[Bah - I typed up a longer response, but lost it when accidentally
trying to send through the wrong SMTP server, so now I have to
remember what I had...]
On Fri, Jun 09, 2023 at 02:45:56PM +0200, Laszlo Ersek wrote:
> On 6/9/23 04:17, Eric Blake wrote:
> > When I added structured replies to the NBD spec, I intentionally chose
> > a wire layout where the magic number and cookie overlap, even while
> > the middle member changes from uint32_t error to the pair uint16_t
> > flags and type. Based only on a strict reading of C rules on
> > effective types and compatible type prefixes, it's probably
> > questionable on whether my reliance on type aliasing to reuse cookie
> > from the same offset of a union, or even the fact that a structured
> > reply is built by first reading bytes into sbuf.simple_reply then
> > following up with only bytes into the tail of sbuf.sr.structured_reply
> > is strictly portable. But since it works in practice, it's worth at
> > least adding some compile- and run-time assertions that our (ab)use of
> > aliasing is accessing the bytes we want under the types we expect.
> > Upcoming patches will restructure part of the sbuf layout to hopefully
> > be a little easier to tie back to strict C standards.
> >
> > Suggested-by: Laszlo Ersek <ler...@redhat.com>
> > Signed-off-by: Eric Blake <ebl...@redhat.com>
> > ---
> > generator/states-reply.c | 17 +++++++++++++----
> > generator/states-reply-structured.c | 13 +++++++++----
> > 2 files changed, 22 insertions(+), 8 deletions(-)
> >
> > diff --git a/generator/states-reply.c b/generator/states-reply.c
> > index 511e5cb1..2c77658b 100644
> > --- a/generator/states-reply.c
> > +++ b/generator/states-reply.c
> > @@ -17,6 +17,7 @@
> > */
> >
> > #include <assert.h>
> > +#include <stddef.h>
> >
> > /* State machine for receiving reply messages from the server.
> > *
> > @@ -63,9 +64,15 @@ REPLY.START:
> > ssize_t r;
> >
> > /* We read all replies initially as if they are simple replies, but
> > - * check the magic in CHECK_SIMPLE_OR_STRUCTURED_REPLY below.
> > - * This works because the structured_reply header is larger.
> > + * check the magic in CHECK_SIMPLE_OR_STRUCTURED_REPLY below. This
> > + * works because the structured_reply header is larger, and because
> > + * the last member of a simple reply, cookie, is coincident between
> > + * the two structs (an intentional design decision in the NBD spec
> > + * when structured replies were added).
> > */
> > + STATIC_ASSERT (offsetof (struct nbd_handle, sbuf.simple_reply.cookie) ==
> > + offsetof (struct nbd_handle,
> > sbuf.sr.structured_reply.cookie),
> > + cookie_aliasing);
>
> Can you perhaps append
>
> ... &&
> sizeof h->sbuf.simple_reply.cookie ==
> sizeof h->sbuf.sr.structured_reply.cookie
>
> (if you agree)?
Yes, that makes sense, and I did so for what got pushed as 29342fedb53
>
> Also, the commit message and the comment talk about the magic number as
> well, not just the cookie, and the static assertion ignores magic.
> However, I can see the magic handling changes in the next patch.
I was a bit less concerned about magic (it is easy to see that it is
at offset 0 in both types and could satisfy the common prefix rules,
while seeing cookie's location and a non-common prefix makes the
latter more imporant to assert). But checking two members instead of
one shouldn't hurt, and in fact, once extended types are in (plus
patch 4/4 of this series also adds an anonymous sub-struct in 'union
reply_header' which is also worth validating), it may make sense to do
a followup patch that adds:
#define ASSERT_MEMBER_OVERLAP(TypeA, memberA, TypeB, memberB) \
STATIC_ASSERT (offsetof (TypeA, memberA) == offsetof (TypeB, memberB) && \
sizeof ((TypeA *)NULL)->memberA == sizeof ((TypeB
*)NULL)->memberB, \
member_overlap)
to be used either as:
ASSERT_MEMBER_OVERLAP (struct nbd_simple_reply, cookie,
struct nbd_structured_reply, cookie);
or as
ASSERT_MEMBER_OVERLAP (struct nbd_handle, sbuf.simple_reply.magic,
struct nbd_handle, sbuf.sr.structured_reply.magic);
Would it make sense to have the macro take only three arguments (since
both of those invocations repeat an argument); if so, is it better to
share the common type name, or the common member name?
I also note that our "static-assert.h" file defines STATIC_ASSERT() as
a do/while statement (that is, it MUST appear inside a function body,
so we can't use it easily in .h files); contrast that with C11's
_Static_assert() or qemu's QEMU_BUILD_BUG_ON() that behave more as a
type declaration (and can therefore appear outside of a function body;
C23 will take it one step further by adding static_assert(expr)
alongside static_assert(expr, msg). I consider myself too tainted,
not only by helping with qemu's implementation, but also by reviewing
gnulib's implementation (which uses __VA_ARGS__ to emulate C23
semantics of an optional message), to be able to feel comfortable
trying to improve our static-assert.h for sharing back to nbdkit, but
I don't mind reviewing anyone else's attempts.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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