookie; Should we document like this? union { __be64 cookie; /* cookie you got from request */ char handle[8]; /* older name */ I think we want future code to use the new term. > + }; > }; > #endif /* _UAPILINUX_NBD_H */ > -- > 2.39.2 > Nir
