Switch to a shared piece of code to compute start and length of a

buffer from start, end and length. The old code miscomputed length
leading to writing and reading from memory past the end of the buffer.
Consolidating the code should make it easier to get right everywhere.

1 files changed, 11 insertions, 24 deletions

diff --git a/shared-bindings/busio/I2C.c b/shared-bindings/busio/I2C.c
index 7383cb104..b1794fdfb 100644
--- a/shared-bindings/busio/I2C.c
+++ b/shared-bindings/busio/I2C.c
@@ -30,6 +30,7 @@
 #include "shared-bindings/microcontroller/Pin.h"
 #include "shared-bindings/busio/I2C.h"
 
+#include "lib/utils/buffer_helper.h"
 #include "lib/utils/context_manager_helpers.h"
 #include "py/runtime.h"
 //| .. currentmodule:: busio
@@ -187,18 +188,11 @@ STATIC mp_obj_t busio_i2c_readfrom_into(size_t n_args, const mp_obj_t *pos_args,
 
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[ARG_buffer].u_obj, &bufinfo, MP_BUFFER_WRITE);
-    int32_t end = args[ARG_end].u_int;
-    if (end < 0) {
-        end += bufinfo.len;
-    }
-    uint32_t start = args[ARG_start].u_int;
-    uint32_t len = end - start;
-    if ((uint32_t) end < start) {
-        len = 0;
-    } else if (len > bufinfo.len) {
-        len = bufinfo.len;
-    }
-    uint8_t status = common_hal_busio_i2c_read(self, args[ARG_address].u_int, ((uint8_t*)bufinfo.buf) + start, len);
+
+    int32_t start = args[ARG_start].u_int;
+    uint32_t length = bufinfo.len;
+    normalize_buffer_bounds(&start, args[ARG_end].u_int, &length);
+    uint8_t status = common_hal_busio_i2c_read(self, args[ARG_address].u_int, ((uint8_t*)bufinfo.buf) + start, length);
     if (status != 0) {
         mp_raise_OSError(status);
     }
@@ -241,21 +235,14 @@ STATIC mp_obj_t busio_i2c_writeto(size_t n_args, const mp_obj_t *pos_args, mp_ma
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[ARG_buffer].u_obj, &bufinfo, MP_BUFFER_READ);
 
-    int32_t end = args[ARG_end].u_int;
-    if (end < 0) {
-        end += bufinfo.len;
-    }
-    uint32_t start = args[ARG_start].u_int;
-    uint32_t len = end - start;
-    if ((uint32_t) end < start) {
-        len = 0;
-    } else if (len > bufinfo.len) {
-        len = bufinfo.len;
-    }
+
+    int32_t start = args[ARG_start].u_int;
+    uint32_t length = bufinfo.len;
+    normalize_buffer_bounds(&start, args[ARG_end].u_int, &length);
 
     // do the transfer
     uint8_t status = common_hal_busio_i2c_write(self, args[ARG_address].u_int,
-        ((uint8_t*) bufinfo.buf) + start, len, args[ARG_stop].u_bool);
+        ((uint8_t*) bufinfo.buf) + start, length, args[ARG_stop].u_bool);
     if (status != 0) {
         mp_raise_OSError(status);
     }