[PATCH v2] xen/hypfs: check the return value of snprintf to avoid leaking stack accidently

[Xenia Ragiadakou <burzalodowa@xxxxxxxxx>]

The function snprintf() returns the number of characters that would have been
written in the buffer if the buffer size had been sufficiently large,
not counting the terminating null character.
Hence, the value returned is not guaranteed to be smaller than the buffer size.
Check the return value of snprintf to prevent leaking stack contents to the
guest by accident.

Also, for debug builds, add an assertion to ensure that the assumption made on
the size of the destination buffer still holds.

Signed-off-by: Xenia Ragiadakou <burzalodowa@xxxxxxxxx>
---

Changes in v2:
- add ASSERT_UNREACHABLE()
- update commit message accordingly

 xen/common/hypfs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xen/common/hypfs.c b/xen/common/hypfs.c
index 66026ad3e0..7b3377d46e 100644
--- a/xen/common/hypfs.c
+++ b/xen/common/hypfs.c
@@ -377,8 +377,10 @@ int hypfs_read_dyndir_id_entry(const struct 
hypfs_entry_dir *template,
     unsigned int e_namelen, e_len;
 
     e_namelen = snprintf(name, sizeof(name), template->e.name, id);
-    if ( e_namelen >= sizeof(name) )
+    if ( e_namelen >= sizeof(name) ) {
+        ASSERT_UNREACHABLE();
         return -ENOBUFS;
+    }
     e_len = DIRENTRY_SIZE(e_namelen);
     direntry.e.pad = 0;
     direntry.e.type = template->e.type;
-- 
2.34.1