This was a bug reported by Roberto Di Cosmo. When he tried to reuse the mmap
library for his own project, Mmap.read occasionally got different result when
reading from the same map. This turned out to be a bug in the binding, where a
C pointer was created pointing to a OCaml value, and the OCaml value was
subsequently moved around by the GC after memory allocation and hence
invalidated the C pointer. This patch removes the indirection of C pointer and
uses OCaml macro to access values directly.
Only Mmap.read function had this problem. The other functions, despite having
the same code style, didn't have memory allocation involved hence wouldn't
intrigue such an error. I've changed all of them to the safer style for future
proof. Directly casting OCaml value's *data block* (rather than the value
itself) as a C pointer is not a common practice either, but I'll leave it as it
is.
The bug hadn't occured on XenServer because XenServer didn't make use of the
Mmap.read function (except in one place for debugging). In XenServer, most mmap
operations were going through another pair of separately implemented functions
(Xs_ring.read/write).
Signed-off-by: Zheng Li <dev@xxxxxxxx>
tools/ocaml/libs/mmap/mmap_stubs.c | 24 +++++++++---------------
1 files changed, 9 insertions(+), 15 deletions(-)
----
diff --git a/tools/ocaml/libs/mmap/mmap_stubs.c
b/tools/ocaml/libs/mmap/mmap_stubs.c
--- a/tools/ocaml/libs/mmap/mmap_stubs.c
+++ b/tools/ocaml/libs/mmap/mmap_stubs.c
@@ -71,12 +71,10 @@ CAMLprim value stub_mmap_init(value fd,
CAMLprim value stub_mmap_final(value interface)
{
CAMLparam1(interface);
- struct mmap_interface *intf;
- intf = GET_C_STRUCT(interface);
- if (intf->addr != MAP_FAILED)
- munmap(intf->addr, intf->len);
- intf->addr = MAP_FAILED;
+ if (GET_C_STRUCT(interface)->addr != MAP_FAILED)
+ munmap(GET_C_STRUCT(interface)->addr,
GET_C_STRUCT(interface)->len);
+ GET_C_STRUCT(interface)->addr = MAP_FAILED;
CAMLreturn(Val_unit);
}
@@ -85,21 +83,19 @@ CAMLprim value stub_mmap_read(value inte
{
CAMLparam3(interface, start, len);
CAMLlocal1(data);
- struct mmap_interface *intf;
int c_start;
int c_len;
c_start = Int_val(start);
c_len = Int_val(len);
- intf = GET_C_STRUCT(interface);
- if (c_start > intf->len)
+ if (c_start > GET_C_STRUCT(interface)->len)
caml_invalid_argument("start invalid");
- if (c_start + c_len > intf->len)
+ if (c_start + c_len > GET_C_STRUCT(interface)->len)
caml_invalid_argument("len invalid");
data = caml_alloc_string(c_len);
- memcpy((char *) data, intf->addr + c_start, c_len);
+ memcpy((char *) data, GET_C_STRUCT(interface)->addr + c_start, c_len);
CAMLreturn(data);
}
@@ -108,20 +104,18 @@ CAMLprim value stub_mmap_write(value int
value start, value len)
{
CAMLparam4(interface, data, start, len);
- struct mmap_interface *intf;
int c_start;
int c_len;
c_start = Int_val(start);
c_len = Int_val(len);
- intf = GET_C_STRUCT(interface);
- if (c_start > intf->len)
+ if (c_start > GET_C_STRUCT(interface)->len)
caml_invalid_argument("start invalid");
- if (c_start + c_len > intf->len)
+ if (c_start + c_len > GET_C_STRUCT(interface)->len)
caml_invalid_argument("len invalid");
- memcpy(intf->addr + c_start, (char *) data, c_len);
+ memcpy(GET_C_STRUCT(interface)->addr + c_start, (char *) data, c_len);
CAMLreturn(Val_unit);
}
xen-unstable.hg-1.patch
Description: Text Data
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