Closed Bug 1207767 Opened 6 years ago Closed 6 years ago
UBSan: left shift of negative value in deblocking
This was found with fuzzing+UBSan build. Output from UBSan: codec/common/src/deblocking_common.cpp:201:17: runtime error: left shift of negative value -5 Which is referring to: Deta = WELS_CLIP3 ((((q0 - p0) << 2) + (p1 - q1) + 4) >> 3, -iTc0, iTc0);
Hi Tyson, we actually know this but we don't think it is a problem we need to fixed, this left shift won't make a problem since the data range of this input is limited.
(In reply to sijchen from comment #1) > Hi Tyson, we actually know this but we don't think it is a problem we need > to fixed, this left shift won't make a problem since the data range of this > input is limited. I'm not clear if you mean that this issue cannot be triggered or that it can be but that is part of normal operation. Left-shifts of negative values are undefined in the C spec. This is a runtime error reported by UBSan and the attached test case does actually trigger this issue. I am not exactly sure what the implications are that depends on how data resulting from this operation is used. I will attach a call stack to help debug this issue.
As same as #1207762 I think it is a common false alarm. The purpose of this implementation is to get the 4 times value of the difference between the values of the two pixels. The range of the value will be -510 to 510. So the calculated results will not exceed the expected value. I think we can ignore this type of alarms.
Again same as bug 1207762. Well it's not a false alarm, things just seem to be working at the moment. The problem is that we are treating a '<< 2' as if it were a '* 4'. The problem isn't the value range, yes it should be a concern, but in this case what is happening in undefined regardless. That means we are at the mercy of the compiler. Different things could happen with different compilers, different build types: x86, arm, 32 or 64-bit, optimized, debug, non-debug, combination of the build types or even different compiler version. So TLDR; a left shift of a negative number by 2 is NOT equal to multiplying it by 4 although it may act that way some or most of the time (even when working within seemingly unaffected value ranges).
after getting more details I tend to think this should be fixed, but the final decision is upon @Haibo, however we may not catch the release 1.5 timeline.
As same as in #1207762. Although I think it is not a bug, I have added a fix to remove it at commit af6a9a8 at master branch and commit 9c88070 at openh264v1.5 branch. Please help to verify it.
Verified with commit: fb61733b2.
Talked to Sijia. Close it here since it is fixed and verified.
Status: NEW → RESOLVED
Closed: 6 years ago
Resolution: --- → FIXED
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