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

[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.
Good analysis. (Bytes.of_string buf) created a buffer for the result 
from read() for which we have no handle.
Reviewing the new patch I believe it is sound. The (new) signature of 
read_mmap is
> val read_mmap : backend_mmap -> 'a -> bytes -> int -> int
The new implementation is below - argument s used to be a string value 
and is now a bytes value. I would suggest to reflect this in the names 
(using b instead of s) as this is about conversion between strings and 
bytes.
>   let read_mmap back con s len =
> -       let rd = Xs_ring.read back.mmap s len in
> +       let stmp = String.make len (char_of_int 0) in
> +       let rd = Xs_ring.read back.mmap stmp len in
> +       Bytes.blit_string stmp 0 s 0 rd;
>         back.work_again <- (rd > 0);
>         if rd > 0 then
>                 back.eventchn_notify ();
Below are the functions that changed their type and where this also 
should be considered:
> -val read_fd : backend_fd -> 'a -> string -> int -> int
> -val read_mmap : backend_mmap -> 'a -> string -> int -> int
> -val read : t -> string -> int -> int
> -val write_fd : backend_fd -> 'a -> string -> int -> int
> +val read_fd : backend_fd -> 'a -> bytes -> int -> int
> +val read_mmap : backend_mmap -> 'a -> bytes -> int -> int
> +val read : t -> bytes -> int -> int
> +val write_fd : backend_fd -> 'a -> bytes -> int -> int
-- Christian


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