Fix alignment issues in FD passing code

A char array on the stack is not guaranteed to have more than byte alignment.
This means that casting it to a 'struct cmsghdr' and accessing its members
may result in unaligned access.  This will generate SIGBUS on struct
alignment architectures like OpenBSD/sparc64.  The canonical solution is to
use a union to force proper alignment.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Matthieu Herrb <matthieu@herrb.eu>
Signed-off-by: Uli Schlachter <psychon@znc.in>

1 files changed, 5 insertions, 2 deletions

diff --git a/src/xcb_conn.c b/src/xcb_conn.c
index 50e7fb6..46390e1 100644
--- a/src/xcb_conn.c
+++ b/src/xcb_conn.c
@@ -216,13 +216,16 @@ static int write_vec(xcb_connection_t *c, struct iovec **vector, int *count)
 
 #if HAVE_SENDMSG
     if (c->out.out_fd.nfd) {
-        char cmsgbuf[CMSG_SPACE(sizeof(int) * XCB_MAX_PASS_FD)];
+        union {
+            struct cmsghdr cmsghdr;
+            char buf[CMSG_SPACE(XCB_MAX_PASS_FD * sizeof(int))];
+        } cmsgbuf;
         struct msghdr msg = {
             .msg_name = NULL,
             .msg_namelen = 0,
             .msg_iov = *vector,
             .msg_iovlen = n,
-            .msg_control = cmsgbuf,
+            .msg_control = cmsgbuf.buf,
             .msg_controllen = CMSG_LEN(c->out.out_fd.nfd * sizeof (int)),
         };
         int i;