Re: [Qemu-devel] [PATCH 2/2] qapi: Fix memleak in string visitors on int lists

[Markus Armbruster]

[I accidentally sent this just to Eric, resending to list...]

Eric Blake  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, especially since
> we are hard-coding things to a list of ranges; so just rewrite
> the thing to open-code the traversal and comparisons, making
> the range_compare() helper function give us a nicer answer,
> avoiding the need to pass any callbacks to g_list_*(). And
> reusing range_extend() ensures 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.
>
> CC: qemu-sta...@nongnu.org
> Signed-off-by: Eric Blake 
> ---
>  include/qemu/range.h | 78 
> +---
>  1 file changed, 31 insertions(+), 47 deletions(-)
>
> diff --git a/include/qemu/range.h b/include/qemu/range.h
> index 4a4801b..9955cca 100644
> --- a/include/qemu/range.h
> +++ b/include/qemu/range.h
> @@ -59,67 +59,51 @@ static inline int ranges_overlap(uint64_t first1, 
> uint64_t len1,
>  return !(last2 < first1 || last1 < first2);
>  }
>
> -/* 0,1 can merge with 1,2 but don't overlap */
> -static inline bool ranges_can_merge(Range *range1, Range *range2)
> +/* Return -1 if @a < b, 1 if greater, and 0 if they overlap. */
> +static inline int range_compare(Range *a, Range *b)
>  {
> -return !(range1->end < range2->begin || range2->end < range1->begin);
> -}
> -
> -static inline 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 inline 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)) {
> +if (a->end && a->end < b->begin) {

This gave me pause.  It's owed to Range's subtle semantics.  Zero @start
means zero, but zero @end means 2^64!  Zero a->end cannot be less than
any b->begin, so this conditional computes "a's end < b's begin", or in
other words "a ends before b".  Correct.

>  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 */
>  static inline GList *range_list_insert(GList *list, Range *data)
>  {
> -GList *l, *next = NULL;
> -Range *r, *nextr;
> +GList *l = list;
>
> -if (!list) {
> -list = g_list_insert_sorted(list, data, range_compare);
> -return list;
> +/* Range lists require no empty ranges */
> +assert(data->begin || data->end);

Uh, wouldn't { begin = 1, end = 1 } be empty, too?  

Do you mean assert(data->begin < data->end || !data->end)?

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

Recommend

   for (l = list; l && range_compare(l->data, data) < 0; l = l->next)
   ;

> +
> +/* If no list, or rest of list exceeds data, insert data and done */

I understand what you mean, but "exceeds" seems less than clear.
Perhaps: "If the rest of the list (if any) is strictly greater than
@data".

> +if (!l || range_compare(l->data, data) > 0) {
> +return g_list_insert_before(list, l, data);
>  }
>
> -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);
> +/* Merge data into current list element */

Suggest: /* 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) {
> +range_extend(l->data, l->next->data);
> +

[Qemu-devel] [PATCH 2/2] qapi: Fix memleak in string visitors on int lists

[Eric Blake]

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, especially since
we are hard-coding things to a list of ranges; so just rewrite
the thing to open-code the traversal and comparisons, making
the range_compare() helper function give us a nicer answer,
avoiding the need to pass any callbacks to g_list_*(). And
reusing range_extend() ensures 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.

CC: qemu-sta...@nongnu.org
Signed-off-by: Eric Blake 
---
 include/qemu/range.h | 78 +---
 1 file changed, 31 insertions(+), 47 deletions(-)

diff --git a/include/qemu/range.h b/include/qemu/range.h
index 4a4801b..9955cca 100644
--- a/include/qemu/range.h
+++ b/include/qemu/range.h
@@ -59,67 +59,51 @@ static inline int ranges_overlap(uint64_t first1, uint64_t 
len1,
 return !(last2 < first1 || last1 < first2);
 }

-/* 0,1 can merge with 1,2 but don't overlap */
-static inline bool ranges_can_merge(Range *range1, Range *range2)
+/* Return -1 if @a < b, 1 if greater, and 0 if they overlap. */
+static inline int range_compare(Range *a, Range *b)
 {
-return !(range1->end < range2->begin || range2->end < range1->begin);
-}
-
-static inline 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 inline 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)) {
+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 */
 static inline GList *range_list_insert(GList *list, Range *data)
 {
-GList *l, *next = NULL;
-Range *r, *nextr;
+GList *l = list;

-if (!list) {
-list = g_list_insert_sorted(list, data, range_compare);
-return list;
+/* Range lists require no empty ranges */
+assert(data->begin || data->end);
+
+/* Skip all list elements strictly less than data */
+while (l && range_compare(l->data, data) < 0) {
+l = l->next;
+}
+
+/* If no list, or rest of list exceeds data, insert data and done */
+if (!l || range_compare(l->data, data) > 0) {
+return g_list_insert_before(list, l, data);
 }

-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);
+/* Merge data into current list element */
+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) {
+range_extend(l->data, l->next->data);
+g_free(l->next->data);
+/* We know we aren't deleting the list head, so shave time
+ * by passing l instead of list */
+if (g_list_delete_link(l, l->next) != l) {
+abort();
 }
 }

-if (!l) {
-list = g_list_insert_sorted(list, data, range_compare);
-}
-
 return list;
 }

-- 
2.5.5