[FFmpeg-devel] [RFC] av_rescale() coverity

[Michael Niedermayer]

Hi all

coverity seems to have started to do a new thing. Namely if theres a
return statement it assumes it can independant of everything occurr

an example would be av_rescale() which on overflow returns INT64_MIN

also with the right flags av_rescale() will pass INT64_MIN and INT64_MAX through
from the input

So coverity since a few days seems to treat every av_rescale() call as if it returns
INT64_MIN and INT64_MAX. coverity doesnt care if that return statement is reachable or
if the flags even include the execution path.

An example is this:
            AVRational time_base_q = AV_TIME_BASE_Q;
            int64_t next_dts = av_rescale_q(ds->next_dts, time_base_q, av_inv_q(ist->framerate));
            ds->next_dts = av_rescale_q(next_dts + 1, av_inv_q(ist->framerate), time_base_q);

Here coverity as a initial statement claims next_dts is INT64_MAX
and next_dts + 1 would overflow


    8. function_return: Function av_rescale_q(ds->next_dts, time_base_q, av_inv_q(ist->framerate)) returns 9223372036854775807.
            9. known_value_assign: next_dts = av_rescale_q(ds->next_dts, time_base_q, av_inv_q(ist->framerate)), its value is now 9223372036854775807.
    331            int64_t next_dts = av_rescale_q(ds->next_dts, time_base_q, av_inv_q(ist->framerate));

    CID 1604545: (#1 of 1): Overflowed constant (INTEGER_OVERFLOW)
    10. overflow_const: Expression next_dts + 1LL, which is equal to -9223372036854775808, where next_dts is known to be equal to 9223372036854775807, overflows the type that receives it, a signed integer 64 bits wide.


another example is this:

    #define AV_TIME_BASE            1000000
    pts = av_rescale(ds->dts, 1000000, AV_TIME_BASE);

coverity hallucinates pts as a tainted negative number here nothing says anything about
the input ds->dts (and thats what would matter)

In the past coverity provided a detailed list of steps on how a
case is reached. One could then check these assumtions and mark things
as false positive when one assumtion is wrong. (coverity was most of the time
wrong)

Now coverity just hallucinates claims out of the blue without any
explanation how that can happen.

Iam a bit at a loss how to deal with this and also why exactly this
new behavior appeared.

Has anyone changed any setting or anything in coverity ?

The number of issues shot up to over 400 on the 22th june
"194 new defect(s) introduced to FFmpeg/FFmpeg found with Coverity Scan."

before this i thought iam mostly done with my coverity work.
now truth is, the STF text speaks about 673 issues at the time and not
what appears after the work started, but it makes me a bit sad if i categorize
~700+ issues and then fix the ones that are bugs just to find coverity
hallucinate 200 new issues a month that ill have to leave open for future
efforts.

I did not expect that years of ignoring coverity accumulate 673 issues and
then suddenly the rate of new issues to shoot up like this. I kind of expected
that i can fix all new issues appearing during the work with insignificant extra effort

thx

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Why not whip the teacher when the pupil misbehaves? -- Diogenes of Sinope
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