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

[Michael Young <m.a.young@xxxxxxxxxxxx>]

On Fri, 9 Mar 2018, Christian Lindig wrote:

> > On 9. Mar 2018, at 22:57, Michael Young <m.a.young@xxxxxxxxxxxx> 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@xxxxxxxxxx>
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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