libxc: check return values on mmap() and madvise() on xc_alloc_hypercall_buffer()

On a Thinkpad T4440p with OpenSUSE tumbleweed with v3.15-rc4
and today's latest xen tip from the git tree strace -f reveals
we end up on a never ending wait shortly after

write(20, "backend/console/5\0", 18 <unfinished ...>

This is right before we just wait on the qemu process which we
had mmap'd for. Without this you'll end up getting stuck on a
loop if mmap() worked but madvise() did not. While at it I noticed
even the mmap() error fail was not being checked, fix that too.

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/libxc/xc_linux_osdep.c

@@ -92,14 +92,32 @@ static void *linux_privcmd_alloc_hypercall_buffer(xc_interface *xch, xc_osdep_ha
 {
     size_t size = npages * XC_PAGE_SIZE;
     void *p;
+    int rc, saved_errno;
 
     /* Address returned by mmap is page aligned. */
     p = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_LOCKED, -1, 0);
+    if ( p == MAP_FAILED )
+    {
+        PERROR("xc_alloc_hypercall_buffer: mmap failed");
+        return NULL;
+    }
 
     /* Do not copy the VMA to child process on fork. Avoid the page being COW
         on hypercall. */
-    madvise(p, npages * XC_PAGE_SIZE, MADV_DONTFORK);
+    rc = madvise(p, npages * XC_PAGE_SIZE, MADV_DONTFORK);
+    if ( rc < 0 )
+    {
+        PERROR("xc_alloc_hypercall_buffer: madvise failed");
+        goto out;
+    }
+
     return p;
+
+out:
+    saved_errno = errno;
+    (void)munmap(p, size);
+    errno = saved_errno;
+    return NULL;
 }
 
 static void linux_privcmd_free_hypercall_buffer(xc_interface *xch, xc_osdep_handle h, void *ptr, int npages)