server: trim extra '\0's in bytes_to_cstr()

Currently bytes_to_cstr() expects one and only one '\0' and the end of
the buffer, but we've seen lookup request with name buffer having
multiple '\0's at the end and caused failures.

To make it more robust, trim extra zeros in buffer, this also follows
what virtiofsd does.

Signed-off-by: Eryu Guan <eguan@linux.alibaba.com>

Author: eryugey

src/api/server.rs

@@ -1395,9 +1395,21 @@ fn reply_error(err: io::Error, unique: u64, mut w: Writer) -> Result<usize> {
 }
 
 fn bytes_to_cstr(buf: &[u8]) -> Result<&CStr> {
-    // Convert to a `CStr` first so that we can drop the '\0' byte at the end
-    // and make sure there are no interior '\0' bytes.
-    CStr::from_bytes_with_nul(buf).map_err(Error::InvalidCString)
+    // There might be multiple 0s at the end of buf, find & use the first one and trim other zeros.
+    let len = buf.len();
+    let mut n: usize = 0;
+    for c in buf.iter() {
+        if c == &0 {
+            break;
+        }
+        n += 1;
+    }
+    let nul_pos = if n + 1 >= len { len } else { n + 1 };
+    let newbuf = &buf[0..nul_pos];
+
+    // Convert to a `CStr` so that we can drop the '\0' byte at the end and make sure there are no
+    // interior '\0' bytes.
+    CStr::from_bytes_with_nul(newbuf).map_err(Error::InvalidCString)
 }
 
 fn add_dirent(