Re: [Qemu-devel] [PATCH v2 3/3] qapi: Fix memleak in string visitors on int lists

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> Commit 7f8f9ef1 introduced the ability to store a list of
> integers as a sorted list of ranges, but when merging ranges,
> it leaks one or more ranges.  It was also using range_get_last()
> incorrectly within range_compare() (a range is a start/end pair,
> but range_get_last() is for start/len pairs), and will also
> mishandle a range ending in UINT64_MAX (remember, we document
> that no range covers 2**64 bytes, but that ranges that end on
> UINT64_MAX have end < begin).
>
> The whole merge algorithm was rather complex, and included
> unnecessary passes over data within glib functions, and enough
> indirection to make it hard to easily plug the data leaks.
> Since we are already hard-coding things to a list of ranges,
> just rewrite the thing to open-code the traversal and
> comparisons, by making the range_compare() helper function give
> us an answer that is easier to use, at which point we avoid the
> need to pass any callbacks to g_list_*(). Then by reusing
> range_extend() instead of duplicating effort with range_merge(),
> we cover the corner cases correctly.
>
> Drop the now-unused range_merge() and ranges_can_merge().
>
> Doing this lets test-string-{input,output}-visitor pass under
> valgrind without leaks.
>
> Signed-off-by: Eric Blake <address@hidden>
> ---
>  util/range.c | 75 
> +++++++++++++++++++++++-------------------------------------
>  1 file changed, 29 insertions(+), 46 deletions(-)
>
> diff --git a/util/range.c b/util/range.c
> index dd46092..56e6baf 100644
> --- a/util/range.c
> +++ b/util/range.c
> @@ -28,65 +28,48 @@
>   *   - this can not represent a full 0 to ~0x0LL range.
>   */
>
> -/* 0,1 can merge with 1,2 but don't overlap */
> -static bool ranges_can_merge(Range *range1, Range *range2)
> +/* Return -1 if @a < @b, 1 if greater, and 0 if they touch or overlap. */
> +static inline int range_compare(Range *a, Range *b)
>  {
> -    return !(range1->end < range2->begin || range2->end < range1->begin);
> -}
> -
> -static void range_merge(Range *range1, Range *range2)
> -{
> -    if (range1->end < range2->end) {
> -        range1->end = range2->end;
> -    }
> -    if (range1->begin > range2->begin) {
> -        range1->begin = range2->begin;
> -    }
> -}
> -
> -static gint range_compare(gconstpointer a, gconstpointer b)
> -{
> -    Range *ra = (Range *)a, *rb = (Range *)b;
> -    if (ra->begin == rb->begin && ra->end == rb->end) {
> -        return 0;
> -    } else if (range_get_last(ra->begin, ra->end) <
> -               range_get_last(rb->begin, rb->end)) {
> +    /* Zero a->end is 2**64, and therefore not less than any b->begin */
> +    if (a->end && a->end < b->begin) {
>          return -1;
> -    } else {
> +    }
> +    if (b->end && a->begin > b->end) {
>          return 1;
>      }
> +    return 0;
>  }
>
> +/* Insert @data into @list of ranges; caller no longer owns @data */
>  GList *range_list_insert(GList *list, Range *data)
>  {
> -    GList *l, *next = NULL;
> -    Range *r, *nextr;
> +    GList *l;
>
> -    if (!list) {
> -        list = g_list_insert_sorted(list, data, range_compare);
> -        return list;
> +    /* Range lists require no empty ranges */
> +    assert(data->begin < data->end || (data->begin && !data->end));

Consider { begin = 0, end = 0 }.

Since zero @end means 2^64, this encodes the (non-empty) range
0..2^64-1.

range.h's comment

 * Notes:
 *   - ranges must not wrap around 0, but can include the last byte ~0x0LL.
 *   - this can not represent a full 0 to ~0x0LL range.

appears to be wrong.  The actual limitation is "can't represent ranges
wrapping around zero, and can't represent the empty range starting at
zero."  Would you like to correct it?

I'm afraid range.h is too clever by half.

> +
> +    for (l = list; l && range_compare(l->data, data) < 0; l = l->next) {
> +        /* Skip all list elements strictly less than data */
>      }

Let's put the comment before the loop.  It describes the whole loop.
Also makes the emptiness of the body more obvious.

>
> -    nextr = data;
> -    l = list;
> -    while (l && l != next && nextr) {
> -        r = l->data;
> -        if (ranges_can_merge(r, nextr)) {
> -            range_merge(r, nextr);
> -            l = g_list_remove_link(l, next);
> -            next = g_list_next(l);
> -            if (next) {
> -                nextr = next->data;
> -            } else {
> -                nextr = NULL;
> -            }
> -        } else {
> -            l = g_list_next(l);
> -        }
> +    if (!l || range_compare(l->data, data) > 0) {
> +        /* Rest of the list (if any) is strictly greater than @data */
> +        return g_list_insert_before(list, l, data);
>      }
>
> -    if (!l) {
> -        list = g_list_insert_sorted(list, data, range_compare);
> +    /* Current list element overlaps @data, merge the two */
> +    range_extend(l->data, data);
> +    g_free(data);
> +
> +    /* Merge any subsequent list elements that now also overlap */
> +    while (l->next && range_compare(l->data, l->next->data) == 0) {
> +        GList *new_l;
> +
> +        range_extend(l->data, l->next->data);
> +        g_free(l->next->data);
> +        new_l = g_list_delete_link(list, l->next);
> +        assert(new_l == list);
>      }
>
>      return list;

I think I could fix up things on commit (assuming we agree on what needs
fixing).