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lib: json: fix arr_encode()

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The JSON library doesn't properly encode arrays whose elements are of
object type. Fix that.

This fix avoids allocating a temporary descriptor on the stack, and
keeps the size of struct json_obj_descr unchanged, by preserving an
unintuitive size optimization made by the library.  See the comments
in the patch for more details.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
This commit is contained in:
Marti Bolivar 2017-06-07 11:50:15 -04:00 • committed by Anas Nashif
commit 2620ae0932
1 changed files with 28 additions and 7 deletions
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35
lib/json/json.c
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@ -697,13 +697,18 @@ ssize_t json_escape(char *str, size_t *len, size_t buf_size)
static int encode(const struct json_obj_descr *descr, const void *val,
json_append_bytes_t append_bytes, void *data);
static int arr_encode(const struct json_obj_descr *descr, const void *field,
const void *val, json_append_bytes_t append_bytes,
void *data)
static int arr_encode(const struct json_obj_descr *elem_descr,
const void *field, const void *val,
json_append_bytes_t append_bytes, void *data)
{
struct json_obj_descr elem_descr = { .type = descr->type };
ptrdiff_t elem_size = get_elem_size(descr);
size_t n_elem = *(size_t *)((char *)val + descr->offset);
ptrdiff_t elem_size = get_elem_size(elem_descr);
/*
* NOTE: Since an element descriptor's offset isn't meaningful
* (array elements occur at multiple offsets in `val'), we use
* its space in elem_descr to store the offset to the field
* containing the number of elements.
*/
size_t n_elem = *(size_t *)((char *)val + elem_descr->offset);
size_t i;
int ret;
@ -713,7 +718,23 @@ static int arr_encode(const struct json_obj_descr *descr, const void *field,
}
for (i = 0; i < n_elem; i++) {
ret = encode(&elem_descr, field, append_bytes, data);
/*
* Though "field" points at the next element in the
* array which we need to encode, the value in
* elem_descr->offset is actually the offset of the
* length field in the "parent" struct containing the
* array.
*
* To patch things up, we lie to encode() about where
* the field is by exactly the amount it will offset
* it. This is a size optimization for struct
* json_obj_descr: the alternative is to keep a
* separate field next to element_descr which is an
* offset to the length field in the parent struct,
* but that would add a size_t to every descriptor.
*/
ret = encode(elem_descr, (char *)field - elem_descr->offset,
append_bytes, data);
if (ret < 0) {
return ret;
}