On Fri, Jun 09, 2023 at 03:39:19PM -0500, Eric Blake wrote: > [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);
This is a nice idea! > 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? We can always start with the 3 arg version and change it if we need to later. At the moment I can't think of a reason to check that fields in two unrelated types overlap, since you'd presumably always want to use them through an actual union type, but I suppose it could happen. > 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. Additionally, we currently only support GCC and Clang, so anything that works for those only is fine. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com nbdkit - Flexible, fast NBD server with plugins https://gitlab.com/nbdkit/nbdkit _______________________________________________ Libguestfs mailing list Libguestfs@redhat.com https://listman.redhat.com/mailman/listinfo/libguestfs
