slirp: Avoid marking naturally packed structs as QEMU_PACKED

Various ipv6 structs in the slirp headers are marked QEMU_PACKED,
but they are actually naturally aligned and will have no padding
in them. Instead of marking them with the 'packed' attribute,
assert at compile time that they are the size we expect. This
allows us to take the address of fields within the structs
without risking undefined behaviour, and suppresses clang
-Waddress-of-packed-member warnings.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

1 files changed, 10 insertions, 2 deletions

diff --git a/slirp/ip6.h b/slirp/ip6.h
index 14e9c78735..1e3e329ce6 100644
--- a/slirp/ip6.h
+++ b/slirp/ip6.h
@@ -133,7 +133,7 @@ struct ip6 {
     uint8_t     ip_nh;               /* next header */
     uint8_t     ip_hl;               /* hop limit */
     struct in6_addr ip_src, ip_dst;  /* source and dest address */
-} QEMU_PACKED;
+};
 
 /*
  * IPv6 pseudo-header used by upper-layer protocols
@@ -145,7 +145,15 @@ struct ip6_pseudohdr {
     uint16_t    ih_zero_hi;       /* zero */
     uint8_t     ih_zero_lo;       /* zero */
     uint8_t     ih_nh;            /* next header */
-} QEMU_PACKED;
+};
 
+/*
+ * We don't want to mark these ip6 structs as packed as they are naturally
+ * correctly aligned; instead assert that there is no stray padding.
+ * If we marked the struct as packed then we would be unable to take
+ * the address of any of the fields in it.
+ */
+QEMU_BUILD_BUG_ON(sizeof(struct ip6) != 40);
+QEMU_BUILD_BUG_ON(sizeof(struct ip6_pseudohdr) != 40);
 
 #endif