Laszlo Ersek
2023-Jun-09 12:45 UTC
[Libguestfs] [libnbd PATCH v4 1/4] states: Document our reliance on type overlaps
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 <lersek at redhat.com> > Signed-off-by: Eric Blake <eblake at 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)? 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. Reviewed-by: Laszlo Ersek <lersek at redhat.com> Thanks Laszlo> assert (h->reply_cmd == NULL); > assert (h->rlen == 0); > > @@ -135,7 +142,8 @@ REPLY.CHECK_SIMPLE_OR_STRUCTURED_REPLY: > } > > /* NB: This works for both simple and structured replies because the > - * handle (our cookie) is stored at the same offset. > + * handle (our cookie) is stored at the same offset. See the > + * STATIC_ASSERT above in state REPLY.START that confirmed this. > */ > h->chunks_received++; > cookie = be64toh (h->sbuf.simple_reply.cookie); > @@ -155,7 +163,8 @@ REPLY.FINISH_COMMAND: > bool retire; > > /* NB: This works for both simple and structured replies because the > - * handle (our cookie) is stored at the same offset. > + * handle (our cookie) is stored at the same offset. See the > + * STATIC_ASSERT above in state REPLY.START that confirmed this. > */ > cookie = be64toh (h->sbuf.simple_reply.cookie); > /* Find the command amongst the commands in flight. */ > diff --git a/generator/states-reply-structured.c b/generator/states-reply-structured.c > index 5aca7262..205a236d 100644 > --- a/generator/states-reply-structured.c > +++ b/generator/states-reply-structured.c > @@ -19,6 +19,7 @@ > /* State machine for parsing structured replies from the server. */ > > #include <stdbool.h> > +#include <stddef.h> > #include <stdint.h> > #include <inttypes.h> > > @@ -45,11 +46,15 @@ structured_reply_in_bounds (uint64_t offset, uint32_t length, > > STATE_MACHINE { > REPLY.STRUCTURED_REPLY.START: > - /* We've only read the simple_reply. The structured_reply is longer, > - * so read the remaining part. > + /* We've only read the bytes that fill simple_reply. The > + * structured_reply is longer, so read the remaining part. We > + * depend on the memory aliasing in union sbuf to overlay the two > + * reply types. > */ > - h->rbuf = &h->sbuf; > - h->rbuf = (char *)h->rbuf + sizeof h->sbuf.simple_reply; > + STATIC_ASSERT (sizeof h->sbuf.simple_reply => + offsetof (struct nbd_structured_reply, length), > + simple_structured_overlap); > + assert (h->rbuf == (char *)&h->sbuf + sizeof h->sbuf.simple_reply); > h->rlen = sizeof h->sbuf.sr.structured_reply; > h->rlen -= sizeof h->sbuf.simple_reply; > SET_NEXT_STATE (%RECV_REMAINING);
Eric Blake
2023-Jun-09 20:39 UTC
[Libguestfs] [libnbd PATCH v4 1/4] states: Document our reliance on type overlaps
[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 <lersek at redhat.com> > > Signed-off-by: Eric Blake <eblake at 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