This also handles mmap errors better by using the `uerror` helper
to raise a proper exception using `errno`.

Changed type of `len` from `int` to `size_t`: at construction time we
ensure the length is >= 0, so we can reflect this by using an unsigned
type. The type is unsigned at the C API level, and a negative integer
would just get translated to a very large unsigned number otherwise.

mmap also takes off_t and size_t, so using int64 would be more generic
here, however we only ever use this interface to map rings, so keeping
the `int` sizes is fine.
OCaml itself only uses `ints` for mapping bigarrays, and int64 for just
the offset.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
---
 tools/ocaml/libs/mmap/mmap_stubs.h    |  4 +++-
 tools/ocaml/libs/mmap/xenmmap_stubs.c | 31 +++++++++++++++++----------
 2 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/tools/ocaml/libs/mmap/mmap_stubs.h b/tools/ocaml/libs/mmap/mmap_stubs.h
index 816ba6a724..3352594e38 100644
--- a/tools/ocaml/libs/mmap/mmap_stubs.h
+++ b/tools/ocaml/libs/mmap/mmap_stubs.h
@@ -27,7 +27,7 @@
 struct mmap_interface
 {
 	void *addr;
-	int len;
+	size_t len;
 };
 
 #ifndef Data_abstract_val
@@ -37,4 +37,6 @@ struct mmap_interface
 #define Intf_val(a) ((struct mmap_interface *) Data_abstract_val(a))
 #define Intf_data_val(a) (Intf_val(a)->addr)
 
+value stub_mmap_alloc(void *addr, size_t len);
+
 #endif
diff --git a/tools/ocaml/libs/mmap/xenmmap_stubs.c b/tools/ocaml/libs/mmap/xenmmap_stubs.c
index 4d09c5a6e6..d7a97c76f5 100644
--- a/tools/ocaml/libs/mmap/xenmmap_stubs.c
+++ b/tools/ocaml/libs/mmap/xenmmap_stubs.c
@@ -27,16 +27,18 @@
 #include <caml/custom.h>
 #include <caml/fail.h>
 #include <caml/callback.h>
+#include <caml/unixsupport.h>
 
 #define Wsize_bsize_round(n) (Wsize_bsize( (n) + sizeof(value) - 1 ))
 
-static int mmap_interface_init(struct mmap_interface *intf,
-                               int fd, int pflag, int mflag,
-                               int len, int offset)
+value stub_mmap_alloc(void *addr, size_t len)
 {
-	intf->len = len;
-	intf->addr = mmap(NULL, len, pflag, mflag, fd, offset);
-	return (intf->addr == MAP_FAILED) ? errno : 0;
+	CAMLparam0();
+	CAMLlocal1(result);
+	result = caml_alloc(Wsize_bsize_round(sizeof(struct mmap_interface)), Abstract_tag);
+	Intf_val(result)->addr = addr;
+	Intf_val(result)->len = len;
+	CAMLreturn(result);
 }
 
 CAMLprim value stub_mmap_init(value fd, value pflag, value mflag,
@@ -45,6 +47,8 @@ CAMLprim value stub_mmap_init(value fd, value pflag, value mflag,
 	CAMLparam5(fd, pflag, mflag, len, offset);
 	CAMLlocal1(result);
 	int c_pflag, c_mflag;
+	void* addr;
+	size_t length;
 
 	switch (Int_val(pflag)) {
 	case 0: c_pflag = PROT_READ; break;
@@ -59,12 +63,17 @@ CAMLprim value stub_mmap_init(value fd, value pflag, value mflag,
 	default: caml_invalid_argument("maptype");
 	}
 
-	result = caml_alloc(Wsize_bsize_round(sizeof(struct mmap_interface)), Abstract_tag);
+	if (Int_val(len) < 0)
+		caml_invalid_argument("negative size");
+	if (Int_val(offset) < 0)
+		caml_invalid_argument("negative offset");
+	length = Int_val(len);
 
-	if (mmap_interface_init(Intf_val(result), Int_val(fd),
-	                        c_pflag, c_mflag,
-	                        Int_val(len), Int_val(offset)))
-		caml_failwith("mmap");
+	addr = mmap(NULL, length, c_pflag, c_mflag, Int_val(fd), Int_val(offset));
+	if (MAP_FAILED == addr)
+		uerror("mmap", Nothing);
+
+	result = stub_mmap_alloc(addr, length);
 	CAMLreturn(result);
 }
 
-- 
2.25.1