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clock: Fix unchecked overflows in linear regression code

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The initial calculation for the precision shift was wrong and would allow for
overflows during the calculations which were not detected and lead to wrong
results.

Also add a test for a case where overflows where previously not detected and
caused a completely wrong result.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7406>
This commit is contained in:
Sebastian Dröge 2024-08-23 15:56:41 +03:00 committed by GStreamer Marge Bot
parent 4106ad4ae6
commit 41f3fab3d4
2 changed files with 52 additions and 5 deletions
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18
subprojects/gstreamer/gst/gstutils.c
View file

@ -4638,10 +4638,20 @@ gst_calculate_linear_regression (const GstClockTime * xy,
*/
/* Guess how many bits we might need for the usual distribution of input,
* with a fallback loop that drops precision if things go pear-shaped */
max_bits = gst_log2 (MAX (xmax - xmin, ymax - ymin)) * 7 / 8 + gst_log2 (n);
if (max_bits > 64)
pshift = max_bits - 64;
* with a fallback loop that drops precision if things go pear-shaped.
*
* Each calculation of tmp during the iteration is multiplying two numbers and
* then adding them together, or adding them together and then multiplying
* them. The second case is worse and means we need at least twice
* (multiplication) as many bits as the biggest number needs, plus another bit
* (addition). At most 63 bits (signed 64 bit integer) are available.
*
* That means that each number must require at most (63 - 1) / 2 bits = 31
* bits of storage.
*/
max_bits = gst_log2 (MAX (1, MAX (xmax - xmin, ymax - ymin)));
if (max_bits > 31)
pshift = max_bits - 31;
i = 0;
do {

39
subprojects/gstreamer/tests/check/gst/gstutils.c
View file

@ -1894,6 +1894,41 @@ static const GstClockTime times4[] = {
60, 100
};
static const GstClockTime times5[] = {
3097492530135419, 3097492507369715,
3097492729101770, 3097492707369715,
3097493128807704, 3097493107369715,
3097493328386576, 3097493307369715,
3097493728105969, 3097493707369715,
3097494207585880, 3097494187369715,
3097495048016903, 3097495027369715,
3097496287625601, 3097496267369715,
3097497447848684, 3097497427369715,
3097498888090268, 3097498867369715,
3097499128283739, 3097499107369715,
3097501168351161, 3097501147369715,
3097502448473187, 3097502427369715,
3097502729226046, 3097502707369715,
3097503009343256, 3097502987369715,
3097503089910694, 3097503067369715,
3097503131675320, 3097503107369715,
3097503179587096, 3097503147369715,
3097504741040566, 3097504707369715,
3097504941855342, 3097504907369715,
3097506820823108, 3097506787369715,
3097508259718567, 3097508227369715,
3097508421611551, 3097508387369715,
3097509422069495, 3097509387369715,
3097511460842652, 3097511427369715,
3097511740612276, 3097511707369715,
3097513169836240, 3097513147369715,
3097514020101841, 3097513987369715,
3097515540180269, 3097515507369715,
3097516618966664, 3097516587369715,
3097518098778648, 3097518067369715,
3097518938513564, 3097518907369715
};
struct test_entry
{
gint n;
@ -1912,7 +1947,9 @@ struct test_entry
2321009753483354451}, {
32, times3, 291922315691409, 162234934150296, 1370930728180888261,
4392719527011673456}, {
6, times4, 60, 100, 2, 1}
6, times4, 60, 100, 2, 1},
{G_N_ELEMENTS (times5) / 2, times5, 3097518938513564, 3097518903890035,
7927902279407500000, 7932215807091104881},
};
GST_START_TEST (test_regression)