[PATCH] 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.

Signed-off-by: Xenia Ragiadakou <burzalodowa@xxxxxxxxx>
---
I 've noticed that in general in xen the return value of snprintf is not
checked. Is there a particular reason for this? I mean if there is no space to
fit the entire string, is it preferable to write only a part of it instead of
failing? If that's the case, then scnprintf could be used instead below.

 xen/common/hypfs.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/common/hypfs.c b/xen/common/hypfs.c
index acd258edf2..66026ad3e0 100644
--- a/xen/common/hypfs.c
+++ b/xen/common/hypfs.c
@@ -377,6 +377,8 @@ 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) )
+        return -ENOBUFS;
     e_len = DIRENTRY_SIZE(e_namelen);
     direntry.e.pad = 0;
     direntry.e.type = template->e.type;
-- 
2.34.1