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Title: cpython and Clang Static Analyzer
Type: enhancement Stage:
Components: Versions: Python 3.7
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: dilyan.palauzov, mark.dickinson, vstinner
Priority: normal Keywords:

Created on 2017-05-05 22:13 by dilyan.palauzov, last changed 2022-04-11 14:58 by admin.

Messages (4)
msg293145 - (view) Author: Дилян Палаузов (dilyan.palauzov) Date: 2017-05-05 22:13
I compiled cpython using Clang 4.0 Static Analyzer with 
scan-build ./configure --enable-loadable-sqlite-extensions --enable-ipv6 --with-system-expat --with-system-ffi --with-system-libmpdec
scan-build make

and here are the results

Please note, that the information is only about what gets actually compiled, code disabled by #if .. #endif is not considered (e.g. when determining whether a variable assignment is useless).  There are probably some false-positives.

Consider this as information, I do not pretend there are any bugs found by the static analyzer.
msg293179 - (view) Author: Mark Dickinson (mark.dickinson) * (Python committer) Date: 2017-05-07 06:59
Thanks for this. I took a look at the 5 reports for Objects/longobject.c, and I don't think there's any action to be taken on any of them. (Two false positives; two "dead assignments" that are used when asserts are enabled, and one division-by-zero that depends on a function being called in a way that never happens in practice.)

* Objects/longobject.c:2823 Assigned value is garbage or undefined

This is a false positive. Here we have:

    a_bits <= a_size * PyLongShift
    shift_digits = (a_bits - DBL_MANT_DIG - 2) / PyLong_SHIFT;

and then we call v_rshift(x_digits, ..., a_size - shift_digits, ...), which fills the first a_size - shift_digits entries of x_digits. Since DBL_MANT_DIG >= PyLong_SHIFT, we have shift_digits < a_size, so x_digits[0] is always initialised by v_rshift.

* Objects/longobject.c:2723 Dead assignment

The value of the assignment is used in a following assert statement; I don't think this should be changed.

* Objects/longobject.c:2463 Dead assignment

Again, the value of the assignment is used in an assert.

* Objects/longobject.c:1828 Division by zero

This function will never get called with bits=0. There are asserts to check this.

* Objects/longobject.c:2830 Assigned value is garbage or undefined

This is another false positive, similar to the first one. Analysing the arithmetic shows that x_digits[0] is always defined.
msg293184 - (view) Author: Дилян Палаузов (dilyan.palauzov) Date: 2017-05-07 09:20
I forgot to pass --with-pydebug to ./configure in order to consider also the asserts.  Here we go:

scan-build ./configure --enable-loadable-sqlite-extensions --enable-ipv6 --with-system-expat --with-system-libmpdec --with-pydebug
scan-build -o /home/didopalauzov/public_html/scan-build-python-3763ea865c make

The output, which has now 9 hints less, is at

The assignments in Modules/socketmodule.c:1456 and Modules/_datetimemodule.c:2232 are correctly detected as superfluous.

In Objects/longobject.c/long_format_binary:

        assert(0); /* shouldn't ever get here*/
        bits = 0; /* to silence gcc warning */

I guess return -1 instead of bits=0 will silent both gcc and the static analyzer warning.
msg293475 - (view) Author: Mark Dickinson (mark.dickinson) * (Python committer) Date: 2017-05-11 08:07
> I guess return -1 instead of bits=0 will silent both gcc and the static analyzer warning.

That's a possibility, though we should also set an exception in that case (since returning an error value without setting an exception is also something that might be picked up by a CPython-oriented static analyser in the future).
Date User Action Args
2022-04-11 14:58:46adminsetgithub: 74473
2017-05-11 08:07:57mark.dickinsonsetmessages: + msg293475
2017-05-11 00:39:22vstinnersetnosy: + vstinner
2017-05-07 09:20:37dilyan.palauzovsetmessages: + msg293184
2017-05-07 06:59:08mark.dickinsonsetnosy: + mark.dickinson
messages: + msg293179
2017-05-05 22:13:21dilyan.palauzovcreate