[PATCH libxcb] Fix alignment issues in FD passing code

[Mark Kettenis]

From: Mark Kettenis <kettenis at openbsd.org>

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 at openbsd.org>
---
 src/xcb_conn.c | 7 +++++--
 src/xcb_in.c   | 7 +++++--
 2 files changed, 10 insertions(+), 4 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;
diff --git a/src/xcb_in.c b/src/xcb_in.c
index 8c3a58c..fd6c2ef 100644
--- a/src/xcb_in.c
+++ b/src/xcb_in.c
@@ -888,13 +888,16 @@ int _xcb_in_read(xcb_connection_t *c)
         .iov_base = c->in.queue + c->in.queue_len,
         .iov_len = sizeof(c->in.queue) - c->in.queue_len,
     };
-    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 = &iov,
         .msg_iovlen = 1,
-        .msg_control = cmsgbuf,
+        .msg_control = cmsgbuf.buf,
         .msg_controllen = CMSG_SPACE(sizeof(int) * (XCB_MAX_PASS_FD - c->in.in_fd.nfd)),
     };
     n = recvmsg(c->fd, &msg, 0);
-- 
1.8.4.2