Re: [PATCH 1/2] make xen ocaml safe-strings compliant

[Michael Young <m.a.young@durham.ac.uk>]

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On Fri, 9 Mar 2018, Christian Lindig wrote:

>
>
>> On 9. Mar 2018, at 22:57, Michael Young <m.a.young@durham.ac.uk> wrote:
>>
>> I have had a go at fixing the patch and my revised attempt is attached. I suspect it could be tidied up, but it works for me.
>
> Thank you for giving this another go. What is the key difference to the previous patch which compiled but lead to lock up at run time? I briefly tried your patch and can confirm that it compiles and looks clean except for two trailing spaces.
>
> — Christian
>
> Acked-by: Christian Lindig <christian.lindig@citrix.com>

The problem with the old patch is illustrated by the following section 
from the old patch for tools/ocaml/xenstored/utils.ml
@@ -85,7 +85,7 @@ let create_unix_socket name =
  let read_file_single_integer filename =
         let fd = Unix.openfile filename [ Unix.O_RDONLY ] 0o640 in
         let buf = String.make 20 (char_of_int 0) in
-       let sz = Unix.read fd buf 0 20 in
+       let sz = Unix.read fd (Bytes.of_string buf) 0 20 in
         Unix.close fd;
         int_of_string (String.sub buf 0 sz)

where the patch makes Unix.read write to a Bytes copy of buf and buf 
itself is unchanged, so int_of_string sees a string of null characters 
rather than a string to convert into a number. The net result is that 
information being read by oxenstored from the hypervisor is corrupted or 
lost.
The same basic problem also occurred in a couple of places in 
the old patch of tools/ocaml/libs/xb/xb.ml.
My fix for this is to switch to Bytes at an earlier stage, so for example 
the corresponding section in the new patch becomes
@@ -84,10 +84,10 @@ let create_unix_socket name =

  let read_file_single_integer filename =
  	let fd = Unix.openfile filename [ Unix.O_RDONLY ] 0o640 in
-	let buf = String.make 20 (char_of_int 0) in
+	let buf = Bytes.make 20 (char_of_int 0) in
  	let sz = Unix.read fd buf 0 20 in
  	Unix.close fd;
-	int_of_string (String.sub buf 0 sz)
+	int_of_string (Bytes.to_string (Bytes.sub buf 0 sz))

  let path_complete path connection_path =
  	if String.get path 0 <> '/' then

 	Michael Young

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