`cpp/missing-return` incorrectly flags functions using C11 `_Noreturn/noreturn` macros

[mrc0mmand]

Description of the false positive
Hello!

Recently in systemd (systemd/systemd#22711) we noticed several warnings to pop-up after moving from -std=gnu99 to -std=gnu11. After closer inspection it looks like the cpp/missing-return query doesn't properly handle functions explicitly marked as no return using C11 macros (_Noreturn and noreturn).

I prepped an example in mrc0mmand/codeql-test#1 but will include the relevant code here as well.

#include <stdio.h>
#include <stdnoreturn.h>

_Noreturn void assert_no_return_c11(void) {
    puts("c11");
}

noreturn void assert_no_return_c11_std(void) {
    puts("c11 std");
}

__attribute__((__noreturn__)) void assert_no_return_c99(void) {
    puts("c99");
}

static int foo_c99(int x) {
    if (x == 42)
        return 0;

    assert_no_return_c99();
}

static int bar_c11(int x) {
    if (x == 42)
        return 0;

    assert_no_return_c11();
}

static int baz_c11_std(int x) {
    if (x == 42)
        return 0;

    assert_no_return_c11_std();
}

int main(void) {
    foo_c99(42);
    bar_c11(42);
    baz_c11_std(42);

    return 0;
}

In the code above, the assert_no_return_c99() function is the only one that's ignored by CodeQL/LGTM, but in practice all three of them should be ignored, as they're explicitly marked as no return functions.

I can reproduce this both in LGTM (see the link below) or manually using the latest CodeQL CLI:

$ codeql database analyze codeql -o results.csv --format=csv -vvv test.qls
$ cat results.csv 
"Missing return statement","All functions that are not void should return a value on every exit path.","error","Function baz_c11_std should return a value of type int but does not return a value here","/codeql-test.c","34","5","34","31"
"Missing return statement","All functions that are not void should return a value on every exit path.","error","Function bar_c11 should return a value of type int but does not return a value here","/codeql-test.c","27","5","27","27"

URL to the alert on the project page on LGTM.com
https://lgtm.com/projects/g/mrc0mmand/codeql-test/snapshot/bc159df6762ccc5d0da28cda09dafb6884439eed/files/codeql-test.c?sort=name&dir=ASC&mode=heatmap#xc61c6be402c3286b:1

[mrc0mmand]

This query shows that indeed only assert_no_return_c99() has a noreturn attribute.

[hvitved]

Hi

Thanks for reporting this. I have handed it over to @github/codeql-c-analysis .

[jketema]

Hi @mrc0mmand. This is indeed a false positive. It should be easy to get this fixed. I'll keep you posted.

[jketema]

Hi @mrc0mmand,

Thanks again for reporting this issue. This is now fixed by #8428.

The fix will propagate to LGTM within a few weeks time, at which point the false positives should disappear.

[mrc0mmand]

Thank you!